US2408826A - Combined frequency modulation radio transmitter and receiver - Google Patents

Description

w. w. VOGEL 2,4085% SMITTER AND RECEIVER Oct. 8, 1946.

COMBINED FREQUENCY MQDULATION RADIO TRAN Filed June 21, 1945 5 Sheets-Sheet l 5 in L m. @3322. 21022355; 55. a Q m 3923 INVENTOR. WlLLlAM W. VOGEL- ATTORNEY Oct. 8, 1946.

w. w. VOGEL COMBINED FREQUENCY MODULATION RADIO TRANSMITTER AND RECEIVER Filed June 21, 1943 5 Sheets-Sheet 2 FIG. 2

HIGH PASS NOISE AMPLIFIER 3O 8 REC AUDIO AMPLIFIER MUTING OSCILLATfi RECTIFIER 53 MICROPHONE 35 5 I2 I 36 l38al24 DISCRIMINATOR 27 INVENTOR. WILLIAM W. VOGEL ATTORN EY Oct. 8, 1946.

w. w. VOGEL 2,408,826

COMBINED FREQUENCY MODULATION RADIO TRANSMITTER AND RECEIVER Filed June 21, 1945 s Sheets-Shet 5 F? no 20 saw 136 as TO MUTING' OSOILLAIQR LIIIIITER 26 AND RECT 33.

TO AFC 21 3mg;- MODULATOR l3 FIG. 5 RELATIVE DISORIHIINIKTOR OUTPUT VOLTAGE FIG. 4 I mmvrox i WILLIAM W. VOGEL BY I 2 3 W RELATIVE F-M VOLTAGE INPUT m NIOROVOLTS AT ANTENNA GROUND CIR. I9

Patented Oct. 8, 1946 CONHHNED FREQUENCY MODULATION RADIO TRANSMITTER AND RECEIVER William W. Vogel, Chicago, 111., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application June 21, 1943, Serial No. 491,595

21 Claims. '1

The present invention relates to improvements in radio communication apparatus and more particularly to improvements in combination radio transmitter and receiver systems of the character use in police and military communication work, for example.

Complete self -contained combination transmitter and receiver units, both of the portable and fixed position types, are now extensively used in many forms of radio communication work and are especially useful in two-way police and military communication work. Such units must of necessity be rugged, light in weight, and easily manufactured in production quantities at low cost. A unit of this type must also be capable of being easily and rapidly conditioned to operate either as a transmitter or as a, receiver, and should be easily tunable to transmit or receive at any desired carrier frequency within an allotted frequency band. Other requirements of such a unit are that the receiving channel of theunit remain quiet during periods when the unit is conditioned for signal reception but a selected signal is not being received, that the unit have sufficient signal radiating power and sufficient sensitivity of reception 'to permit high quality two-way communication to be held over substantial distances, that the unit be capable of radiating a signal at a selected carrier frequency with a minimunl of drift in the carrier frequency, that the receiving channel be automatically blocked against signal detection therein during signal transmission, and that the receiving channel of the unit be capable of maintaining its sensitivity in receiving a signal carrier having a drifting carrier frequency.

In general, it is an object of the present invention to provide an improved combination radio transmitting and receiving system which meets all of the requirements outlined above in a highly satisfactory manner.

it is another object of the invention to provide system of 'the'frequency modulated type which meets all of the requirements outlined above.

it is a further object of the invention to provide a combined transmitting and receiving system which is provided with a receiving channel of the superheterodyne type, and in which porti ans of both the transmission and receiving channels ar used both during signal transmission and signal reception, thereby to minimize the number of component parts of the system without sacriflcing desirable operating features.

' According to still another object of the invention, an improved method is provided for producing a signal modulated carrier, for radiating the signal at a particular carrier frequency, and for minimizing fluctuations in the carrier frequency.

In the illustrated embodiment of the invention, the transmission channel is provided with a mixer stage which follows th master carrier producing oscillator and utilizes a crystal controlled oscillator having a resonant frequency equaling the center intermediate frequency of the first intermediate frequency section of the receiving channel, in order to permit signal transmission and reception at the same carrier frequency without altering the tuning of any of the tunable stages of the system. More specifically, the master os-' cillator of the transmission channel is coupled to the first mixer stage of the receiving channel to operate as a, local oscillator during signal reception. and a mixer stage is provided in the transmission channel for effecting radiation of a signal modulation carrier produced in the transmission channel at the same carrier frequency as that which the receiving channel is conditioned to receive.

According to another object of the invention, energy derived from the crystal controlledbscillater of the transmission channel is utilized to control the master oscillator so that variations in the center frequency of the voltage developed at the output side of the master oscillator are minimized during signal transmission.

According to a still further object of the invention, energy derived from the crystal oscillator of the transmission channel is also utilized to block the receiving channel against signal detection therein when the system is conditioned for signal transmission.

In accordance with a further and more specific object of the invention, muting facilities are'provided for automatically blocking the audio section of the receiving channel to prevent noise voltages which appear in this channel from being reproduced when the system is conditioned for signal reception but a selected signal is not being received, and energy derived from the transmission channel is utilized automatically to render the muting facilities inactive to block the audio section of the receiving channel when the system is conditioned for signal transmission.

In accordance with still another object of the invention, a coupling path is provided between the crystal controlled oscillator of the transmission channel and the first mixer stage of the receiving channel to inject a carrier voltage into the receiving channel during signal transmission for the stated purposes of blocking the receiving channel against signal detection during signal transmission, stabilizing the carrier output frequency of the master oscillator during signal transmission, and rendering the muting facilities inactive during signal transmission.

It is another object of the invention to provide an improved radio receiving system of the double superheterodyne type which is provided with an oscillator of the crystal controlled type at the second mixer stage thereof.

According to a further object of the invention, the crystal controlled oscillator provided at the second mixer stage of the receiving channel is arranged to coact with the crystal controlled oscillator of the transmission channel to minimize fluctuations in the carrier output frequency of the master oscillator during signal transmission.

It is a still further object of the invention to provide an improved arrangement for electively rendering the transmission and receiving channels active and inactive in a manner such that all of the facilities mentioned above are selectively, automatically and appropriately rendered active and inactive as the two channels are selectively conditioned for signal transmission and signal reception.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Figs. 1 and 2, when laid end to end in the order named, illustrate a combined frequency modulated radio transmitting and receiving system characterized by the features of the invention briefly referred to above;

Fig. 3 diagrammatically illustrates the circuit arrangement of the cathode heaters of the electron discharge tubes included in the system shown in Figs. 1 and 2;

Fig. 4 is a graph illustrating the noise and signal response characteristics of the receiver; and

Fig. 5 is a circuit diagram illustrating a modification of the receiving equipment forming a part of the system shown in Figs. 1 and 2.

Referring now more particularly to Figs. 1 and 2 of the drawings, there is illustrated, par-- tially in schematic form, a combination frequency modulated radio transmission and receiving system which is well adapted for use as a complete portable unit and includes a transmitting section l8 and a receiving section H commonly coupled to an antenna ground circuit l9 through a tunable antenna circuit l3. Briefly considered, the transmitting section I 0 comprises a combination automatic frequency control and reactance modulator stage l3, a tunable master oscillator H, a tunable frequency doubler network IS, a tunable transmitter mixer It, a power amplifier l1, and the tunable antenna circuit l8, connected in tandem in the order named. The receiving section I of the system comprises the tunable antenna circuit l8, a tunable radio frequency amplifier 28, a first mixer or converter stage 2|, a first intermediate frequency amplifier 22, a second mixer or converter stage 23, a second intermediate frequency amplifier 24, a first limiter 25, a second limiter 26, a frequency discriminator 21, an audio frequency amplifier 28,

.and a loud speaker 29, all connected in cascade in the order named. As pointed out below the system may be selectively controlled to operate either as a transmitter or a receiver and, when conditioned for operation, is set to operate as a receiver. In order to render the audio section of the receiving channel inoperative to pass noise signals appearing in this channel during intervals when a desired signal is not being received, muting or squelch apparatus is provided which comprises a high pass filter network at coupled to the output side of the frequency discriminator 21, a noise amplifier and rectifier 3|, a direct current amplifier 32 and a muting oscillator and rectifier section 33. These stages are connected in tandem in the order named, and respond to noise voltages appearing in the receiver channel to impress a blocking bias voltage upon the audio amplifier 28 in the manner explained below.

More specifically considered, the transmitting section of the system comprises a microphone I! which is arranged to impress audio frequency voltages developed during operation thereof between the input electrodes of the combination frequency control and modulator tube 38 through a coupling network which comprises a microphone transformer 35, a condenser 36, and a resistor 31.

