US2492795A - Frequency shift signaling system - Google Patents

Description

Dec. 27, 1949. H. E. GOLDSTINE I FREQUENCY SHIFT SIGNALING SYSTEM 2 Sheets-Sheet 1 Filed Jan. 29, 1946 INVENTORD flallanlfialdztuw,

Q ATTORNEY Dec. 27, 1949 H. E. G'OLDSTINE 2,492,795

FREQUENCY SHIFT SIGNALING SYSTEM Filed Jan. 29, 1946 2 Sheets-Sheet 2 igga 055 701v: Kim) SEA Him 7011/5 JKEYi/Z mr/flaz I I;

INVENTOR BY n g/ww ATTORNEY mitted. Obviously,

Patented Dec. 27, 1949 FREQUENCY smrr SIGNALING SYSTEM Hallan E. Goldstine, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 29; 1946, Serial No. 644,147

11 Claims.

This application discloses an improved method of and means forproducing telegraphy signals or facsimile signals of the frequency shift type, or of the on-off type or telephony signals.

Frequency shift signalling systems are known in the prior art and the nature of these signals and known method of producing the same will not be discussed in detail herein. Frequency shift signals comprise two currents of different frequency alternatively usable. The shifting from one current to the other current is usually in accordance with intelligence represented by code. The current frequencies differ by several hundred cycles, and one may represent mark and the other space when telegraphy signals are being sent. When facsimile signals are involved one frequency may represent black or white. The character of telegraphy or facsimile systems of the on-oif or amplitude modulation type are well known, as is the character of amplitude modulation telephony signals, and the same will not be described herein.

In frequency shift systems, as stated above, current is shifted from one frequency to the other in accordance or the same impressed on carrier current is transin view of the already crowded condition of the radio-frequency spectrum it is of considerable importance to have a system of 'this type wherein the mean or averbe maintained conage or center frequency can stant, so that the frequency shifted current will not deviate from its assigned band in the frequency spectrum. An object of my invention is to provide center frequency is fixed. In my improved systerm this is accomplished by modulating an unstable oscillator from a first frequency to a second frequency and using the oscillators output to modulate a crystal controlled oscillator of fixed frequency. The sum or difference side band is then selected for use at the fundamental frequency or a harmonic thereof.

My frequency shift means is to comprise a unit for use in new or existing transmitter sys-- terns. These transmitter systems may have or may be provided with crystal oscillators supplying the carrier current of fixed frequency used in providing frequency shifted side band energy of fixed center frequency. The modulated oscillator also must have the proper stable center frequency about which it is shifted to provide 1e final desired output frequency which may be harmonically related to the selected side band frequency which in the embodiment described is with signals and this current an improved system wherein the tone keyed in accordance the sum of the crystal frequency and the modulated oscillator frequency. A further object of my invention is to provide means for tuning the modulated oscillator to the exact desired center frequency without putting the transmitter on the air. This permits adjustment of the frequency shift unit in the laboratory and is accomplished b supplying a crystal controlled monitor oscillator of a frequency equal to the desired center frequency of the modulated oscillator and arranging the circuits so that the modulated oscillator frequency may be compared with and brought into synchronism with the crystal controlled oscillator frequency. This crystal controlled monitoring oscillator is also made use of as the modulated oscillator when amplitude keying or amplitude modulated telephony signals are to be generated. 7

Many telegraphy systems today make use of a with intelligence. The tone is rectified to provide voltage having a direct current component varying between zero potential and a negative value, say minus 180 volts. The so varying direct current potential is then used to key the telegraphy system. My frequency shift unit is so arranged that it may be controlled by the output of these existing keyed tone converters. However, the output of the keyers supplying the tone for conversion is subject to variations due to variations in power supply voltage. This results in variations in the potential at the converter output. Since this tone converter output varies between about zero and a negative value signal limiting is inherent in the positive direction. The direct current varying potential in said prior systems is for use in amplitude keying and in such use signal limiting in the negative direction is not so important and is provided in the system. In my system, however, this direct current potential is to control a reactance tube modulating an oscillator and it is important that the signalling be also limited in the negative direction. A further object of my invention is to provide means which in cooperation with the existing keyed tone converters accomplishes signal limiting in both directions of the frequency shift operation. In accordance with my invention these variations are removed by provision of a gas discharge voltage regulator tube shunting the keyed tone converter output which is also in effect the reactance tube control input.

