US2398793A - Radio receiving system - Google Patents

Description

April 23', 1946;

H. MAGNU SKI RADIO RECEIVING SYSTEM Filed July 3,}943 4 Sheets-Sheet l ow @02 3c mums v cuts. 552mm. .65. mhsowzmwkz l INVENTOR.

HEN RY MAGNUSKI ATTO R NEY April 23, 19460 H. MAGNUSKI RADIO REQEIYING -SYSTEJVI 4 Sheets-Sheet 2 Filed July 3, 1943 NN mmhdm wmm INVENTOR. HENRY MAGNUSKI w. mozziiwma BY aim ATTORNEY April 23, 1946.

H. MAGNUSKI RADIO RECEIVING SYSTEM Filed July 5, 1943 4 Sheets-Sheet 4 R. F. S ELEGTIVITY FREQUENCY .FIG..5

INVENTOR. HENRY MAGNUSKI FIG. 6 ,uouss RELATIVE- F-M VOLTAGE INPUT m MICROVOLTS AT ANTENNA- GROUND CIRCUIT [9' ATTO R NEY Patented Apr. 23, 1946 name RECEIVING SYSTEM. Y

Henry Magnuski, Chicago, 111 assignor to G alvin-i Manufacturing Corporatiomcl' icago, Ill.,.a.c.0rf-- poration of Illinois Applicationiuly. 3,1943, serial bio-{493,323

19 Claims,

era'tiom so thatthey will alwayspick up and reproduce: at any time signals of the selected frequency which:. may be radiatedfrom oneor more transmittingpoints eithercontinuously or intermittentlyfi. usual receiver. of this character is of:thetwell'known:superheterodyne'type and. is relatively highly: selective,.so that it will not reproduce or otherwise respond. to. signals which are radiated-.atlcarrier frequencies" other than the particular carrier: frequency'to which the receiver is tuned. In order tomaintain a receiver of this type accurately tunedto a signal which is being received; regardless of'any driftinthe carrier frequency of: the. signaL it is. common practice to utilize automatic frequency control facilitieshaving: thetfunctlon' of automatically shifting the local oscillatoroutput frequency. of the receiver as. the received; carrier: frequency drifts. With this arrangement, the: signal. modulated carrier trans.- mitted; through the highly selective intermediate frequency'section of thereceiver is held substantially constant: at. a. value. substantially equaling the; center. frequency to which this selective section of the receiver'is: tuned. This expedientis fullyv satisfactoryin its operation to prevent distortion or-loss of signal reproduction by thereceiver when the signal carrier is continuouslyradiatedat the transmitting: point. If, however,

the'signal is only intermittently'radiated' and the] frequencyzof: signal: carrier radiation is, during an off-signal period,either deliberateor unknowinglyshift'ed away from the particular frequency to which the-receiver is tuned, theselectivestages of the intermediate frequency section of the recei'ver prevent the signal from being. passed fore to provide'improvedi and' exceedingly simple) methods. and. apparatusafor; so. controlling a radio receiving: system astooliviatethe problem briefly outlined above. 7 I

Itis another object; of the present invention'to provide animproved radio: receiving system wherein provisions are made foriautomatically and continuously: adjusting the system for the reception of a? signal radiated: at any frequency within a predetermined frequency range during a period. when the signal is not; being received.

According to a further object of the invention, an. improved arrangement is provided: for automatically arresting. the adjustment oi the system. whena desired signalii'sireceived.

Itv isanothen object: of the invention to provide in1thet radio r'eceivingsystemlof the superheterodynez type, aciricuit. arrangement. for automatical-1y sweeping the outputl frequency of the local oscillator; back. and forth over av predetermined frequency range in orden continuously to adjust the system for the reception of any signal radiated within: 2..COIIIBSDOI1diHg. frequency band.

According to: still another object of the'invention;. the receiver is provided: with automaticfre quency: control facilities, and animproved arrangement isutilized= for automatically transferringthe: control: of thedocalE oscillator outputfrequency' from the frequency sweep circuit to' the automatic frequency control facilities when a signal is transmitted through the intermediate frecontrol thGl'IIOiSBFVOItagQ appearing at. the output side of: the frequency discriminator that the 0scillator sweep circuit is; appropriately controlled tooperate only: during ofi-signal periods;

The: invention, both as to its organization. and method of: operation, together with further objects and-advantages: thereof,.will best beunderstood by; reference. to the: following specification taken rinlconnection with the accompanying drawings', in which":-

Figs; 1 and- 2; when laid-end toend in the'order' tude modulated type.

named, illustrate a radio receiving system of the frequency modulated type characterized by the features of the invention briefly outlined above;

Fig. 3 illustrates a modification of that portion of the circuit equipment shown in Fig. 2;

Figs. 4 and 5 are graphs which, in combination, illustrate the method of operation of the improved sweep circuit arrangement forming a part of the system; and

Fig. 6 is a graph illustrating the'n'oise response characteristic of the system.

Referring now more particularly to Figs. 1 and 2 of the drawings, the invention is illustrated in its embodiment in a radio receiving system of the frequency modulated type, although it will be understood from the following explanation that the invention is not so limited and may, if desired, be employed in a receiver of the ampli- Briefiy considered, the illustrated system comprises an antenna-ground circuit H), a tunable; radio frequency amplifier H, a tunable mixer stage l2, anintermediate frequency amplifier I3, first and second limiter stages 14 and [5, a frequency discriminator [6, an audio. frequency amplifier 11 and a loud speaker Hi, all connected in cascade in the order named. As indicated by the dash line U, the tuning elements of the two tunable stages H and I2 .are ganged together, for unicontrol operation in an entirely conventional manner.

For the purpose ofmuting or blocking the system against the reproduction of noise voltages appearing therein during an off-signal period, improved muting or squelch apparatus is providedwhich is coupled to the output side of the frequency discriminator l6 and is utilized selectively to impress a blocking bias voltage upon the control grid of the first tube provided in the audio frequency amplifier 11 during off-signal periods. In general, this apparatus comprises a high pass filter 22 coupled tothe output side of the discriminator iii, a noise amplifier and rectifier 23, a direct current amplifier 24, and a combination muting oscillator and rectifier 25. The four enumerated stages are connected in tandem in the order named and respond to noise voltages appearing in the receiving channel to block the audio frequency amplifier H in the manner explained below. I

More specifically to consider certain of the elements provided in the receiver, it will be noted that the tunable mixer stage 12 comprises a pentagrid converter tube l2a having its third control grid coupled to the output circuit of the radio frequency amplifier H, and having its output electrodes, namely the anode and cathode thereof, coupled to the first tube of the intermediate frequency amplifier l3 through a frequency selective network which comprises the two inductively coupled tuned circuits 26 and 21.

These circuits are relatively sharply tuned to the center intermediate frequency which the intermediate frequency amplifier I3 is designed to pass. Anode and screen potentials are applied to the tube 12a through the usual blocking resistors Hand 30 which are by-passed to ground by means of by- pass condensers 29 and 3|, respectively. The converter tube 12a is also provided with a local oscillator section which includes the two inner grids and the cathode thereof. More specifically, a tunable frequency determining circuit 34 comprising the shunt connected inductance element 34a, the terminal condenser 340 and the tuning condenser 34b, is connected between the second grid and cathode of the tube l2a in series with the available source of anode potential, which source is suitably bypassed by a condenser 35. This circuit is variably tunable by means of the tuning element 341) over a frequency range which corresponds to the desired tuning range of the system, and is regeneratively coupled to the control grid and cathode of the tube I2a by means of a feed back circuit which comprises an inductance element 36 inductively coupled to the inductance element 34a and connected in series with a parallel -connected grid leak resistor 32 and. condenser 33 between the control grid and cathode of the tube. The local oscillator just described may be designed to oscillate at a frequency which is either greater or less than the center frequency to which the radio frequency amplifier II is tuned by an 7 amount equal to the center frequency to which the intermediate frequency section of the receiver is fixedly tuned. In the present case it may be assumed that the oscillator output frequency is higher than the centerfrequency to which the radio frequencyamplifier H is tuned.