The space current path through the tube 38 is connected in shunt with the space current path through the tube 40 of the master oscillator l4, and also shunts the tunable frequency determining circuit 4| of the master oscillator I4. This oscillator is of the conventional tuned plate circuit type, the fresuency determining circuit 4| thereof comprising a fixed inductance element 4|a whichis tuned to the desired resonant frequency by means of the shunt connected fixed condenser 4|b and an adjustable tuning condenser 4|c. Operating potentials are supplied to the anodes of the tubes 40 and 38 through a resistor 46 and the inductance element 4|a, a low impedance direct current blocking condenser 4 Id being provided in the tunable circuit 4| in order to isolate this direct current path from ground. The tunable frequency determining circuit 4| of the oscillator I4 is regeneratively coupled to the input electrodes of the tube 40 by means of an inductance element 42 which is inductively coupled to the inductance element 4|. It is also coupled to the input electrodes of the frequency control and modulator tube 38 by means of the inductance element 42 and a. phase shifting network which includes the condenser 31a and the resistor 31b. A suitable grid condenser 44 shunted by a grid leak resistor 43 is serially included in the input circuit of the oscillator tube I4 for the purpose of maintaining the control grid of this tube at the proper operating potential with respect to the cathode of the tube.

The signal modulated carrier voltage developed across the tunable frequency determining circuit 4|, is impressed between the input electrodes of a tube 41 included in the frequency doubler 5 through a network which comprises the coupling condenser 49 and a resistor 48. This tube is provided at its output side with a tunable frequency selective circuit 52, which includes a fixed inductance element 52a shunted by a fixed tuning condenser 52b and an adjustable tuning condenser 52c, and is tuned to a center frequency substantially twice the center resonant frequency of the frequency determining network 4| forming a part of the master oscillator stage I4. Anode current is supplied to the tube 41 through a filter resistor 54 and the inductance element 52a, and the usual direct current isolating condenser 52d is provided in the circuit 52 to isolate the anode current path from ground.

In accordance with the present invention, the

carrier voltage developed through operation of the tunable master oscillator l4 and the tunable frequency doubler I5 is utilized as a heterodyning frequency source for converting a received. frequency modulated radio frequency carrier into a correspondingly modulated intermediate frequency carrier in the first mixer stage 2! of the receiver channel ll. When, therefore, it is desired to utilize the system to transmit and receive signals at a fixed and preestablished carrier frequency without altering the tuning of the tunable circuits inthe system incident to a change from transmission to reception, or vice versa, it is necessary to increase or decrease the output frequency of the frequency doubler i5 by an amount equal to the value of the intermediate frequency utilized in the first intermediate frequency amplifier section 22 of the receiver channel I I. Whether or not the output frequency of the doubler I5 is raised or lowered to provide the desired frequency of carrier transmission, will, of course depend upon whether the doubler output frequency is above'or below the particular carrier frequency at which transmission is to be effected. In the particular arrangement illustrated, a piezoelectric crystal 58 having a resonant frequency equal to the intermediate frequency utilized in the first intermediate amplifier 22 of the receiver is utilized to increase .the frequency of a transmited signal carrier above the carrier frequency appearing across the frequency selective circuit 52 by an amount equal to the intermediate frequency utilized in the first intermediate frequency amplifier 22. More specifically considered, the tunable circuit 52 is coupled to the input electrodes of the transmitter mixer tube 63 through the shunt connected crystal 58 and condenser 59. A grid leak and condenser network comprising the two resistors 55 and 56 and a condenserfvl is provided for maintaining the proper bias potential between the input electrodes of the mixer tube 63. For the purpose of driving the crystal 58 to maintain oscillation of the crystal at its resonant frequency, a tuned circuit 69 is provided which is suitably designed to resonate at the same frequency as the crystal 58 and comprises ,a fixed condenser 60a shunted by an adjustable inductance element 601). This network is included in the screen electrode circuit of the mixer tube 63 and also in the path comprising the'resistor 52 over which the required operating potential is positively applied to the screen electrode of the tube 63.

The mixer tube 53 is provided with a tunable frequency selective output circuit 64, whichcomprises a fixed inductance element E lashunted by the fixed condenser Mb and the adjustable tuning condenser this through the low impedance direct current isolating condensers 64d and .Gde. This circuit is normally maintainedtuned to a frequency which is equal to twice the output frequency of the oscillator it plus the resonant 'frequency of the crystal 53, which latter frequency equals the first intermediate frequency used in the receiver channel ll, The output voltage appearing across the circuit GA is impressed across the input circuit of the power amplifier I! through a coupling network which includes the condenser 65.

Referring now more in detail to the signal receiving channel H of the system, the first mixer stage 2! is illustrated as being resistance-capacitance coupled to the output circuit of the tunable radio frequency amplifier 20 through a network which includes the coupling condenser {wand tem is conditioned for reception,

a filter vresistor l3 whichiis quency utilized in the first condensersBI-b and 6 As indicated above, when the sys the tunable master oscillator 14 and the tunable frequency doubler I5 are utilized as a heterodyning freresistor 69.

.quency source required to effect the desired carrier frequency conversion in the first mixer stage 2 i. To this end, an inductance element 53 which is inductively coupled to the inductance element 52a of the frequency selecting circuit 52 is included'in the cathode-ground circuit of the mixer tube Hi. The output electrodes of this tube are coupled to a fixed tuned frequency selecting'circult .l'i which comprises a fixed condenser Hb shunted by anadjustable inductance element Ha and is tuned to the desired first intermediate frequency of 4.3 megacycles, for example, Anode potential is supplied to the tube 10 over a path which includes the inductance element lla and shunted by a by-pass condenser 12. The tuned output circuit H of the tube Hi is coupled to the input electrodes of the first tube-in the intermediate frequency amplifier 22 through a network which comprises the coupling condenser'l land resistor 15.

The output side of the first intermediate frequency amplifier 22 is coupled to the input electrodes of the mixer tube 16 provided at the second mixer or converter stage 23 in an obvious manner. This tube is provided with output electrodes which are bridged by a frequency selective circuit 8! tuned to the second intermediate frequency of 2.515 megacycles, for example, and comprising a condenser 8H) shunted by an adjustable inductance element 81a. Anode potential is supplied to the tube 16 over a path which includes the inductance element Ma and a filter resistor 83 shunted by a by-pass condenser 82. The voltage appearing across the frequency selective circuit 8| is impressed across the input side of the second intermediate frequency amplifier it through a network which comprises the coupling condenser'ad and a resistor 85. For the urpose of effecting the required carrier frequen'cyi onversion at the second mixer stage 23, the mixer tube section of the Pierce type which includes a piezoelectric crystal ll connected between the con- !6 is provided with an oscillator trol and screen electrodes of the tube. This crystal has a resonant frequency of 6.815 megacycles which is greater than the intermediate freintermediate frequencyamplifier 22 by an amount equal tothe intermediate frequency utilized in' the second intermediate frequency am lifier 23. A suitable biasing network comprising the series connected resistors 18a and 18b shunted by the grid condenser "i9 is provided between the input electrodes of the tube 16 for maintaining the proper bias voltage between these electrodes.

Noise and signal voltages appearing at the output side of the second limiter 26 are introduced into the frequency discriminator 21. Briefly considered, this discriminator comprises a tuned circuit 81, a pair of diode rectifier tubes 88 and '89, the space current paths of which are respectively shunted by load resistors 90 and 9!, a radio frequency by-pass condenser 93 having substantially negligible impedance to frequencies of the order of the second intermediate frequency, and a stabilizing condenser 92. More specifically, the resonant circuit '87 serves totune the frequency discriminator network to .a center frequency equal to the second intermediate 'frequency and comprises .a pair of series connected 810' which are shunted "byian 7 adjustable inductance element 81a. Preferably the last mentioned element is of the variable permeability type being provided with an adjustable powdered ferrous metal core, the position of which ma be changed to alter the inductance of the element within the desired limits. The circuit constants of the resonant circuit 81 ar so chosen that the discriminator network is provided with a band pass characteristic such that all' desired signal components of a frequency modulated carrier appearing in the second intermediate frequency channel 24, 25, 26 may be detected and impressed upon the input circuit of the audio amplifier 28. The voltage appearing across the output side of the second limiter 26 is impressed upon the discriminator network 21 through a coupling condenser 86 which is connected at one side thereof to the junction point between the two condensers 81b and 810. Audio frequency voltages detected through operation of the discriminator 21 appear across the condenser 93 and are impressed upon the input side of the audio frequency amplifier 28 through a coupling circuit which includes radio frequency decoupling resistor 94, an audio frequency filter comprising the resistor 35 and condenser 96, an audio frequency coupling condenser 91, and a volume control voltage dividing network comprising the two resistors 98 and I and a direct current blocking condenser IOI. It will be understood in this regard that the proportion of the availabl audio frequency voltage appearing across the series connected resistors 98 and I00 which is impressed upon the input circuit of the audio frequency amplifier 28, is determined by the etting of the wiper 90 along the resistor 98.