In on-oif telegraphy systems the transmitted wave by necessity covers a considerable frequency band'since the keying on and off of the carrier current sets up numerous side frequencies. Where frequency shift signals are used considerable band width reduction is obtained. This is of considerable advantage and an object of my improved system is further reduction of the band width occupied by the frequency shifted currents. This is accomplished by supplying a low pass pi-section filter between the keyed tone converter and the reactance tube input. This filter shapes the signal wave and controls the rate of build-up of the keying characteristics and reduces the sideband spread and thereby utilizes less band width for the same keying speed and thus provides a reduction of adjacent sideband interference.

The extent of swing of the reactance tube modulated oscillator depends upon the variation of current through the reactance tube and this in turn depends upon the potential on the reactance tube grid. In my improved system the keying unit is to supply transmitters operating at the fundamental frequency or at various multiples thereof. The frequency shift at the modulated oscillator is then related integrally to the desired total frequency shift at the output. If the total swing is to be the same or substantially the same when the multiplication factor is changed the swing at the modulated oscillator must be reduced. An object of my invention is to provide simple and improved means for changing the total swing at the modulated oscillator an exact and known amount when the multiplication factor is changed. This means comprises an attenuation network which may be included in the circuit between the keyed tone converter and the reactance tube input to reduce the frequency shift swing.

When it is desired to change the extent of frequency swing or shift in systems of this nature the voltage applied to the reactance tube is varied more or less negative relative to zero. This not only changes the swing but may also change the center frequency. A further object of my invention is to provide improved means for recalibrating the modulated oscillator. In my system the arrangement is such that in the space condition the reactance tube is biased to be fully conductive so that the modulated oscillator supplies output of lowest frequency. In the mark condition the reactance tube is least conductive and supplies oscillations of a higher frequency representing mark. In order to facilitate centering of the oscillator frequency I provide a potentiometer arrangement such that half of the steady mark voltage may be applied to the reactance tube while its output frequency is being compared with the monitoring crystal oscillator frequency and the modulated oscillator is tuned to provide zero beat, as indicated by meter or other means, with the monitoring oscillator.

In an embodiment the center frequency is automatically corrected when the control potential applied to the reactance tube is changed. This is accomplished by simultaneously changing the reactance tube bias.

In an additional embodiment the center frequency is properly reset when the frequency swing is changed by changing the reactance in the controlled generator circuit.

In describing my invention in detail reference will be made to the attached drawings wherein Fig. 1 illustrates schematically and by block diagram, circuit element and circuit connections,

a frequency shift keying unit arranged in ac I circuit. The reactance tube modulates an oscillator 60 which supplies excitation to an amplifier 76 of the cathode follower type, which in turn excites differentially a balanced modulator Hit. The tubes 82 and 82' of the balanced modulator Hill are also excited cophasally by carrier current or voltage from a selected one of the two crystal controlled oscillator units I20 and Mil. A side band resulting from the balanced modulation process is selected by tuned circuit H34 and fed to an amplifier stage I60 and from the amplifier stage to amplifiers and/or frequency multipliers H0 as desired. The crystal oscillator for monitoring purposes and for use in telephony and on-ofi telegraphy has been designated 2%. The alternating current source for the heaters of the various tubes and for the rectifier supplying the direct current potentials and for the temperature regulating units have been omitted for the sake of simplicity. The direct current potential source, not shown, has its output coupled to voltage regulators 221! so that direct current potentials of constant amplitude are provided for certain of the tube electrodes in the modulator stage and modulated oscillator circuits and in the crystal controlled monitoring oscillator 29%. A switching and metering arrangement for use in making the necessary adjustments and tuning operations is designated generally at 256.