The second limiter l5 and the frequency discriminator l6 are entirely conventional in design and, accordingly, no reference characters have been applied to the component parts thereof, although the details of the two stages have been illustrated in the interests of facilitating the following explanation pertaining to the mode of operation of the system, In this regard it is noted that the audio frequency voltage which appears at the output side of the frequency discriminator l6 during operation of the system to receive a radiated signal, is applied between the input electrodesof the audio frequency amplifier Ila over a path which includes the audio frequency coupling condenser ill. Briefly considered, this amplifier comprises a first amplifier tube Ila which is resistance-capacitance coupled to a second amplifier tube Ill), and the output electrodes of the latter tube are coupled to the input terminals of the loud speaker l8 through a suitable audio frequency coupling transformer 18. The usual cathode biasing circuits H and 11 are provided in the respective cathode legs of the output circuits forthe two tubes Ila and All). The coupling network employed between these two tubes comprises a resistor'15 and a coupling condenser 14. Screen and anode potentials are applied to the tube [1a through the resistor l3 and the resistors 13 and 12 in series,

respectively. Similarly, screen and anode potentials are applied to the tube llb through the primary winding of the coupling transformer 18. Asindicated' above, noise voltages appearing in the signal transmission channel of the receiver in the absence of a received signal modulated carrier are passed through the discriminator l6 and appear as detected audio'and superaudible voltages across the resistors Ilia at the output side of the discriminator. Such detected voltages are impressed across the high pass filter 22 and those components thereof having frequencies above the cutofi frequency of the filter network are impressed between the input electrodes of the tube 23a included in the noise amplifier'and rectifier 23. More specifically, the high pass filter 22 corriprises a pair of series condensers A! and .48 and a pair of shunt resistors 35 and 46, and is designed to pass those components of noise voltages having frequencies above the normal signal reproducing band of the receiver. The noise amplifier section of the tube 23a works into a noise rectifier-circuit 'whichcomprises the diode section of the tube and a load resistor 52, This rectifier circuit is coupled to the anode of the tube 23a through a coupling condenser 54 which is of appropriate impedance to pass any noise current that may be transmitted through the high pass filter 22. Anode and screen potentials are supconnected resistors 58a, 53b and 580 bridged across the available source of anode potential, and is provided with a tap 51a adjustable along the resistor 58b to impress a variable positive potential upon the control electrode of the tube 24a through the filter resistor 51. A filter network comprising th'e resistor 55 and the condenser 56 is provided for preventing alternating components of the voltage appearing across the load resistor 52 from being impressed between the input electrodes of the tube 24a. Screen and anode potentials are supplied to the amplifier tube 24a through the resistor to and the resistors 60 and 59 in series, respectively. The direct current amplifier 24, as controlled by the variable bias voltage derived from the load resistor 52, is utilized to control the starting and stopping of the muting oscillator and rectifier 25. This stage of the muting or squelch apparatus comprises a dual purpose tube 25a having an oscillator section which includes a tuned frequency determining circuit 63 coupled between the output electrodes of the tube through a condenser 64. cult 63 is fixedly tuned'to a particular frequency of from 200 to 300 kilocycles and comprises an inductance element 63a shunted by a tuning condenser 63b. It is regeneratively coupled to the I input electrodes of the tube 25a by means of a feed back circuit which comprises an inductance element 68 inductively coupled to the inductance element 63a and connected in series with a parallel connected grid lead resistor 61' and condenser 66 between the control grid and cathode of the tube 254:. Anode potential is supplied to the tube 25a over a path which includes the inductance element 63a and a filter resistor 65. The oscillator section of the tube 25a is coupled to the rectifying circuit of the tube through a coupling condenser 62, and the indicated rectifyin circuit serially includes the diode rectifier section of the tube and a load resistor 69. Any bias voltage appearing across the load resistor 69 during operation of the oscillator and rectifier stage 25 is negatively applied to the control grid of the first audio frequency amplifier tube l'i'a over a path which includes the resistor 6 I.

For the purpose of automatically controlling the output frequency of the local oscillator embodied in the tunable mixer stage I2 during operation of the system to receive a frequency modulated signal carrier, automatic frequency control facilities are provided which include the frequency control unit I9. This unit comprises a control tube I9a provided with input electrodes coupled to the output side of the discriminator It over a path which includes the resistor 84 and the audio frequency decoupling resistor 81 and filter condenser 83. The output electrodes of this tube are bridged across the tunablefrequency The resonant cirdetermining circuit 34 of the local oscillator, and a phase shifting network comprising the cathodegrid capacitance of the tube I9a, the resistor and the condenser 85 is provided between the electrodes of the tubefor the purpose of providing the required phase displacement between the current drawn by the tube ISa and the currents respectively traversing the capacitive and inductive legs of the tunable circuit 34.

In order to control the frequency control unit I 9 so that the output frequency of the local oscillator provided in the tunable mixer I2 is adjusted back and forth over a given frequency range during off-signal periods, there is provided in accordance with the present invention a sweep oscillator 20 of the relaxation type which is coupled between the output circuit of the first audio amplifier tube Ila and the input electrodes of the frequency control tube I911. In brief, this oscillator comprises an electron discharge tube 20a of the gaseous type, the electrodes of which are shunted by a timing condenser 80. This tube may, if desired, be of the neon filled type, and is connected through the resistor I9 to have applied thereacross the voltage appearing between the anode of the first audio frequency amplifiertube Ila and ground. It possesses the characteristic of being rendered conductive only when a voltage exceeding a predetermined magnitude is impressed across the electrodes thereof and of thereafter being rendered nonconductive only when this voltage is reduced to a predetermined low value. The voltage appearing across the electrodes of the tube ZDaisimpressed between the input electrodes of the frequency control tube ISa through the condenser E! and the resistors 82 and 84. i

In considering the operation of the system shown in Figs. 1 and 2 of the drawings, it may be assumed that the tuning elements of the two tunable stages II and I2 are tuned to center frequencies which correspond to the center carrier frequency at which a signal is to be radiatedfrom a specific transmitting point, and that the signal modulated carrier as radiated from this oint appears across the antenna-ground circuit Iii. This signal voltage as amplifiedthrough the radio, frequency amplifier I I. is mixed with the carrier output of the local oscillator provided at the, tunable mixer stage I2 to be converted into a signal modulated intermediate frequency carrier which is selected by the two-frequency selective circuits 2-5 and Z1 and impressed upon the input side of the intermediate frequency amplifier I3. In this re gard it will be understood that when the illustrated system is conditioned for operation, the oscillator section of the tunable mixer stage I2 is in continuous operation. More specifically, the oscillator section of the mixer I2 is in the nature of a tuned plate circuit oscillator, a portion of the oscillatory voltage developed across the tunable frequency determining circuit 34 being impressed between the cathode and the first control grid of the tube In through the mutual inductance couplingbetween the two inductanc elements 34a and 36. It will be understood, therefore, that when cathode heating current is, supplied to the tube !2a the oscillator section of this tube is maintained in continuous operation. The intermediate frequency carrier voltage as amplified through the intermediate frequency amplifier I3 is transmitted through the first andsecond limiter stages I4 and I5 and impressed upon the frequency discriminator I6. In the two limiter stages I4 and I5, any amplitude modulation components of the carrier are eliminated in an en tirely conventional manner so that the amplitude of the frequency modulated carrier voltage impressed upon the frequency discriminator I6 is substantially constant. The audio signals, as represented by the frequency modulation components of this voltage, are detected through operation of the discriminator I6 and appear as an audio frequency voltage across the output resistors Ilia. This audio frequencyvoltage is impressed between the input electrodes of the first amplifier tube I 1a through the coupling condenser IO; is amplified by the two tubes Ila and Ill); and is impressed through the coupling transformer 18 across the loud speaker I8 for reproduction.