As will be explained mor fully below, noise signals appearing in the signal transmission channel of the receiver in the absence of a received signal modulated carrier are passed through the discriminator 21 and appear as detected audio voltages across the condenser 93. Such detected voltage are impressed across the high pass filter network 30, and those components thereof having frequencies above the cutoff frequency of the filter network are impressed between the input electrodes of the tub I06 included in noise amplifier and rectifier 3I. More specifically considered, the high pass filter 30 comprises a pair of series condensers I04 and I05 and a pair of shunt resistors I02 and I03, and is designed to pass those components of noise voltages which have frequencies above the normal signal reproducing band of the receiver. The noise amplifier section of the tube I06 works into a noise rectifier circuit which comprises the diode section of the tube and a load resistor IIO. This rectifier circuit is coupled to the anode of the tube I06 through a coupling condenser I08 which is of appropriate impedance to pass any noise currents which may be transmitted through the high pass filter 30. Anode and screen potentials are supplied to the tube I06 through the resistors I01 and I09, the second of which is by-passed to ground through a condenser III.

Rectified noise voltages appearing across the load resistor IIO are utilized to control the bias between the input electrodes of the tube II3 provided in the direct current amplifier 32. The initial or threshold bias established between the electrodes of this tube is derived from a voltage dividing network, which comprises the series connected resistors Ha, I IBD and H60 bridged across the available source of anode potential,

and is Provided with a .tap H1 adjustable along the resistor II6b to impress a variable positive potential upon the control electrode of the tube II 3 through the filter resistor H5. The biasing circuits connected between the input electrodes of the tube I I3 are by-passed for audio frequency currents b means of a condenser II 4. Screen and anode potentials are applied to the amplifier tube II3 through the resistor II 8 and the resistors II 8 and H9, in series, respectively.

The direct current amplifier 32 as controlled by the variable bias voltage derived from the load resistor H0, is utilized to control the starting and stopping of the muting oscillator and rectifier 33. This stage of the muting or squelch apparatus comprises a dual purpose tube I 20 having an oscillator section which includes a tuned frequency determining circuit I H connected between the output electrodes of the tube through a .by-pass condenser I22. The resonant circuit I2I is fixed tuned to a particular frequency of from 200 to 300 kilocycles and comprises an inductance element I 2| a shunted by a tuning condenser I2Ib. It is regeneratively coupled to the input electrodes of the tube I 20 by means of a feed back circuit which comprises an inductance element I23 inductively coupled to the inductance element I2Ia and connected in series with a parallel connected grid leak resistor I 24 and condenser I 25 between the control grid and cathode of the tube I 20. Anode potential is supplied to the tube I20 over a path which includes the inductance element I2I a and a resistor I26. The oscillator section of the tube I20 is coupled to the rectifyin circuit of the tube through a coupling condenser I29, and the indicated rectifying circuit serially includes the diode rectifier section of the tube and the resistors I28, I32 and 18b. Any bias voltage appearing across the load resistors I28, I 32 and 18b during operation of the oscillator and rectifier stage 33 is negatively applied to the control grid of the first tube in the audio frequency amplifier 28 over a path which comprises the resistor I21, the resistor I00 and the lower portion of the resistor 98.

In order to insure that the system will be speedily conditioned for operation when cathode heating current is supplied to the cathode of the various tubes provided in the system, all of the tubes, with the exception of the discriminator diode 88, are of the filamentary cathode type. The diode rectifier 88 must of necessity be of the indirectly heated cathode type since the cathode thereof is, during operation of the discriminator 21, maintained at potentials substantially above the reference ground potential present upon the filamentary cathodes of the remaining tubes provided in the system. More specifically, the circuit arrangement of the cathodes provided in the various electron discharge tubes referred to above and also provided in the dia grammatically illustrated sections of the system, is shown in Fig. 3 of the drawings. In this circuit, reference characters corresponding to those used in Figs. 1 and 2, but having the differentiating subscripts a and b, are used to identify the relationship between the cathodes and the respective associated circuit sections, as shown in Figs. 1 and 2. From a consideration of the circuit arrangement shown in Fig. 3, it will be noted that the various cathodes are effectively isolated at radio and audio frequencies by means of the separating filter networks comprising the illustrated high impedance choke coils and the low impedance by-pass condensers. It

'9 will also be noted that current for energizing the various cathodes in the series-parallel circuit -is 'supplied'bya direct'current source I-SB-through the contacts of a manually operable on and ofi switch The cathodeslfiaand-lia'of the electrondischargetubes respectivelyprovided in the mixer IS and the power amplifier "i"? are arranged to be energized in series with each other and with a suitable current limiting resistor it! through the contacts "of -a--manually operable press-to-talk switch l 38. This switch is normally spring biased'to'its open circuit posi- 'tion and may be "utilized in the manner 'explained below selectively toconditionthesystem for signal transmission or signal reception, as desired. It'is provided witha'pair of normally open contacts 138a which are closed to connect the microphone 12 across the primary winding of the transformer 35 only when the switch is operated to condition the system for signal transmission. From'an inspection of thecathode circuit arrangement, it 'will be apparentthatthis circuit has been carefully arranged to utilize "the voltage drops across certain of the cathodes as bias voltages between certain of the other tubes provided in the system. "For example, the voltage drop appearing across the cathode "25a of the tube-provided in the firstlimiter stage 251s impressed :between the filamentary cathode 25a o'fthe tube ll in the proper direction to bias this cathode positively with respect to the control grid of the tube. These bias voltages, asderived from the circuit network shown in Fig.3, are appropriately indicated in Figs. the illustrated battery symbols, and the relationship between the respective battery symbols and the voltage drops across certain of the cathodes shown in Fig. 3 will'be readily apparent from a careful comparison of the circuit shown in Fig. 3 with that shown inFigs. 1 and2.

Preferably, the transmitter mixer tube263 is a pentode o'fthe well'known commercial 3A4 type, the frequency doubler tube 41 and the master scillator tube pcnto'des, the automatic frequency control and reactance modulator tube "idandthefirst mixer tube Ti] are commercial 'type 1L4 pentodes, the

second mixer tube 16 is a commercial type 1R5,

pentagrid converter, the two diodes '88 and "89 are or" the commercial'typelAB and'lSS espectively, thenoise amplifier and rectifier tube Hi5 andthe muting oscillator and rectifiertube F29 are commercial type 185 pentodes, 'and'the direct current "amplifier tube i ii! is a commercial "type 1L4 pentode. Suitable screenjp'otentials are applied to the tubes 63, 11, "til, '38 and T6 over direct currentpaths -which respectively "include thG'filtGfIfiSlStOlS 6'2, '56, 4ii,'-39 and"8fi,-respectively. The potential applied" to the screen elecrode of the muting oscillator and 'rectifiertube 129 is controlled in -the 'mannenmore fully explainedbelow to efiect the d'esired'staiting and stopping of the oscillator section of this tube. It will be understood that "the tuning elements of the various tunable circuits 'provided in the system are gang controlled to be operated 'in unison, so that frequency alignmentbetween'the various resonant frequencies thereof 'is maintained during each tuning operation. More specifically, the tuning element s of the 'antenna circuit -i8, the tuning element of the-radio frequency amplifier 20,-andthe adjustable condensers-64c, 52c and ile, respectively provided at the-tunablestages +6, 1 5-and l Mare mechanically the input electrodes of l and 2 of the drawings by '66 are commercial type 1T4 1 hey transmission at connected-in the manner indicated by the dash line U,sothat allof the enumerated tuning elements may be'operated in'unison.

Briefly toconsider-the operation'of the system, it -will be understood that when the switch I36 is-operated'to its closed circuit position, the cathcdes of all tubes provided in the system, with the exception of the-cathodes tea and lla'of-the 'tubes-providedin the mixer it and the power amplifier 41, are energized from the current source 135. If now the push-to-talk-swi-tch I38 =-is operated to itsclosedcircuit-position, the oathodes l dd and Haare alsoenergized. Due to the filamentary character of the energized cathodes,

are rapidly heated to-electron emitting temperatures following-the -energization thereof.

When the two switches 136 andl'38 are thus operated, the system is conditioned for signa1 the particular carrier frequency established by the tuning of the five tunable stages H5, l5, 1%, I'Land 48 of the transmission channel. In thisregard it will be understood that when-space current ilow through the tube 40 is initiated, the master oscillator I4 starts to oscillate at a-carrier frequency which is primarily determined by the setting of the tuning condenser Me and is secondarily determined by the mag-nitudecf the bias voltage between the control-gridand-cathode of-the tube38. More generally considered, if-the'receiving channel H of the system is designed to operate with a first intermediate frequency-M433 megacycles and signals are to betransmitted and received-at a carrier frequency of 44.3 megacycles, thecondenser 'Mc is=soadjusted that with zero (bias upon the control grid of the-tube'3 8, the master oscillator l liwvill produce a carrier voltage having'a frequenc of -megacycles and-the other tunable circuits of the transmission channel Ill are adjustedaccordingly. With the frequency of the signal-carrier thus determined, an audio frequency voltage 7 developed through operation of the microphone i2 is impressed through the microphone-transformer 3'5 and the coupling condenser 36 between the control grid and cathode of the modulator tube 38. The "resulting audio frequency variation of the voltage between the control-grid and cathode of the tube '38 effectivelych-anges the reactance of the tunable frequency deter-mining circuit-'4 lof the master oscillator l4 ata corresponding rate. In other words, varying the voltage applied between the input electrodes of the tube -38 effectively serves to vary the tuning of the network in like manner, whereby the carrier outputof the oscillator 14 is reactance modulated in accordance with-the audio signal voltage impressed between the input electrodes "of the tube 38. This modulated carrier voltage isimpressed betweenthe input'electrodes of the tube' l'l of the frequency doubler i5 through the coupling condenser'49. Due to the action of the tube 41 in distorting the signal modulated carrier voltage and the action of the tunablefrequency selecting circuit-52 in selecting only'signal'modulated carrier components having twice thefrequen'cy-of the carriervoltage developed at the output side of the oscillator '14, the modulated carrier voltage appearing across the output circuit of the doubler-lli has acarrier frequency'which is twice that of -the'oscillator-carrier output frequency, -i.- e. megacycles in the caseassumed'above. Thesignal modulated carrier voltage appearing across the 'frequencyse- 'lecting-circuit '52 is impressed between the input electrodes of the transmitter mixer tube '63 over 1 l a path which includes the coupling condenser 59 and the heterodyning piezoelectric crystal 58. As previously explained, this crystal has a resonant frequency which is equal to the first intermediate frequency used in the receiving channel II of the system. Accordingly, this crystal, act- Dig in conjunction with the tuned circuit 60, functions to produce a carrier voltage which is electronically mixed in the tube 63 with the carrier frequency output across the tuned circuit 52, so that a carrier is produced at the output side of the mixer tube .63 having a frequency equal.

to twice the output frequency of the oscillator l4 plus the first intermediate frequency. This carrier voltage is frequency modulated in accordance with the audio frequency voltage applied to the input electrodes of the modulator tube 38. At the output side of the tube 63, this particular signal modulated carrier voltage is selected through the action of the tuned frequency selecting circuit 64 and is impressed across the input circuit of the power amplifier I 'I through the coupling condenser 65. After being amplified by the amplifier II, the voltage is transmitted through the tunable antenna circuit I8 and impressed across the antenna ground circuit I9 for radiation.

Referring now more specifically to the function performed by the tunable transmitter mixer I6, it is pointed out above that-the tunable frequency selecting circuit 64 is tuned to respond only to a signal modulated carrier having a carrier frequency which is greater than twice the output carrier frequency of the oscillator I4 by an amount equal to the intermediate frequency utilized in the first intermediate frequency channel 22 of the receiver. Since the carrier voltage appearing across the tuned output circuit 52 of the frequency doubler I is used as a heterodyning frequency source at the first mixer stage 2| during reception and this frequency is mixed with the frequency produced by the crystal 58 to produce a frequency of carrier radiation which is equal to the sum of the two frequencies, signal reception and transmission may be held at the same carrier frequency. Thus if the crystal 58 has a resonant frequency of 4.3 megacycles, equaling the center intermediate frequency used in the first intermediate frequency section of the receiver chamiel II, and the tuning elements of the tunable stages I4, I5, IS, IT, I8 and 20 are adjusted by means of the adjusting element U to a setting wherein the carrier output frequency of the oscillator I4 is 20 megacycles and the carrier output frequency of the doubler I5 is 40 megacycles, then the tunable stages I I, I8 and 20 are tuned to a carrier frequency of 44.3 megacycles. This of course means that if the tunable stages of two remotely located sets of the character illustrated are tuned for transmission and reception at the same carrier frequency, it is unnecessary to alter the settings of the tuning elements of either set when the direction of transmission between the two systems is changed. Thus, the systems of the two sets may rapidly be altered for transmission in either direction with a minimum number of manual operations on the part of the persons using the respective sets for two-way communication.

As indicated above, the desired increase in the frequency of the radiated carrier over the carrier frequency appearing at the output side of the frequency doubler I5 is provided through the action ofthe piezoelectric crystal 58. In considering the manner in which this crystal is driven at its resonant frequency, it is pointed out that at this resonant frequency, the upper terminal of the tuned circuit 52 is effectively at ground potential due to the low impedance of this 5 circuit at the particular frequency in question. The resonant circuit 60 which is coupled between the cathode and screen electrode of the tube 63 is precisely tuned to the resonant frequency of the crystal 58. Due to the electronic and capacitance coupling between the upper terminal of the tuned circuit 60 and the lower terminal of the crystal 58, a sufficient driving voltage is applied across the crystal 58 through the tuned circuit 52 to maintain the oscillation of the crystal. This coupling also serves to maintain the tuned circuit, 50 oscillating at its resonant frequency.

In order to condition the system for signal reception after signal transmission has been'ef fected in the manner explained above, the pushto-talk switch I38 is released. Incident to the restoration of this spring biased switch to its normal position, the cathodes I So and Na of the tube 63 and the tube provided in the power amplifier I! are deenergized in an obvious manner. Thus, the transmitter mixer stage I 6 and the power amplifier stage II of the transmitter channel III are rendered inactive without in any way interrupting or otherwise affecting the operation of the preceding stages I3, I4 and I5. In this regard it is pointed out that when space current flow through the mixer tube 63 is interrupted, the operation of the oscillator section of this tube, i. e. that portion of the tube input circuit which comprises the intercoupled crystal 58 and resonant circuit 60, is arrested. Thus, no carrier voltage is produced in the transmission channel Ill having a frequency approaching the intermediate frequency used at either the first or second intermediate frequency sections of the receiving channel II. Accordingly, the continued operation of the three stages I3, I4 and I 5 of the transmission channel II can in no way interfere with the reception of a selected signal modulated carrier.

Assuming that the system is conditioned for signal reception in the manner explained above, and that the tunable stages of the system are appropriately tuned to the center frequency of a desired frequency modulated signal carrier, the 50 signal carrier voltage appearing across the antenna ground circuit I9 is transmitted through the tunable circuit I8 and the coupling condenser 61 to the input side of the tunable radio frequency amplifier 20. This voltage, as amplified 55 by the amplifier 20, is mixed with the carrier output voltage of theirequency doubler I5, which output voltage is impressed between the cathode and control grid of the tubeIIi over a coupling path including the inductance element 53. It is 60 thus converted into a signal modulated intermediate frequency carrier which is amplified through the first intermediate amplifier 22 and impressed between the input electrodes of the tube 16 provided in the second mixer stage 23.

In accordance with the present invention, the intermediate frequency carrier output from the amplifier 22 is, in the second mixer stage 23, mixed with the carrier frequency produced through operation of the oscillator section of the 70 tube 16 as controlled by the crystal 11, so that a beat frequency carrier, modulated with the signal voltage and of the desired second interme diate frequency, appears across the tuned output circuit 8|. This modulated carrier, as se- 75 lected through the action of the tuned circuit III,

13' is transmitted through the condenser 84 tothe second intermediate frequency amplifier 24 where it is amplified and transmitted successively through the limiter stages 25 and 26 to the input side of the discriminator 21. In this discriminator the modulation components of the second intermediate frequency carrier, as represented by deviations in the carrier frequency from the established center frequency, are detected in the manner pointed out below. The detected signal voltage appears across the condenser 93, which condenser is possessed of exceedingly low impedance at the center carrier frequency and exceedingly high impedance at the audio frequencies. This voltage is impressed across the voltage dividing network comprising the resistors 98 and 100 through the carrier frequency decoupling resistor 94 and the audio frequency coupling condenser 91. The portion of thisvoltage which appears between the wiper 99 and ground is impressed across the input circuit of the audio frequency amplifier 28 in an obvious manner. The audio frequency signal voltage as impressed across the input side of the audio frequency amplifier and transmitted production.