Automatic frequency control Briefly to consider the manner in which the a frequency control unit I9, as controlled by the frequency discriminator I6, functions during si nal reception to automatically vary the output frequency of the local oscillator provided at the tunable mixer stage I2 so that the center frequency of the carrier transmitted through the intermediate frequency section of the receiver is maintained substantially constant, it is pointed out that the two frequency selective circuits 26 and 21', the intermediate frequency amplifier I3, the two limiter stages I4 and I5 and the frequency discriminator I6 are somewhat broadly tuned to a particular center frequency. It is noted, moreover, that regardless of the settings of the tuning elements provided in the tunable radio frequency amplifier II, this amplifier is broadly tuned to the center carrier frequency which corresponds to the settings of the tuning elements, so that regardless of any drift in the center frequency of the received carrier, all modulation components of the received signal are passed through this stage of the receiver. It will be understood, therefore, that by providing the automatic frequency control arrangement described below, any drift in the output frequency of the local oscillator or in the center frequency noted that the audio frequency filter comprising the decoupling resistor 81 and the by-pass condenser 83 prevents the audio frequency components of the voltage appearing at the output side of the discriminator I6 from being impressed between the input electrodes of the frequency control tube I9a. Since the bias voltage applied to the grid of. the tube I9a is of negative polarity, it has the effect of decreasing the space current traverving the anode and cathode of this tube. Thus, the out of phase component of current traversing the space current path of the tube I9a and the circuit elements of the tuned circuit 34 is decreased, thereby efiectively to increase the output frequency of the local oscillator provided at the tunable mixer stage I2. As a result, the center frequency of the signal carrier traversing thefintermediate section of the receiver is increased. When the center frequency of the signal carrier is thus changed to more closely approach the frequency to which the intermediate frequency section of the receiver is center tuned, the rate of increase of the negative bias voltage appearing at the output side of the frequency discriminator I6 is correspondingly decreased to produce a corresponding decrease in the rate of change of the space current flow through the control tube I9a. This effects a corresponding decrease in the rate of change in the output frequency of the local oscillator section of the mixer stage I2. The discriminator IS, the frequency control unit I9, the local oscillator of the tunable mixer stage I2 and the intermediate frequency section of the receiver continue to interact until a point is reached at which the output frequency of the local oscillator is stabilized.

In a similar manner, if the center frequency of the signal carrier traversing the intermedi- 7 ate section of the receiver starts to increase above stages of the receiver. are concerned. In this connection it will be understood that the frequency discriminator I6, which is of conventional design is operative to produce a direct voltage across the output resistors I6a thereof which varies in magnitude and polarity in accordance with departures of the center frequency of a carrier traversing the intermediate section of the receiver from the particular center frequency to which the resonant circuits of this receiver section are tuned. Thus, if the center frequency of a signal modulated carrier traversing the intermediate frequency section of the receiver starts to drift to a value below the particular center frequency to which the selective circuits of this receiver section are tuned, a direct voltage of negative polarity appears at the upper terminal of the two resistors I6a having a magnitude which is directly proportional to the extent of deviation of the carrier center frequency from the particular frequency to which the intermediate frequency section of the receiver is center tuned and increases as the extent of frequency drift increases.

frequency control tube I9a, being applied to the control grid of this tube through the radio frequencydecoupling resistor 84 and the audio frequencydecoupling resistor 81. At this point it is V This voltage is utilized as a bias voltage for the V the particular Value to which this section of the receiver is center tuned, a, bias voltage which is positive with respect to ground appears across the resistors Ifia at the output side of the discriminator I6, the magnitude of this voltage being directly proportional to the extent of deviation of the signal carrier center frequency from the particular frequency to which the intermediate frequency section of the receiver is center tuned. This bias voltage has the effect of increasing the space current traversing the control tube I9a with the result that the out of phase component of current drawn by this tube through the circuit elements of the local oscillator frequency determining circuit 34 is increased. Accordingly, the output frequency of the local oscillator provided at the tunable mixer stage I2 is decreased to produce a corresponding decrease in rate at which the center frequency of the signal carrier traversing the intermediate section of the receiver is shifting from the center frequency to which this section of the receiver is tuned, and

a corresponding decrease in rate of increase of the magnitude of the negative bias voltage appearing across the resistors I611 at the output side of the discriminator. This decrease in the rate-of increase of the positive bias applied to the control tube I9 produces a corresponding dethe intermediate frequency section of the receiver through the action of the amplitude limiters M continue to interact until such time as a point of stability is. reached between the bias voltage applied to the control tube Illa and the ouput frequency of the local oscillator included in the mixer stage It. By appropriate design of the circuit elements entering into the frequency control operation described above, the center frequency of the signal carrier traversing the intermediate frequency section of the receiver may be stabilized at a value which closely approximates the particular center frequency to which the selective circuits of. the intermediate frequency section of the receiver are center tuned. This is true, moreover, even though the center carrier frequency of the received signal departs to a substantial degree from the particular frequency of reception to which the system is initially tuned through operation of the tuning element 34b and the tuning element or elements of the radio frequency amplifier II. It will be understood, therefore, that so long as a frequency modulated carrier is being received, the automatic frequency control facilities function to maintain substantially constant and at a value closely approximating that to which the intermediate frequency section of the receiver is center tuned, the signal carrier which is impressed upon the input side of this receiver section.

Operation of the muting apparatus Briefly to consider the operation of the muting apparatus shown in the lower portion of Fig. 2 of the drawings, it may be pointed out that when the system is conditioned for signal reception but is not receiving a desired signal, noise signal voltages appearing in those stages of the receiver which precede the discriminator It appear as audio and super-audible frequency voltages at the ouput side of this discriminator. More specifically, these voltages are transmitted through the radio frequency, mixer and intermediate frequency sections of the receiver and are detected by the discriminator 6 to appear as audio frequency and super a'udible' frequency voltages at the output side of the discriminator. Such noise voltages maybe produced as a result of thermal agitation within the tubes provided in the system, shot effects, extraneous noise voltages appearing across the antenna-ground circuit IE, or by physical shock to the circuit elements provided in thereceiver. Regardless of the origin thereof, however, the noise signals are manifested as audio and super-audible frequency voltages appearing across the resistors lfia at the output side of the discriminator, and in the absence of the described muting apparatus would be passed through the audio frequency amplifier H to the loud speaker l8 for reproduction.

More specifically considered, the noise response of the receiver is graphically illustrated in Fig. 6 of the drawings wherein the noise voltage appearing across the resistors Ilia is plotted as a function of the selected signal carrier input voltage appearing across the antenna-ground circuit It. From a consideration of this curve it will be noted that when no signal carrier is being received, the noise voltage appearin at the output side of the discriminator I6 is high and that the magnitude of this voltage is sharply reduced in response to the application of a selected signal'carrier to the antenna-ground circuit Ill. The decrease in the level of the noise voltage which accompanies the transmission of a selected signal through the receiver is largely effected "age drops across the two identified resistors.

and [5 and the frequency discriminator l6.