Referring now more particularly to the operation of the discriminator 21, it will be noted that this circuit is essentially a circuit two arms of which respectively include the condensers 81b and 810 of equal capacitances. A third arm of th'e'bridge comprises the capacitive impedance of the diode 8.8. The fourth arm of the bridge comprises the combined capacitive impedance of the diode 89 and the condenser :2. The inductance element 81a is bridged between two terminals of the bridge circuit and the frequency modulated signal voltage is applied to the circuit across the other two terminals thereof. Since the load resistors 90 and El have impedances far in excess of the capacitive impedances oi the diode legs of the bridge circuit at the frequencies involved, they may be neglected in analyzing the circuit. Again the capacitance of the condenser 93 is so much greater than that of. either diode leg of the circuit, that this condenser may also be'neglected in analyzing the circuit, With this bridge circuit arrangement the voltage appearing at the output side of the discriminator is the difference between the absolute values of the voltages to'ground at the upper and lower terminals of the inductance element Ea. From an examination of the bridge, it will be understood that if the capacitance of the condenser Slb equals that of the condenser 810, which it does, and the capacitance of the two diode legs of the circuit are equal, such that the bridge is balanced, the currents respectively traversing the condensers 81b and 810 are equal so that equal voltage drops appear across these condensers. Accordingly, no difierence between the voltages to ground is developed at the upper and lower terminals of the inductance element 81a, regardless of the frequency of the exciting voltage applied to the circuit. In the actual circuit, however, the capacitance of the leg which includes the diode 89 is greater than the capacitance of the leg including the diode 88 by an amount equal to the capacitance value of the condenser 92, such that the bridge is unbalanced. Accordingly during excitation of the circuit, the current traversing the condenser 81c exceeds the current traversing the condenser 81b sothat a to the loud speaker 29 for re- 28 is amplified in this amplifier four terminal bridge current is caused to how through theinductance element 81a.

The magnitude of this current obviously depends upon the reactive impedance of the inductance element 81a at the particular frequency of excitation, and the direction of current-flow is such that the voltage drop across the condenser il'ib is enhanced and that across the condenser iilc is decreased. It willbe understood, therefore, that by suitably proportioning the impedance of the inductance element 81a relative to the reactive impedances of the condensers 81b and Bic at a particular center frequency, to establish a given relationship between the currents traversing the circuit elements 81a, 81b and 81c, the absolute voltages between the upper and lower terminals of the inductance element 81a and ground become equal, In their relationship to each other, however, these voltages are out of phase so that a difference voltage actually exists between the upper and lower terminals of the circuit 87. This difference voltage is, of course, equal to the vector sum of the absolute voltages from the upper and lower terminals of th inductance element 87a to ground. The particular frequency at which these absolute voltages become equal to balance the bridge represents the center frequency at which the voltage appearing at the output side of the discriminator between the cathode of the diode 88 and ground becomes zero. In this regard it is pointed out that when the bridge is balanced so that the voltages from the upper and lower terminals of the inductance element 81a to ground are equal, equal direct voltages are produced across'the' load resistors 90 and 9|. These voltages are opposingly combined in a direct current path through the inductance element 81a so that when equal, no direct voltage appears between the cathode of the diode 88 and ground.

As the exciting voltage for the resonant circuit 8? is increased above the center frequency, due to the signa1 modulation thereof at an audio rate,

4 the reactive impedance of the circuit constants change to alter the relative magnitudes of the currents traversing the circuit elements 81a, 81b and 810, so that the voltage from the upper terminal of the inductance element 81a to ground exceeds that between the lower terminal of the inductance element 81a and ground. Accordinely, a voltage which is positive with respect to ground is produced between the cathode of the diode 86 and ground. If, on the other hand, the exciting frequency for the circuit 81 is decreased below the center frequency, the reactive impedances of the circuit constants change to alter the relative magnitudes of the currents traversing the circuit elements Bid, 81?) and 810 so that the voltage between the lower terminal of the inductance element 81a and ground exceeds that between the upper terminal of the inductance element 811; and ground. As a result, an output voltage which is negative with respect to ground is produced between the cathode of the diode 8B and ground. It has been found that the extent or magnitude of the discriminator output voltage varies in accordance with the departure of the exciting frequency from the center intermediate frequency to which the discriminator network 21 is center tuned. It will be understood, therefore, that if the frequency of the carrier appearing at the output side of the limiter 26 is frequency modulated in accordance with a given audio signal, a corresponding audiofrequency voltage is accurately reproduced across the condenser 93 at the output side of the discriminator 21.

To consider somewhat more fully the action of the condenser 92 in stabilizing the operation of the discriminator network 27, it may be pointed out that if the impedances of the four legs of the bridge circuit are perfectly balanced, changes in the exciting frequency will not produce the desired differences of potential between the upper and lower terminals of the inductance element 81 and ground. By providing the condenser 92 connected in the manner illustrated, however, thereby to insure that the over-all capacitance between the lower terminal of the inductance element 81a and ground exceeds that between the upper terminal of this element and ground, the desired circulating current within the resonant circuit 81 will always be produced to insure stability of circuit operation. In this regard it is pointed out that the unbalancing or stabilizing condenser 92 may be connected either between the lower terminal of the resonant circuit 81 and ground or between the upper terminal of this circuit and ground. In either case, the desired operation of the network is produced. It is noted, however, that when a condenser 92 of appropriate capacitance value is connected between the upper terminal of the circuit 8'! and ground, the direction of circulating current flow within the circuit is reversed. Accordingly, the polarity of the output voltage produced across the condenser 93 incident to a given departure of the exciting frequency from the center intermediate frequency is the reverse of that which is obtained for the same frequency departure when the condenser 92 is connected between the lower terminal of the resonant circuit and ground.

If desired, one rectifying section of the improved discriminator 21 may be combined with the audio frequency amplifier 28 in the manner illustrated in Fig. of the drawings, wherein reference characters corresponding to those used in Fig. 2 identify the same circuit elements. From an examination of the Fig. 5 arrangement,

it will be seen that the diode section of the tube 89 is utilized as one of the rectifying aths of the discriminator, and that the cathode, anode and three grids of the tube are used to amplify the audio frequency voltage which is developed between the wiper 99 and ground during reception 50 of a selected signal. This audio voltage is transmitted to the loud speaker I29 for reproduction through a coupling transformer I39. The manner in which the audio section of the tube is blocked under the control of the muting oscillator 33 and mode of operation of the discriminator 2'! are exactly the same as explained herein with reference to the system shown in Figs. 1 and 2. In fact, the circuit of Fig. 5 may be directly substituted for the discriminator 2! and the audio 60 frequency amplifier 28 in the system of Figs. 1 and 2 to perform in the same manner, when the indicated connections are made between this circuit and the limiter 26, the high pass filter 30, the transmitter mixer I6, the muting oscillator and rectifier 33, and the modulator stage I3.

Automatic frequency control As previously indicated, provisions including the ground, to be produced between stantially constant value which substantially equals the center intermediate frequency to which the resonant circuit of the first intermediate frequency section of the receiving channel I I are tuned. The purpose of this arrangement is to correct for any drift in the output frequency of the oscillator I 4 or in the center frequency of the received signal carrier. In this regard, it is noted that regardless of the settings of the tuning elements provided in the tunable stages I8 and 20 of the receiving channel II, these stages are broadly tuned to the center carrier frequency which correspond to the settings of the tuning elements, so that irrespective of any drift in the center frequency of the received carrier all modulation components of the received signal are passed through these stages of the receiving channel. In a similar manner, the fixed tuned stages of the first and second intermediate frequency sections of the channel II are somewhat broadly tuned in order to permit, within limits, deviations in the center carrier frequencies appearing therein without cutting off the modulation components of the frequency modulated carriers which are transmitted therethrough. It will be understood, therefore, that by providing the improved automatic frequency control arrangement described below, any drift in the output frequency of the oscillator I4 or in the center frequency of a received signal carrier is substantially corrected in so far as the intermediate frequency sections of the receiving channel and the discriminator 21 are concerned.

Briefly to consider the manner in which the output frequency of the oscillator I4 is automatically controlled, it may be assumed that the center frequency of the received signal carrier starts to drift to a value higher than the center frequency to which the resonant circuits of the tunable stages I8 and 20 are tuned, or that whil this center carrier frequency remains constant, the output frequency of the oscillator I4 starts to drift from an established value to a lower value. As a result of the frequency drift and regardless of where it originates, the center frequency of the carrier transmitted through the first intermediate frequency amplifier 22 increases to decrease the center frequency of the carrier transmitted through the second intermediate frequency stages 24, 25 and 26. As will be apparent from the above explanation, this departure in the exciting frequency of the tuned circuit 8! from the center frequency to which this circuit is tuned, causes a bias voltage, which is negative with respect to the cathode of the diode D8 and ground. This bias voltage is negatively applied to the control grid of the modulator tube 38 over a path which includes the radio frequency decoupling resistor 94, the audio frequency decoupling resistor 95 and the filter resistors l3| and 31. At this point it is noted that the audio frequency filter comprising the decoupling resistor 95 and the by-pass condenser 96 prevents the audio frequency components of the voltage appearing at the output side of the discriminator 2! from being impressed between the input electrodes of the modulator tube 38. This filter also prevent audio frequency voltages developed during signal transmission by the microdiscriminator 21 and the modulator stage I3 of 7 phone I2 from being impressed upon the input the transmission channel I, are made in the system for automatically adjusting the output frequency of the frequency doubler l5 so that the difference between this frequency and the center frequency of a-selected carrier is held at a sub 28 through produces a corresponding. decrease in therate of increase of the negative bias voltage app-lied between? the control grid and cathode of the modulator tube 38. The bias applied to the tube 38 continues to increase at a constantly decreasing rate until it is balanced by the center frequencies of the signal carriers traversing the first and second intermediate frequency section of the receiving channel I Lat which point the center frequencies and the bias remain balanced against each other. If the circuit constants of th system areproperly chosen,'the:bias.voltage will in. each instance be stabilized at a, value such that the center frequencies of the signal carriers traversing the first and second intermediate sections of the receiving channel II will be held at values which closely approximate the center frequencies at which these sections of the receiving channel and thediscriminator '21 are designed to operate.

If the center frequency of a selected signal carrier drifts to a value below the center frequency to which the stages I8 and 20 are tuned, or the output frequency of the oscillator I-l drifts from its established value to a higher value, the center frequencies of the signal carriers traversing the iirst and second intermediate frequency sections of the receiving channel II are decreased and increased respectively. As a result, a positive bias voltage appear-sat the output side of the discriminator 27 which is applied to the tube 38 to produce a decrease in the output frequency of the oscillator'l'd. Thecenter frequencies of the signal carriers traversing the intermediate frequency sections of the receiving channel are increased and decreased accordingly. Thus, the frequency correcting action proceeds in the exact manner explained above until a'point of stability is reached at which the center frequency of the signal carrier voltage impressed upon the discriminator network 21 closely approximates the center frequenc to which the resonant circuit 8'! is tuned.

Operation of the muting apparatus Referring now more particularly to the manner in which the audio section of the receiving channel I I is muted or squelched during periods when the system is conditioned for operation but is not being used either for signal transmission or reception, it may be pointed out that at all times when the system is conditioned for reception but is not receiving a desired signal, noise signal voltages appear in those stages of the receiver channel which precede the discriminator 21. These Voltages are transmitted through the intermediate frequency and mixer stages Of'the channel I I and are detected by the discriminator 21 to appear as audio frequency voltages at'the output side .of the discriminator. They may be produced as a result of thermal agitation within the tubes provided in the receiving channel, shot effects,

extraneous noise voltages "appearing across the antenna-ground circuit I9, or'by physical shock decrease in the level of the noise developed across 18 tolthe :circuit elementsprovided in the receiving channel. :Regardless of the origin thereof, howeverythe noise signals are manifested as audio frequency voltages acrossthe output side of the discriminator which, in the absence of the muting apparatus .iprovided in the system, would be passed through the audio frequency amplifier 28 .to the loud speaker 129 for reproduction,

More specifically considered, the noise response .of the receiver is graphically illustrated in Fig. 4

noted that.when no signal carrier is being received, the noise voltage appearing at the output side of the discriminatorZl is high and that the magnitude ofthisvoltage is sharply reduced in response to the application of a selected signal carrier to the antenna-ground circuit I9. The voltage which accompanies the transmission of a selected signal through the receiving channel II, is largely effectedlin the amplitude limiters 25 and 26.

To consider the action of the muting apparatus, it is pointed out that the noise voltage appearing between the cathode of the diode 88 andground at the output side of the discriminator 2'! is impressed upon the input sideof the highpassfilter '30. This filter acts to pass only those components of the noise voltage having frequencies above .the

normal signal reproducing band of the receiver. For example, this filter may be designed to pass frequencies above 20 kilocycles. The noise voltage appearing across the output side of the filter 30 is impressed beteween the input electrodes of the noise amplifier and rectifier tube I06 and appears in amplified'form across the coupling condenser I08 and the diode section of the tube IE5 in series. diode section of the tube llldadirect voltage is Due to the rectifying action of the produced across the load resistor I Ill which Varies .in magnitude in accordance with the magnitude of the noise voltage impressed between the input electrodes of the tube I08, This direct'voltage,

i. esthatacross the resistor I I0, is negatively applied to the control grid of the direct voltage amplifier tube I 13 through the resistor I I2 in op position to thefixed bias voltage normally positively applied to the control grid of the tube II-3 through the resistor H5. The negative voltage appearing across the resistor III) so greatly predominates over that positively applied to the control grid of the tube H3 that this tube is biased beyond its space current cutoifpoint. Accordingly, the voltage drops across th two resisters II8 and H9 are sharply decreased to very low values, with the result that the full voltage 'of the available-source of anode current is positivelyapplied to the screen electrode of the oscillator :and rectifier tube I20. Theapplication of this voltage to the screen electrode of the tube I26 initiates the operation of the .oscillator section of .this tube, so that an oscillatory voltage is the series connected coupling condenser I29 and the space current path between the diode electrodes of the tube. Due to the action of the'diode section of the tube I20 in rectifying the oscillatory voltage, a direct bias voltage is produced across comprising the series resistors I28, I32 and 18b. This bias voltage :is negatively applied tothe controlgrid-of the first tube provided in the audio frequency amplifier the diode load circuit 28, over a ipath which includes the resistors I21 and I and the encircuited portion of the resistor 98, The magnitude thereof is sufiicient to bias the first audio frequency amplifier tube beyond cutoff, whereby the noise signals are prevented from being transmitted through the audio channel of the receiver to the loud speaker 29 for reproduction.

As will be apparent from further consideration of the curve shown in Fig. 4 of the drawings, when a selected signal carrier of substantial magnitude appears across the antenna ground circuit I 9, the limiters and 26 function sharply todecrease the noise voltage developed at the output side of the discriminator 21. This produces a corresponding decrease in the bias voltage developed across the load resistor IIU. When the negative bias applied to the control grid of the tube II 3 is thus reduced to a low value, the current flow through the resistors H8 and I I9 and the space current path of the tube I I3 is sharply increased to produce a corresponding increase in the voltage drops across the two identified resistors. As a result, the voltage which is positively applied to the screen electrode of the oscillator and rectifier tube I20 through the two resistors H8 and H9 is sharply decreased to a value such that operation of the oscillator section of this tube cannot continue. When the production of an oscillatory voltage across the space current path of the tube I20 is thus arrested, the negative bias voltage across the. rectifier load circuit resistors I28, I32 and 18b is reduced to zero, permitting the normal negative bias voltage as developed across the resistor 78b to be impressed upon the control electrode of the first tube provided in the audio frequency amplifier 28. When this amplifier tube isthus unblocked or biased to a normal value, the audio section of the receiving channel is rendered operative to amplify the audio frequency components of the received signal and to transmit the same to the loud speaker 29 for reproduction.

From the foregoing explanation it will be understood that normally, i. e. when the system is conditioned for signal reception, the noise signals appearing in the receiving channel II are utilized to completely block the audio section of the receiving channel against the transmission of noise signals to the loud speaker 29. Morespecifically, the component circuit elements. of the muting apparatus should be so chosen that in the absence of a desired signal, the negative bias voltage developed at the upper terminal of the resistor I28 is approximately 20 volts. To this end, from to volts must be positively applied to the screen electrode of the oscillator and rectifier tube I20 when a tube of the commercial 1S5 type is employed in the oscillator and rectifier stage 33. Further, the component circuit elements of the muting apparatus should be such that when a selected frequency modulated carrier is received having a magnitude exceeding a predetermined low value, the voltage positively applied to the screen electrode of the tube I20 is dropped to approximately 20 volts such that operation of th oscillator section of the tube I20 is arrested. In the absence of an oscillatory voltage between the anode and cathode of this tube, the only negative bias voltage applied to the control grid of the first tube in the audio frequency amplifier 28 is that developed across the grid leak resistor 18b, which voltag is of the orderof one volt. 4

When the apparatus is designed to have the characteristics just described, the audio channel of the receiver will at all times remain blocked during periods when a selected signal is not being received and will. be automatically unblocked when a selected signal is transmitted through the receiving channel of the system to the discriminator 2'! for detection. In this regard it will be understood that since the high pass filter 30 will only pass frequencies outside of the normal signal reproducing frequency band of the receiving channel, the muting apparatus is not responsive to the audio frequency components of a received signal carrier and thus this apparatus i prevented from blocking the audio section of the receiving channel against the transmission of detected signal voltages to the loud speaker 29.