To consider the action of the muting apparatus, itis pointed out that the noise voltage appearing across the discriminator output resistors I 6a. is impressed upon the input side of the high pass filter 22. This filter acts to pass only those componentsof 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 22 is impressed between the input electrodes of the noise amplifier and rectifier tube 23a and appears in amplified form across the coupling condenser 54 and the diode section of this tube. Due to the rectifying action of the diode section of the tube 23a, a direct voltage is produced across the load resistor 52 which varies in magnitude in accordance with the magnitude of the noise voltage impressed between the input electrodes of the tube. This direct voltage, '1. -e., that across the resistor 52, is negatively applied to the control grid of the direct current amplifier tube 24a, through the resistor 55 in opposition to the fixed bias voltage derived from the voltage dividing resistors 58 and normally positively applied to the control grid of the tube 24a through the resistor 51. The negative voltage appearing across the resistor 52 so'greatly predominates over that positively applied to the control grid of the tube 24a that this tube is biased beyond its space current cutoif point. Accordingly, the voltage drops across the two resistors 59 and 60 are sharply decreased to very low values with the result that the full voltage of the available source of anode current is positively applied to the screen electrode of the oscillator and rectifier tube 25a. When the full voltage of the anode current source is thus applied to the screen grid' of the tube -25a, operation of the oscillator section of this tube is initiated, so that an oscillatory voltage is developed across the series connected coupling condenser 62 and the space current path between the diode electrodes of this tube. Due to the action of the diode section of the tube 25a in rectifying this oscillatory voltage, a direct bias voltage is produced across the diode load circuit comprising the resistor 69. This bias voltage is negatively applied to the control grid of the first audio amplifier tube Ila over a path which includes the resistor 6|, and the magnitude thereof is sufficient to bias the tube Ila beyond cutoff, whereby the noise signals are prevented from being transmitted through the audio frequency amplifier I! to the loud speaker [8 for reproduction.

As will be apparent from further consideration of the curve shown in Fig. -6 of the'drawings, when a selected signal carrier of substantial magnitude is transmitted through the intermediate frequency section of the receiver, the limiters I4 and I5, together with the discriminator l6, function sharply to decrease the noise voltage developed at the output side of the discriminator. This produces a corresponding decrease in the bias voltage developed across the load resistor 52. When the negative bias voltage applied to the control grid of the tube 24a is thus redu'ce'd to'a low value, the current flow through the resistors 60 and 59 and the space current path of the tube 24a is sharply increased to produce a corresponding increase in the volta result, the voltage which is positively applied to the screen electrodes of the oscillator and rectifier tube 25a through the two resistors 60 and 59 is sharply decreased to a value such that operation of the oscillator section of the tube 25a cannot continue. When the production of an oscillatory voltage across the space current path of the tube 25a is thus arrested, the ne ative bias voltage developed across the rectifier load circuit comprising the resistor 69 is reduced to zero, permitting the normal negative bias' voltage appearing across the cathode biasing circuit H to appear between the input electrodes of the first audio amplifier tube Ha. When this amplifier tube is thus unblocked or biased to a normal value, the audio section I! of the receiver is rendered operative to amplify the signal frequency components of the received signal and to transmit the same to the loud speaker 18 for reproduction.

From the foregoing explanation it will be understood that normally, 1. a, when the system is conditioned for signal reception, the noise signals appearing in the receiver are utilized to completely block the audio section of the receiver against the transmission of noise signals to the loud speaker 18 or any other signal responsive device which may be utilized in lieu of the loud speaker l8. More specifically, 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 6| is approximately 20 volts. To this end, from 40 to 50 volts must be positively applied to the screen electrode of the oscillator and rectifier tube a when a tube of the commercial 185 type is employed in the oscillator and rectifier stage 25. Further, the component circuit elements of the muting apparatus should be such that when a selected frequency modulated carrier having a magnitude exceeding a predetermined low value is received, the voltage positively applied to the screen electrode of the tube 25a is dropped to approximately 20 volts such that operation of the oscillator section of the tube is arrested. In the absence of an oscillatory voltage between the anode and cathode of this tube, the only negative bias voltage applied between the input electrodes of the first audio tube Ila is that developed across the cathode biasing circuit H.

When the muting 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 receiver to the discriminator It for detection. In this regard it will be understood that since the high pass filter 22 will not pass frequencies within the normal frequency reproducing band of the receiver, the muting apparatus is not responsive to the audio frequency components of a received signal carrier and thus this apparatus is prevented from blocking the audio section of the receiver against the transmission of detected si nal voltages to the loud speaker l8. It will also be understood that the average magnitude of the voltage developed across the load resistor 52 in the noise amplifier and rectifier stage 23 varies directly with the magnitude of the noise voltage components which are passed through the high pass filter 22 and impressed between the input electrodes of the tube 2311. Accordingly, the greater the magnitude of the noise voltages appearing in the receiver, the greater the bias volt age impressed between the cathode and control grid of the direct current amplifier tube 24a. Once this tube is biased beyond cutoff, the voltage drops across the two resistors 59 and 60 are unafiected by changes in the noise level and the positive potential applied to the screen electrode of the muting oscillator and rectifier tube 25a remains constant. Accordingly, the bias voltage negatively applied to the control grid of the audio amplifier control grid 20a remains constant with increases in the magnitude of the noise voltage appearing at the output side of the discriminator 19 above a predetermined value.

From the above explanation it will also be apparent that during periods when a signal is being received, such that no blocking bias voltage appears across the resistor 69 and the normal bias voltage appearing across the cathode biasing network ll is alone impressed between the input electrodes of the first amplifier tube Ila, space current of substantial magnitude traverses the space current path of this tube. Accordingly, the voltage drops across the series connected resistors l2 and 13 provided in the output circuit of the tube Ila are of substantial magnitude and absorb a substantial portion of the available voltage of the anode current source. It is evident, therefore, that during periods of signal reception, the voltage between the resistor terminal 12a and ground is of a low order of magnitude. During off-signal periods, on the other hand, when the high negative voltage appearing across the load resistor 69 is utilized to block space current flow through the first audio amplifier tube Ila, the voltage drops across the two series connected resistors l2 and 13 are negligible such that substantially the full voltage of the available anode current source appears between the resistor terminal 12a and ground. This change in the voltage between the terminal 12a and ground is utilized to control the sweep oscillator 20 in the novel manner pointed out immediately below.

Sweeping the local osillator frequency during ofisignal periods As indicated above, facilities including the sweep oscillator 20 are provided in the system for the purpose of continuously sweeping the output frequency of the local oscillator of the tunable mixer stage l2 back and forth over a given frequency band during oiT-signal periods, thereby continuously to adjust the system for the reception of a radiated signal having a center frequency anywhere within a corresponding frequency range. The advantages of this arrangement will .be more or less apparent from the preceding explanation, but will be considerably more apparent by examining the selectivity curves for the radio frequency and intermediate frequency sections of the receiver. From a consideration of these curves, which are shown in Fig. 4 of the drawings, it will be noted that the characteristic selectivity curve A for the radio frequency section ll of the receiver is substantially flat over a relatively broad frequency range of approximately kilocycles. This of course means that insofar as the radio frequency section of the re ceiver is concerned, any received signal carrier having a center frequency within a band of approximately 140 kilocycles may be transmitted through this section of the receiver without substantial distortion and with approximately the same degree of amplification. In the other hand, relatively highly selective tuned circuits are employed in the intermediate frequency section of the receiver, as indicated by the narrow frequency range embraced within the characteristic selectivity curve B for this section of the receiver. It will of course be understood that the center or zero frequency of the selectivity curve A depends upon the particular setting of the tuning element or elements provided in the radio frequency amplifier l I and may be varied by adjusting the tuning element or elements. The center frequency of the selectivity curve B, on the other hand, is fixed at a particular value, due to the fact that the frequency selective circuits provided in the intermediate frequency section of the receiver are fixedly tuned to a particular center frequency. Thus, if the tunable stages of the re- Under these circumstances, if a signal, which is to be reproduced by the receiver and is normally radiated at a center carrier frequency of 3'0 megacycles, is in fact radiated, following an off-signal period, at a center carrier frequency 60 kilocycles above or below the center carrier frequency of 30 megacycles at which the tunable stages of the receiver are set for signal reception, the center frequency of the beat frequency carrier which is produced by heterodyning the received signal carrier with the local oscillator output is either 60 kilocycles less than, or 60 kilocycles greater than, the center frequency to which the resonant circuits of the intermediate frequency section of the receiver are tuned. By referring to the selectivity curve B, it will now be seen that this section of the receiver cannot pass the beat frequency carrier which is thus produced. The receiver is, therefore, incapable of reproducing the radiated signal in the absence of suitable adjustment of the tuning elements to correct for the changed center carrier frequency of signal radiation.