Blocking the receiving channel during transmission In considering the manner in which the receiving channel I I is blocked against reproduction of the signal components of the modulated carrier radiated during operation of the transmission channel II, it is pointed out that in accordance with the present invention, the equipment is deliberately designed and is physically so arranged that a relatively large amount of stray capacitance couplin exists between the circuit elements provided in the input and output circuits of the first mixer 2|, and the electrodes of the crystal 58 and the circuit conductors connecting these electrodes with the input electrodes of the tube 63 and the terminals of the tuned circuit 52. More specifically, the electrodes of the crystal 58, the elements of the tuned circuit 60 and the circuit elements forming the input circuit for the mixer tube 70 are unshielded; and the control grid of the tube III is spaced approximately one inch from the circuit conductor which connects the control grid of the tube 63,

the lower electrode of the condenser 59 and the lower electrode of the crystal 58. Energy derived from the oscillator section of the mixer tube 63 is also delivered to the cathode-grid circuit of the mixer tube 10 through the coupling between the two inductance elements 52a and 53. The resulting coupling path is indicated by the broken line C which extends between the two stages I6 and 2I.. .With this arrangement and during sig nal transmission, when the crystal 58 and the tuned circuit 60 are oscillating, a strong unmodulated carrier voltage appears at the output side 'of the first mixer 2|, having a frequency equal to the center frequency to which the resonant circuits of the first intermediate frequency section of the receiving channel are tuned. This strong carrier voltage as transmitted through the first intermediate frequency amplifier 22, th second mixer 23, the second intermediate frequency amplifier 24, and the two limiter stages 25 and 26, to the input side of the discriminator 21, effectively blocks the enumerated stages of the receiver against the transmission of the signal modulated carrier which is impressed upon the input sid of the tunable radio amplifier 20 through the condenser 61. More particularly, the blocking carrier voltage which appears across the output side of the first mixer 2I in the receiver, as a result of the coupling path provided by thestray capacitance coupling between the circuit elements of this mixer and the circuit elements associated with the crystal 58,'exceeds by several times the modulated signal carrier which appears at the input the mixer 2!, those stages of canoe-see .21 side of the mixer 2| dueto the couplinggbetween the transmitting and receiving channelsthroug the condenser 67. :Since the carrier voltage as derived from the crystal 58 so greatly predominates over that transmitted through th tunable radio frequency amplifier 20 to the input side of the receiver which follow the mixer 2| are efiectively blocked against the transmission of the signal modulated carrier to the discriminator. 27. Thus,.the.loud speaker 29 is prevented'from reproducing the audio frequency voltage developed through operation :of .the microphone 12 when the system is conditioned for transmission.

Stabilizing the carrier frequency daring transmission In accordance with another feature of the present invention, the crystal 58 is alsoutilized in conjunction with th fixed tuned stages of the receiver to set the radiated carriercenterfrequency so that this frequency cannot be changed by the discriminator 21 and is maintained-at desired value. Thus, due to the capacitance coupling between the circuit elements of the first mixer '21 and the circuit .elements of the fixed tuned crystal 58 and'resonant circuit til, a carrier havinga frequency exactly equaling the first intermediate frequency is injected into thefirst intermediate frequency'section of the receiving channel. This carrier is mixed Withthe frequency'produced by the oscillater section of the secondmixer tube'lfi .to produce .a carrier in the second intermediate frequency section of the receiving exactly equals the center frequency to which the resonant circuitL81 is tuned. When thiscarrier voltage is applied'to the discriminator.2.l,.the bias voltage appearing at the output side ofthe discriminator between the lower terminal of the resistor 85 and ground is reduced to anegligible or zero value. Moreover, since the crystal 58 and the crystal ll whichcontrols the oscillator section of the second mixer tube I5 are invariably fixed to oscillate at set frequencies, the negligible bias voltage appearing at th output side-f the discriminator 21 cannot be changed or alteredin the slightest degree during transmission. Accordingly, the bias applied to the input electrodes of the modulator tube 38 through the resistors i3! and 31 is held at a fixed negligible value during transmission with the result that the modulator oscillators l3, M are infiexibly set to produce a signal modulated carrier voltage having a'fixed center frequency. Thus, the crystal 58, acting in conjunction with the stages-HA2,23,24,2-5, 26 andZ'l of the receiving channel ii, functions to stabilize the center frequency of the radiated signal carrier at the definite and fixed value desired. This center frequency value can, however, be altered by adjustment of the adjusting element U to alter the settings of the tuning elements provided in'the tunable stages [4, i5, 1-6, H and E8 of the transmission channel, but once th desired value is established it is maintained by the crystal 58 in'the manner just explained.

Controlling the mating apparatus to unblock the audio frequency section of the receiving channel during signal transmission substantially the exact In accordance with another feature of the present invention, the strong carrier injected-by the crystal 58 and its associated circuit-elements into the first intermediate frequency channel of the receiving channel H through the channel which i --cap.acitance coupling lbetween 1the circuit :ele- .ments associated withitheicrystalzfl 'andzth qcir- .cuit: elements :ofathe first :mixcr. 2 his also; utilized --l]0'lCOIltI'O1".-t'hB mutingapparatusesoxthat -the audio 5 .section -.23 :of :the'zrreceiving channel :11 ;is ;un- ::blocked cor :ren-dered vactive :during :signalxtrans- -mission. iMorezspecifically, this :carri'erEhas1the same reflect, in .so .faras :the :reduct-ion of 74110158 voltages at the .outputrside of the discriminator 19 21 is concerne1d,..as does the application ,ofisa strong signal of a' selected -center ,carrier Efrequencyaizo the 'antennargroundcircuit J9. .,:Accordingly, andas willwbezapparentr by reconsider- ..ing the I curve shown ::in Eig. A 1 of the drawings,

when the. strong :carrier 'is. injected into rthefirst intermediate frequency section ".of the ;,receiving channel, theincisevoltage: developed. betweenethe cathode of the. tube :88' and igroundizat thezoutput side of the'discriminator 21.drops'tovanegligible value. -.As a result, the" oscillator :SBQtiOIlwOfithG tube I20 stops oscillating, for ireasonsrexplained above, and the negative blocking bias is removed from the control grid of the firsttube inrtheaudio frequency amplifier 128. Thus, this ,amplifiergis rendered operative, and man: if desired; 1 used to amplify 1 and transmit to ethe loud-speaker .129 for reproduction, any side tone voltage-suitably derived from the audio channel 10f the "transmission channel H]. l

:From the foregoing explanation it will ibe apparent that an improved arrangement, including the crystal controlled oscillator section :of zthe mixer tube 63, the coupling epathbetweenithis oscillator section and .the first mixer istageizi and the crystal controlled oscillaton section ofithe mixer tube 16, is provided. for minimizing .fiuctuations in the output carrier. frequency. of .thesmaster oscillator l4 during signal transmission. These portions of .the .system are also utilized to perform other functions, such .that unnecessary .duplica- -tion .of circuit elements is avoided. :ThllS, the crystal controlled oscillator zsectionaof ;the mixer .tube .63 and the coupling path'between this oscillator' section .and the first mixer stage 2 l, func- -tion to block :the :receiving :channel againstisignal detection therein :duringl'signalitransmission. All

three of the identifiedzportions of the assist-em .coact to .controlthe mutinglapparatusrso that-the audio section of the receiving channel :is ,unbloclced during signal transmission. The -importanceof providing a crystalcontrolledpscillator section in :the second .mixenstageof: the receivin channel will befully. apparent iniviewpf the;pre- "ceding iexplanation. :Thus, sby-.using.;;a:1fixed :output frequency oscillator of 1this character sat .a point in the control channel connecting the crystal oscillatorsection of the mixer [6 with the input electrodes of the modulator :tube 38, itubecomes impossible to introduce any drift'intozthe center carrier frequency appearing at the input side of the discriminator-'21. This WOll'ldiIlOil-hfi the case if an oscillator of the (tank circuit type were used at the second mixer stagmofthereceiving channel.

While one embodiment of the invention has been disclosed, it will be understood that-various modifications may be made therein, which are within the true spirit and scope of the invention.

I claim:

1. In a combined radio transmitting -and receiving system which includes means for selectively conditioning said system'for'si'gnaltransmission or reception, a receiving channel, a-signal responsive device operative in response to the 7 transmission of areceivedsignal through said oscillator for'controlling said master oscillator through a section of said receiving channel to govern the output frequency of said master oscillator and for controlling said muting means through the same section of said receiving channel 'to render said muting means inoperative to prevent noise or signal voltages from being transmitted to said signal responsive device.

2. In a combined radio transmitting and receiving system which includes means for selectively conditioning said system for signal transmission or reception, a receiving channel, a signal responsive device operative in response to the transmission of a received signal through said channel, muting means responsive to noise voltages appearing in said channel in the absence of a received signal and having frequencies different from the frequencies of received signals transmitted I through said receiving channel to prevent said signal responsive device from responding to the noise voltage, a transmission channel including a master oscillator, and means associated with said transmission channel for controlling the output frequency of said master oscillator and for rendering said muting means inoperative to prevent noise or signal voltages from being transmitted to said signal responsive device.

3. In a radio receiving system which includes a receiving channel provided with an audio frequency section, a signal responsive device operative in response to a signal transmitted through said channel, means responsive to noise voltages appearing in said channel in the absence of a received signal modulated carrier and having frequencies different from the frequencies of received signals transmitted through said receiving channel for blocking theaudio section of said channel and responsive to a received carrier for unblocking said audio section, and means for injecting a carrier voltage into said channel at a point preceding said audio section, thereby to block a portion of said channel preceding said audio section and to control said last-named means so that said audio section is unblocked.

4. In a radio receiving system which includes a receiving channel and a signal responsive device operative in response to a Signal transmitted through said channel, means responsive to noise voltages appearing in said channel in the absence of a received signal and having frequencies different from the frequencies of received signals transmitted through said receiving channel for rendering said signal responsive device nonresponsive to the noise voltages, and means for blocking said channel against the transmission of a received signal to said signal responsive device and for concurrently rendering said lastnamed means ineil'ective.

5. In a combined radio transmitting and receiving system which includes means for selectively conditioning said systemfor signal transmission or reception, a receiving channel including an audio frequency section, meansassociated with said channel for blocking. said audio section against the transmission of noise voltages appearing in said channel in the absence of a received signal, and means operative during signal transmission for automatically controlling said last-named means to open said audio section and for concurrently blocking against signal transmission a portion of said channel preceding said audio section.

6. Ina combined radio transmitting and receiving system which includes means for selectively conditioning said system for signal transmission cept a receiving channel, a signal responsive device operative in response to a signal transmitted through said channel, means associated with said channel for rendering said signal responsive device nonresponsive to noise voltages appearing in said channel in the absence of a received signal, and means for automatically rendering said last-named means ineffective during signal transmission.

7. In a combined radio transmitting and receiving system which includes means for selectively conditioning the system for signal transmission or reception, a receiving channel including a mixer stage followed by intermediate and audio frequency sections and provided with at least one tunable frequency selective stage, a transmission channel provided with a plurality of tunable stages and including a tunable carrier producing oscillator which is coupled to said mixer stage to operate as a local oscillator during signal reception, means associated with one of said channels for adapting said system for signal transmission and reception at the same carrier frequency without altering the setting of any of the tunable stages of either chamiel, and means controlled by said last-named means for normally blocking the audio section of said receiving channel and for opening said audio section during signal transmission.

8. In a combined radio transmitting and re ceiving system which is adapted to be selectively conditioned for signal transmission or signal reception, transmission and receiving channels, a

plurality of coupling paths extending between said channels at points intermediate the ends thereof, means including one of said paths for transferring energy from said transmission channel to said receiving channel during signal reception, and means including another of said paths for transferring energy from said transmission channel to said receiving channel to block said receiving channel against signal detection therein when said system is conditioned for signal transmission.

9. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a receiving channel including a mixer stage followed by an intermediate frequency section, a transmission channel provided with a transmission mixer stage preceded by a carrier producing oscillator which is coupled to aid first-named mixer stage to operate as a local oscillator during signal reception, a second oscillator included in said transmission mixer stage and tuned to resonate at a frequency equal to the center frequency to which the intermediate frequency section of said receiving channel is tuned, a coupling path for injecting an oscillatory voltage derived from said second oscillator into the intermediate frequency section of said receiving channel during signal transmission, thereby to block said receiving channel against signal detection therein, and means for prevent- 25- ing said voltage from'being injected into themtermediate. frequency section of, said receiving channel when said system is conditioned for sig nal reception.

10. In a combined radio transmittingand receiving system which is adapted to be selectively conditioned for signal transmission or 'signalre ception, a receiving channel including a mixer Stage followed. by an intermediate frequency section, a transmission channel provided with a transmission mixer stage preceded by a carrier producing oscillator whichis coupled to said first named mixer stage to operate as a local oscillator during signal reception, a second oscillator. included, in said transmission mixer stage andtuned to' resonate at a frequency equal to the center frequency to which the intermediate frequency section of said receiving channel is tuned, and a coupling path for injecting an oscillatory voltage derived from said second oscillator into the intermediate frequency section of said receiving channel during signal transmission, the energy transferred over said coupling path being sufficient to block said receiving channel against signal detection therein.

11. In a combined radio transmitting and receiving system which includes means for selectively conditioning the system for signal transmission or reception, a receiving channel including a mixer stage followed by an intermediate frequency section and provided with at least one tunable frequency selective stage, a transmission channel provided with a plurality of tunable stages and including a tunable carrier producing oscillator which is coupled to said mixer stage to operate as a local oscillator during signal reception, means associated with one of said channels for adapting said system for signal transmission and reception at the same carrier frequency without altering the tuning of any of the tunable stages of either channel, and means controlled by said last-named means through a portion of said receiving channel for preventing variations in the carrier frequency of a transmitted carrier.

12. In a combined radio transmitting and receiving system which includes means for selectively conditioning the system for transmission or reception, a transmission channel including an oscillator and a mixer stage arranged in tandem in the order named, a crystal controlled oscillator included in said mixer stage to convert the output from said first named oscillator into a carrier having a frequency difierent from the output frequency of said first-named oscillator, a receiving channel, and means including a portion of said receiving channel and said crystal controlled oscillator for holding the frequency of thevoltage appearing at the output side of said firstnamed oscillator substantially constant.

13. In a radio transmitter, a transmission channel including a modulator, a carrier producing oscillator and a mixer stage intercoupled in tandem in the order named, a crystal controlled oscillator included in said mixer stage to convert the modulated carrier output from said first-named oscillator into a correspondingly modulated carrier having a carrier frequency different from the carrier frequency at the output side of said first-named oscillator, and means including a control channel extending from said crystal controlled oscillator to said modulator and having a frequency discriminator therein for controlling said first-named oscillator so that the carrier frequency appearing at the output side of saidfirst-named oscillator is held substantially constant.

14-. In a radio transmitten, a transmission cluding said crystal controlledoscillator and a frequency discriminator controlled thereby for controlling said first-named oscillator-to governthe output frequency of said first-named oscillator.

15. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a transmission channel including a carrier producing oscillator, a receiving channel including an intermediate frequency section, means for automatically controlling the carrier frequency of a signal carrier transmitted through said intermediate frequency section during signal reception, and means responsive to the conditioning of said system for signal transmission for minimizing fluctuations in the output carrier frequency of said oscillator and for rendering said last-named means inactive.

16. In a combined radio transmitting and re-. ceiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a transmission channel including a carrier producing oscillator; a receiving channel including an intermediate frequency section, means for automatically controlling the carrier frequency of a signal carrier transmitted through said intermediate frequency section during signal reception, means for controlling said oscillator to minimize fluctuations in the output carrier frequency thereof, and means for selectively rendering said two last named means operative to perform their stated functions.

1'7. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a transmission channel including a carrier producing oscillator, a receiving channel including a mixer stage followed by an intermediate frequency section, a crystal controlled oscillator provided in said mixer stage, and means including said oscillator for minimizing fluctuations in the output frequency of said carrier producing oscillator when said system is conditioned for signal transmission.

18. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a transmission channel including a carrier producing oscillator followed by a mixer stage which includes a crystal controlled oscillator, a receiving channel including a mixer stage followed by an intermediate frequency section, a crystal controlled oscillator provided in the mixer stage of said receiving channel, and means including said crystal controlled oscillators for minimizing fluctuations in the output frequency of said carrier producing oscillator when said system is conditioned for signal transmission.

19. Transmitting apparatus comprising a first oscillator, means including a second oscillator for converting the output voltage of said first oscillator into a signal carrier voltage having a fre- 27 quency different from the output frequencies of either of said oscillators, and means including a frequency discriminator controlled by said second oscillator for maintaining the mean or center frequency of said first oscillator substantially constant.

20. A wave signal transmission system comprising a first oscillator, means including a second oscillator for converting the output voltage of said first oscillator into a signal carrier voltage having a frequency different from the output frequency of either of said oscillators, and means including a frequency discriminator controlled by said second oscillator for minimizing variations in the mean or center frequency of said first oscillator.

21. In a combined radio transmitting and receiving system, a receiving channel including a reception.

WILLIAM W. VOGEL.

Images