From the above explanation it will be clearly parent that the selectivity of the receiver is primarily determined byQthe output frequency of the local oscillator provided in'the tunable mixer stage I2. Thus, if this frequency is so adjusted that the center frequency of the developed beat frequency carrier is substantially equal to the center frequency to which the resonant circuits of the intermedaite frequency section of the receiver are tuned, the received signal will be transmitted through this section of the receiver even though the center frequency of the received signal is substantially removed from the center frequency to which the radio frequency section of the receiver is tuned. Accordingly, by sweeping the output frequency of this oscillator back and forth over a predetermined and selected .frequency band during off-signal periods the receiver may be automatically conditioned to pick up and reproduce any signal which is radiated at a centor-frequency disposed within a corresponding frequency band. It is to this end that the sweep oscillator 20 is provided in the system.

'In considering the operation of the oscillator .29 to control the output frequency of the local oscillator provided in the tunable mixer stage [2,

it is pointed out above that during a period of no signal reception, the muting apparatus, as illustrated in Fig. 2 of the drawings, operates to bias the first audio frequency amplifier. tube Ha beyond cutoff, with the result that substantially th full voltage of the available anode current 7 source is impressed across the electrodes of the discharge tube 20a through the resistor '19. When this voltage is initially applied to the tube 20o following a period of signal reception, it divides between the resistor 19 and the condenser with the result that the condenser is charged from the anode current source. After a predetermined time interval, which is determined by the resistive and capacitive constants of the two circuit elements 19 and 8D, the voltage appearing across the condenser 80 increases to a value equaling that required to produce space current flow through the discharge tube 20a. Immediately this tube is rendered conductive, the condenser 8!] discharges through the tube to sustain pace current therethrough for a short interval. When the condenser 80 is sufliciently discharged, the voltage drop thereacross falls to a value insufiicient to sustain s ace current flow through the tube 20a, with the result that the tube is rendered nonconductive. The condenser 80 is now recharged from the anode current source through thethree series connected resistors 13, I2 and I9 until the voltage thereacross again rises to a value sufiicient to produce space current flow through the tube 20a. s

From the above explanation, it will be apparout that so long as the voltage between the terminal 12a and ground is held at a value exceeding the characteristic breakdown voltage of the discharge tube 20a, this tube is intermittently and periodically rendered conductive and nonconductive, and in combination with the condenser 80 and the resistor 19 operates much in the manner of the well known relaxation oscillator. More specifically, the voltage which appears across the shunt connected terminals of the condenser 80 and the electrodes of the tube 259a is of saw-tooth wave form. The frequency of the saw-tooth voltage which is produced may of course be adjusted as desired by suitably proportioning the resistance of the resistor 19 relative t the capacitance of the condenser 88 to alter the time constant of the network formed by these two series connected circuit elements. The extent or amplitude of the saw-tooth voltage is determined by the design of the discharge tube 20c and may be altered by employing different tubes having different differences between the breakdown and cut-oil voltages thereof. I

As indicated above, the voltage appearing between the upper electrode of the discharge tube 20a and ground during off-signal periods is of substantially saw-tooth form. This voltage is utilized to frequency modu ate the output frequency of the local oscillator provided in the mixer stage l2 at the frequency of the modulating voltage, which is preferably of the order of one cycle per second. To that end it is impressed betweenthe input electrodes of the frequency control tube Ha over a, path which includes the coupling condenser 8|. the resistor 82 and the radio frequency decoupling resistor M. More specifically, this saw-tooth voltagelisin the nature of a constantly changing bias voltage and serves to constantl and correspondingly change the magnitude of space current flow through the tube Illa, thereby to produce a corresponding change in the magnitude of the out ofvphase component of current traversing the circuit elements of the frequency determining circuit 34 provided in the local oscillator section of the mixer tube lZa. It will be understood, therefore, that the output frequency of this local oscillator is varied directly in accordance with changes in the voltage appearing between the upper electrode of the discharge tube 20a and gr und. In other words, since the modulating voltage is of saw-tooth form, the output frequency of the local oscillator provided in the mixer stage I2 is continuously adjusted back and forth over a predetermined frequency band. The width of this band is determined by the magnitude of the modulating v ltage. More specifically, the output frequency of the local oscillator is, through the action of the sweep oscillator 29, continuously adjusted about a predetermined center frequency in the manner clearly shown by the output frequenc curve C illustrated in Fig. 5 of the drawings. The particular center frequency about which the output frequency of the local oscillator is adjusted is of course determined by the setting of the tuning element 341). From an examination of the curve C it will be apparent that as the voltage between the upper electrode of the discharge tube 20a and ground increases, a corresponding increase is produced in the output frequency of the local oscillator. On the other hand, during each interval when the voltage between the upper electrode of the discharge tube 28a and ground is decreasing a corresponding decrease is produced in the output frequency of the local oscillator. Thus, the local oscillator frequency is swept back and forth over a given frequency band, the extent of'which is determined by the magnitude of the modulating voltage appearing between the upper electrode of the discharge tube 20a and ground.

As will be apparent from a consideration of the related curves A, B and C, shown in Figs. 4 and 5 of the drawings, the continuous adjustment of the local oscillator output frequency over a given frequency band of approximately 60 kilocycles, as exhibited by the curve 'C, is tantamount to shifting the intermediate frequency selectivity curve bodily back and forth over a corresponding frequency band. Thus, when the output frequency of the local oscillator is adjusted by the sweep oscillator 20 to a value substantially 40'kilocycles lower than the center frequency to which the local oscillator is tuned by the tuning element 341), it can be heterodyned with a received signal carrier having a center frequency 40 kilocycles lower than the center frequency to which the radio frequency section of the receiver is tuned, to produce a beat frequency carrier having a center frequency of 4.3 megacycles exactly equaling thatto which the more highly selective stages of the intermediate frequency section of the receiver are center tuned. The result, therefore, is the 40 kilocycles above the center frequency to which the radio frequency amplifier H is tuned, to produce a beat frequency carrier of 4.3 megacycles exactl equaling the center frequenc to which .the selective circuits of the intermediate fre-V quency section of the receiver are tuned. In effect, therefore, the intermediate frequency selectivity curve B of the receiver is shifted to the position illustrated by the dash line selectivity curve Ba shown in Fig. 4 of the drawings. At all points along the output frequency curve C of the local oscillator which are intermediate the maximum and minimum values just discussed, the receiver is automatically conditioned to pick up and transmit through the intermediate frequency section thereof a radiat d signal carrier having a center frequency which is greater or less than the center frequency to which the radio frequency section of the receiver is tuned by an amount equal to the deviation of the local oscillator output frequency from its center frequency,

From the above explanation it will be apparent that the sweep oscillator 20 Operating in conjunction with the frequency control unit l9 functions continuously to adjust the receiver for the selection of a radiated frequency modulated carrier having a center carrier frequency which may be of any value within a given frequency range.

In this regard it is noted that the extent of the variation in the output frequency of the local oscillator which may occur during off-signal periods is only limited by the frequency separation of adjacent assigned channels. Thus, if the center frequencies of the adjacent channels are kilocycles apart, and the side band frequencies extend plus and minus 20 kilocycles on either side of each channel center frequency, the output frequency of the local oscillator provided in the tunable mixer stage IZ may safely be adjusted by the sweep oscillator to values approximately 50 kilocycles above or below any channel center frequency so that the band of adjustment is approximately kilocycles in width. In such case, the receiver is automatically and continuously adjusted to receive any radiated signal within a band of 100 kilocycles which embraces as the I of course amplified by the radio frequency amplifier l l mixed with the local oscillator output in the mixer stage i2 and appears across the tuned frequency selective circuit 26. Due to the selective action of this circuit, the following selective circuit 21 and the selective circuits provided in the succeeding portions of the intermediate frequency section of the receiver, the signal modulated beat frequency carrier produced in the mixer stage l2 cannot be transmitted through the intermediate frequency section of the receiver until such time as the output frequency of the local oscillator approaches a value substantially 20 kilocycles below the center frequency about which the frequency, of this oscillator is being adjusted.

The reasons for this will be fully apparent from the preceding explanation. When, however, the

output frequency of the local oscillator approaches from either direction the value d, for example, which is substantially 15 kilocycles less than the center frequency of the band over which the local oscillator frequency is adjusted, the signal modulated beat frequency carrier, as produced by mixing the local oscillator output with the incoming signal carrier, approaches the correct fre quency to be passed through the intermediate frequency section'of the receiver. center frequency of the signal carrier which is transmitted through the intermediate frequency section of the receiver is somewhat removed from the center frequency to which this section of the receiver is tuned. When the signal voltage is transmitted to the frequency discriminator I6, two results are obtained simultaneously. First, and as will be apparent by reconsidering the charac teristic noise curve shown in Fig. 6 of the drawings, the noise output voltage across the discriminator output resistors I Go is reduced to a low value. Secondly, a direct bias voltage is developed across the discriminator output resistors Ilia, due to the fact that the center frequency of the signal modulated carrier transmitted through the intermediate frequency section of the receiver is removed from the center frequency to which this receiver section is tuned. As previously'indicated, this bias voltage varies in magnitude in accordance with the magnitude of the difference between the center intermediate frequency and" the transmitted carrier frequency and in polarity in accordance with the direction'o-f deviation of the carrier frequency from the center intermediate frequency. It is applied: to the control grid of the frequency control tube I9a to assume control of the local oscillator output frequency in the manner explained above. Incident tothe reduction in the magnitude of the. noise voltage appearing at the output side of the frequency discriminator I6, the muting apparatus is rendered inactive-so that the blocking bias voltage developedacross the resistor 69 is reduced to zero in the manner previously explained. Thus, the first amplifier tube Iia. of the audiofrequency amplifier I1 is biased to a normal value such that space current fiow therethrough is reinitiated, thereby to permit the signal components of the received signal modulated carrier to be transmitted through this amplifier for reproduction by the loud speaker I8. As space current flow through the amplifier tube I la is reinitiated, a substantial portion of the voltage of the anode current source is dissipated as a voltage dropacross the two resistors 12 and 73, with the result that the voltage betweenthe terminal 120. and. ground is reduced below the value required to render the discharge tube 200. of the sweep oscillator'20 conductive. Accordingly, the operation of this oscillator is arrested immediately the blocking bias voltage is removed from the first audio frequency amplifier tube Ila. After the operation of this oscillator is arrested, the frequency control tube Illa is controlled by the bias voltage appearing at the output side of the frequency discriminator I6 to maintain the local oscillator output frequency at the value required for transmission of the signal carrier through the intermediate frequency section of the receiver. In this regard it is noted that the transfer of control of the local oscillator outputfrequen'cy from the sweep oscillator 20 to the automatic ire quency controlfacilities occurs concurrently and simultaneously, so that immediately the signal carrier is located through operation of the sweep oscillator 20'and is transmitted through the intermediate frequency section of the receiventhe reception thereof is retained through the action of the automatic frequency control facilities.

From the above explanation, it will be understood that the sweep oscillator 20 is in the nature of an automatic tuningdevice. which fund- Initially the tions only during'ofi-sigrial periods and then only for the purpose of locating asigna1 carrier which may be radiated at any frequency within a small frequency range. It will also be understood that when asignal within this range is detected through the action of the sweep oscillator, the

operation of this oscillator is immediately arrested under the control'of the muting apparatus'. Thus themuting apparatus performs the function of preventing undesirednoise voltages from being reproduced by the loud speaker I8 during off-signal periods and the-additional func tion of controlling the sweep oscillator 29 soth-at it is only active during off-signal periods. More fundamentally, the noise response of the receiver is primarily relied upon not only to controlthe operation of the muting apparatus but also the operation of the sweeposcillator 20. Thus it is clear that the operation of the sweep oscillator 20 is directly dependent upon the action of the muting apparatus. The operation of the mutin 'apparatus, on the other hand, is directly dependout upon the magnitude of noise voltages appearing at the output side of the frequency discriminator I6. By virtue of the arrangement described above, thenecessityforan' operator in the field continuously to adjust the tuning elements of the receiver over a-= given frequency band for long periodsof time in'an effort'to locate a signal which may be infrequency radiated at any frequency' within a band for only short intervals, is entirely eliminated Moreover, the action of the sweep oscillator insweeping the local oscillator output'frequency during off'-signalperiods; positively prevents the loss of any signal which may be radiated from a specific and important-transmitting point'at any time within a long period.

As previously pointed out, the-apparatus shown in Fig. 3 of thedrawings may be combined with that shown in Fig. 1 of 'the'drawings to produce a system of simplified form having all of the operating characteristics described above. In general, the apparatus shown inFie. 3 is-Substantially identical with that illustrated in'Fig. .2, the only difference being that in the" Fig. 3 arrangement asimplified version of the muting apparatusis provided. Thus it-will be noted that the arrangement of the second limiter I5, the frequencydiscriminator I6, the high'pass filter zz and the audio frequency'amplifier II; is: identical with thearrangement of thecorresponding elements provided in the apparatus shown in Fig. 2 ofthe drawings. The'muting' apparatus is-simplified; however, in that the direct current amplifier 24 and themutingoscillator and rectifier 25 have been omitted therefrom. Thus, inthe Fig.*3"arrangement, the-noise voltage appearing at the output side of the high pass filter is impressed between the input electrodes of the noise ampliefiertube; 23d, which in turn, is directly coupled-"toadioderectifier circuit comprising the diode 231). A suitable" cathodebiasing circuit comprising the 'fi'xed "resistor and theadj ustabIe-resistor 88 is bridged? across the available source of anode current-for the purpose of impressing a threshold bias upon the amplifiertube 23a which determines the magnitude of noise voltage required toeif'ect muting of the audio frequency amplifier IT. The adjustable resistor 88 is by-passed for noise voltages by means-of a con denser 89. Anode-potentialis-applied to theamplifler tube 23d through a;resistor SI and the amplified =noise voltage appearing across-this res'istor is 'c'oup'led irito the diode rectifier circuit through; the-two" coupling "condensers 92" and 93.

The indicated diode rectifier circuit includes, in

addition to the diode 23b, a load resistor 94, and

the voltage appearing across this resistor is applied to the control grid of the first audio frequency amplifier tube Ila. through a filter resistor 95. Y Y r In considering the operation of the muting apparatus shown in Fig. 3 of the drawings, it will be understood by reference to Fig. 6 of the drawings that in the total absence of a received signal, a substantial noise voltage appears across the output resistors'I-Ba of. the frequency discriminator Hi. The components ofthis voltage which are above the normal signal reproducing band of the receiver are transmitted through the high pass filter 22 and impressedbetween the input electrodes of the amplifiertube 23a. When of sulficient magnitudeto overcome the threshold bias established by the setting of the adjustable resistor 88, the noise voltage components appear in amplified form at the output side of the tube 23a across the resistor SI. Y This noise voltage is impressed across the space current path of the rectifier tube 23b througn the coupling condensers 93 and 92 sothat a rectified voltage is produced across the load resistor. More specifically, the direction of noise current flow through the space current path of the diode 23b is such that a potential which is negative with respect to ground is producedat the upper terminal of the load resistor 94. This potential is of substantial magnitude duringoff-signal periods when the noise voltage appearing at the output side of the frequency discriminator I6 is of large magnitude, and is impressed through the filter resistor 95 between the input electrodes of the first audio frequency amplifier tube Ila. The extent of this bias voltage is, in a properly designed noise amplifier and rectifier 23, of asufficient order of unagnitude to bias the first amplifier tube I la beyond cutoff, thereby to arrest space, current flow through the two resistors 12 and I3 and to block the audio-section of the receiver against signal transmission. Y r

. If, following a period of no signal reception a desired signal is transmitted through the intermediate frequency section of the receiver, the noise voltage appearing at the output side'of the frequency discriminator. I6 is reduced to a very low value, as will be apparent from areconsideration of the characteristic noise curve shown in I Fig. 6 of the drawings. Incident to this reduction of the noise voltage and dueto the action of the cathode biasing resistor 88, no noise voltage appears across the resistor 9 l, with the result that the bias voltage developed across the diode load resistor 94 is reduced substantially to zero. When this voltage disappears, the first audio fre-' quency amplifier'tube Ila is biased toa normal value, with the result that space current flow through this tube is resumed and the audio frequency amplifier I1 is unblocked or opened to permit the audio frequency components of the received signal to be transmitted to the loud speaker [8 for reproduction.

From the above explanation it will be understood that during off-signal periods or in the total absence of a received signal carrier, space current flow through the amplifier tube lla is arrested, with the result that substantially the full voltage of the available anode current source appears between the' terminal 12a of the audio frequency amplifier and ground. This voltage isof sufficientmagnitude to effect the operation of; the sweep oscillator 20in the exact manner explained above. On the other hand, immediately a signal is received, the bias applied between the input electrodes of the tube Ila is reduced to normal,

space current flow through this tube is resumed,

and the voltage between the terminal 12a and ground is reduced to a low value which is less than that required to sustain the operation of the sweep oscillator 29. Thus, the operation of the sweep oscillator 20 is arrested when a signal is received. It will be understood, therefore, that the muting apparatus, as shownin Fig. 3 of the drawing, is capable of controlling the sweep oscillator 29 to effect the operation thereof during offsignal periods and to arrest the operation thereof during signal reception, in the same manner as was described above with reference to the system formed by combining the circuits shown in Figs. 1 and 2 of the drawings. It will also be understood that the mode of operation of. the automatic frequency control facilities during the period of signal reception is exactly the same, regardless of whether the Fig. 2 arrangement or the Fig. 3 arrangement of the muting apparatus is employed in the system.

From the preceding explanation it will be understood that an exceedingly simple arrangement which is thoroughly positive and reliable in operation is provided for continuously adjusting the receiver for the reception of a signal which may be only intermittently transmitted at any center carrier frequency within a given frequency band. It will also be understood that the provision of such facilities is of particular utility in receivers designed for military, police and other emer ency work, which may be set to receive at a particular frequency over along period of time during which signal transmission may occur only intermittently.

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 radio receiving system of the superheterodyne type which includes a mixer stage followed by an intermediate frequency channel, a local oscillator associated with said mixer stage and the output frequency of which determines whether a received carrier will be transmitted through or excluded from said channel, means for automatically controlling the output frequency of said oscillator to maintain the frequency of a carrier traversing said channel substantially equal to the intermediate frequency, additional means for automatically and continuously adjusting the output frequency of said oscillator back and forth over a predetermined frequency range during a period of no signal reception, and muting means for selectively rendering said two last-named means active and inactive and for preventing noise voltage translation in said system during a period of no signal reception. Y

2. In a radio receiving system of the superheterodyne type which includes a mixer stage provided with a local oscillator, means for muting said system during periods of no signal reception, and means responsive to operation of said muting means for changing the output frequency of sale, local oscillator.

3. In a radio receivingsystem of the superheterodyne type which includes a mixer stage provided with a local oscillator, means for muting said system during periods of no signal reception, and means responsive to operation of aid muting tinuously varyingthe output frequency of said means for continuously varying the output frequency of said oscillator back and forth over a predetermined frequency range during periods of nosignal reception.

4. In a wave signal receiving systemwhich includes a tunable circuit the tuning of which determines the frequency at which a signal may be received by said system, means for muting said systemduring periods of no signal reception, and mean responsive to operation of said muting means for changing the tuning of said circuit.

5. In a radio receiving system which includes a tunable circuit the tuning of which determines thefrequency at which a signal may be received bysaid system, means for muting said system during period of no signal reception, and means responsive to operation of said muting means for continuously varying the tuning of said circuit back and forth over a predetermined tuning range during periods of no signal reception.

'6. In a radio receiving system which is provided with a signal responsive device and includes a tunable circuit the tuning of which determines the frequency at which asignal may be received, muting means responsive to noise voltages appearing in said system in the absence of a received signal for preventing said device from responding to the noise voltages and responsive to the receptionof a desired signal in the presence of noise voltages for permitting said device to respond to the desired signal, and means responsive'to operation of said muting means for automatically and continuously varying the tuning of said circuit over a predetermined tuning range a only during a period of no signal reception in order continuously to adjust said system for the reception of any signal within a given frequency range.

7. In a radio receiving system which is provided with a signal responsive device and includes a tunable circuit the tuning of which determines the frequency at which a signal may be received, muting means responsive to noise voltage appearing in said system in the absence of a received signal for preventing said device from responding to the noise voltages and responsive to the reception of a desired signal in the presence of noise voltages for permitting said device to respond to the desired signal, means responsive to the operation of said muting means for continu ously adjusting the tuning of said circuit over a predeterminedtuning range during an off-signal period when a signal is not being received, and automatic tuning means responsive to, the transmission of a signal through said system for maintaining said circuit tuned substantially to the frequency to which it is adjusted by said last-named means but controlled by variations in the frequency-of the received signa1 to correspondingly adjust the tuning of said circuit.

8. In a radio receiving system of the superheterodyne type which includes an intermediate frequency section preceded by a mixer stage and followed by a signal responsive device, a local oscillator associated with said mixer stage and the output frequency of which determines the frequency at which a signal may be received, muting means responsive to noise voltages appearing in aid system in the absence of a received signal for preventing said device from responding to the noise voltages and responsive to the reception of a desired signal in the presence of noise voltages for'permitting said device to respond to the desired "signal, and means responsive to operation ofesaidmuting means for automatically and-conoscillator over a redetermined frequency range only during a period of .no signal reception in order continuously to adjust said system for the reception of any signal within a given frequency range. 9, In a radio receiving system of .the super heterodyne type which includes an intermediate frequency section preceded by a mixer stage and followed by 'a'signal responsive'device, a local oscillator associated'with said mixer stage and the output frequency of which determines the frequency at which a signal :may be received, muting means responsiveto noise voltages appearing in said system .in the absence of a received signal for preventing said device from responding to the noise voltages andv responsive to the reception of a desired signal in the presence of noise voltages for permitting said device to respond to the desired signal, means responsive to operation of said muting means for automatically and continuously varying the output frequency of said oscillator over a predetermined frequency range only during-a period of no signal reception in order continuously to adjust said system for the reception of any signal within a given frequency range, and automatic frequency control means operative during signal reception to maintain the output frequency of said oscillatorat substantially'the value to which itis adjusted by said last-named means but controlled by lvariationsiin the frequency of the received signal to correspondingly adjustthe output frequency of said oscillator.

10. Ina radio receiving system of the frequency modulated type which includes a mixer stage, an intermediate frequency amplifier, limiter means and a frequency discriminator 'intercoupled in tandem in the order named, alocal oscillatorlprovided at said mixer stage, andmeans coupled to the output side of said discriminatorfor automatically and continuously sweepingthe output frequency of said oscillator back and-forth over a predetermined frequency rangeduring a, period of no signal reception in order continuously to adjust-the system for the reception of any frequencymodulated carrier havin a center frequency 'withinia given frequency range.

11. .In'a radio receiving system of the frequency modulated type which includes a mixer stage, an intermediate frequency amplifier, limiter means and. a frequency discriminator intercoupled in tandem in the *ordernamed, a local oscillator provided at said mixer stage, means coupled to the output side of said discriminator and responsive to noise voltages appearing in said system in the absence of a receivedsignal for muting said system, andrmeansresponsive to operation of said muting means :for automatically and continuously sweeping the "output frequency of said oscillator back and forth over a predetermined frequency rangeduring a period of no signal receptionin; order continuously to adjust the system for thejreception of any frequency modulated carrier having a center frequency within a given frequencyrange. C

12. In a radio receiving system of' the frequency modulated type which includes a mixer stage, an intermediate frequency amplifier, limiter means and a frequency discriminator intercoupled in tandemin the order named, a local oscillator provided at said mixer stage, means coupledto the output side of said discriminator for automatically andcontinuously sweeping the out put frequency "of said oscillator back'and 'forth over a predeterminedfrequency range during a period of no signal reception in order continuously to adjust the system for the reception of any frequency modulated carrier having a center frequency within a given frequency range,'and automatic frequency control means for taking over the control of said oscillator output frequency during a period of signal reception.

13. Ina radio receiving system of the frequency modulated type which includes a mixer stage provided with a local oscillator and a tuning oscillator, limiter means and a frequency discriminator intercoupled in tandem in the order named, the method of operation which comprises selecting and rectifying noise voltages appearing at the output side of said system during a period of no'signal reception, utilizing the rectified noise voltages to mute the system against response to the noise voltages, utilizing said rectified noise voltages to so control said tuning oscillatoras' to continuously vary the output frequency back and forthover a predetermined frequency range, and utilizing a component of the voltage appearing at the output side of said discriminator during a period of signal reception to control the output frequency of said oscillator in accordance with variations in the center frequency of the receive signal from a predetermined value.

1,4. In a radio receiving channel which includes a plurality of tandem connected stages, muting apparatus coupled to said channel at a point therealong and automatically operative to mute said channel during periods of no signa1 reception, a. tunable signal selecting circuit provided in at least one of the stages which precedes the pointat which said muting apparatus is coupled to said channel, andmeans responsive to operation of said muting apparatus for automatically varying the tuning of said circuit over a predetermined tuning range. V V

15. In a radio receiving system which is provided with a signal responsive device and includes a tunable circuit the tuning of which determines the signal frequency at .whicha signal may be received, means for preventing said devi'cefrom responding to noise voltages appearing in said system during off-signal periods a'ndresponsive to the reception of a desired signal for permitting said device to respond to the'desired signal, and means responsive to the operation of said lastnamed means for varying the tuning of said 'circuit over a predetermined tuning range during off-signal periods, thereby continuously to adjust the'system for the reception of any signal within a given frequency range.

16. In a radio receiving system which includes a signal transmission channel and at least one tunable circuit the tuning of which determines the signal frequency at which a signal may be received, an oscillator, means responsive to operation of said oscillatorfor varying the tuning of said circuit over'a predetermined tuningrange, and means responsive solely to noise voltages appearing in said system in the absence of a received signal for initiating operationof said oscillator and responsive to the appearance of a signal voltage in said system for arresting the operation of said oscillator.

17. In a frequency modulated radio receiver which includes, in the order named, a mixer stage, an intermediate frequency section provided with frequency selective circuits tuned to a predetermined center frequency, and a frequency discriminatoroperative to develop a bias voltage which variesfin polarity with deviations in the center frequency of .an intermediate frequency signal in different senses from said predetermined frequency and in magnitude with the extent of variation of the center frequency of said intermediate frequency signal away from said predetermined frequency; a local oscillator included in said mixer stage to effectefrequency conversion at saidstage and provided with a circuit tunable over a predetermined frequency range to effect signal'selection, an electron discharge tube including a control grid and provided with a space current path intercoupled with said circuit to vary the tuning of said circuit in opposite senses as the voltage on said control grid is varied in correspondingly opposite senses, and a conductive path for impressing the bias voltage developed by said discriminator on said grid in the correct sense to minimize variations in the center frequency of an intermediate frequency signal traversing the intermediate frequency section of said receiver, and means responsive to the cessation of a received signal for continuously varying the bias on said control grid to vary the tuning of said circuit over a predetermined tuning range.

18. In a'frequency modulated radioreceiver which includes, in the order named, a mixer stage, anintermediate frequency section provided with frequency selective circuits tuned to a predetermined center frequency, and a frequency discriminator operative to develop a bias voltage which varies in polarity with deviations in the center frequency of an intermediate frequency signal in different senses from said predetermined frequency and in magnitude with the extent of variation of thecenter frequency of said intermediate frequency signal away from said predetermined frequency; a local oscillator included in said mixer stage to effect frequency conversion at said stage and provided with a circuit tunable over a predetermined frequency range to effect signal'selection, an electron discharge tube including a control grid and provided with a space current path intercoupled with said circuit to vary the tuning of said circuit in opposite senses as the voltage on said control grid is varied in correspondingly opposite senses, and a conductive path for impressing the bias voltage developed by said discriminator on said grid in the correct sense to minimize variations in the center frequency of an intermediate frequency signal traversing the intermediate frequency section of said receiver, means responsive to the cessation of a received signal for preventing translation of noise voltages appearing in said receiver, and means responsive to the operation of said last-named means for continuously varying the bias on said control grid to vary the tuning of said circuit over a predetermined tuning range.

19. In a frequency modulated radio receiver which includes, in the order named, a mixer stage, an intermediate frequency section provided with frequency selective circuits tuned to a predetermined center frequency, and a frequency discriminator operative to develop a bias voltage which varies in polarity with deviations in the center frequency of an intermediate frequency signal in different senses from said predetermined frequency and in magnitude with the extent of variation of the center frequency of said intermediate frequency signal away from said predetermined frequency; a local oscillator included in said mixer stage and provided with a circuit tunable over a predetermined frequency range to effect signal selection, an electron discharge. tube including a control grid and provided with a space current path intercoupled with said circuit to vary the tuning of said circuit in opposite senses as the voltage on said control grid is varied in correspondingly opposite senses, and a conductive path for impressing the bias voltage developed by said discriminator on said grid in the correct sense to minimize variations in the center frequency of an-intermediate frequency signal traversing the intermediate frequency section of said receiver, means responsive solely to 10 noise voltages appearing in said receiver in the absence of a received signal for preventing translation of the noise voltages by the receiver, and an oscillator operative in response to operation of said last-named means to continuously vary the bias on said control grid so that the tuning of said circuit is varied over a predetermined tuning range. HENRY MAGNUSKI.

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