The frequency shift modulation is carried out as follows. Keyed potentials filteredand modified and used as will be described hereinafter are applied to the control electrode 30 of a reactance tube it to vary the conductivity thereof. The reactance tube has its anode 34 and cathode 36 shunting a portion of tuned tank circuit 39 of the oscillation generator including one electron discharge system of tube 60. The oscillator per se is substantially conventional and will not be described in detail herein. It includes, however, inductance 4|, a portion of which connects the grid and the anode of tube 60 in a regenerative circuit. This inductance is shunted by a plurality of tuning condensers 44, which are of temperature compensating type, and the entire inductance Al is shunted also by a trimmer condenser C6 in series with the condenser C40, also used to tune the oscillator. The grid bias supply for the triode 53 includes a radio frequency choke inductance L2 and resistor R3. In the embodiment being described oscillator 60 operates at about 200 kc. The reactance tube has its output electrodes shunting a portion of the inductance 4| so that the reactance of the tube 40 is included in the tank circuit 39 and takes part in determining the frequency of operation of the oscillator. The reactance tube 49 is of a type well known in the prior art, having its anode 34 operating at a first radio frequency voltage by virtue of its connection to the tank circuit 39 and its control grid 30 operating at an R. F. voltage in quadrature phase relation to said anode voltage by virtue of the operation of the phase shifting network including condenser C, resistance RI and inductance 21.

The amplifier tube system has its control grid H coupled by condenser C8 to the anode of tube 60 to be excited by oscillatory energy developed in the generator including tube 60. The tube 10 is in a cathode follower circuit with a radio frequency choking inductance L3 and resistor R5 in its cathode return circuit. The cathode follower stage is used to keep the wave form of the oscillator-amplifier stage reasonably clear of harmonics. Since the cathode follower has a large amount of degeneration, the cathode feedback will tend to maintain the grid-cathode voltage constant. This effect makes the amplifier more linear. Moreover, the low impedance output of the cathode output keeps the R. F. voltage lower due to the low gain and helps prevent feedback effect on the oscillator.

The amplified oscillations are supplied from the radio frequency choking inductance L3 by coupling condenser C5 to the primary winding 13 of a transformer Tl having its secondary winding coupled by a tuned circuit 15 in pushpull relation to the first grid electrodes 80 and so' of the two tubes 82 and 82' in the balanced modulator I00. The balanced modulator I00 also has the tube grids B0 and 80' excited in like phase by crystal controlled oscillations from tuned circuit 83, the inductance of which forms the secondary winding of tuned trans-former T2. The primary winding of the transformer T2 is connected to the anode of the tube I IQ of the high frequency crystal generator I26 and to the anode of the tube I39 of the high frequency crystal controlled oscillator 2 I49. One or the other of these oscillators, here oscillator I20, is operative to supply oscillations of the fixed desired carrier wave frequency depending upon the position of selector switch S4. The energy is fed from the high frequency crys tal oscillator through the transformer T2. The secondary of T2 is tuned to the crystal frequency and the energy is fed through condensers Cl 0, C! I and CIZ and (M3 to the grids of tubes 82 and 82' in parallel. The resistors R1 and R8 are high impedance grid leaks and prevent the tube from drawing excessive grid current. The condensers CH CH and Cl! and CH3 tune the secondary winding of transformer TI to the low frequency (200 kc.) but offer a low impedance path for the oscillatory energy fed from the selected crystal oscillator through transformer T2 and. the harmonies of these oscillations.

The oscillators I and I4!) are not being claimed herein and will not be described in detail. They are, however, very good stable oscillators in temperature controlled units so that the carrier oscillations supplied to the balanced modulator are of good strength and of substantially constant frequency.

The anodes of the two modulator tubes of the balanced modulator I00 are coupled differentially to a tuned tank circuit I04 the inductance of which forms the primary winding of a transformer T3. This circuit W4 is tuned to pass a side band resulting from intermodulation of the oscillations from the oscillator tube 60 and from the selected crystal oscillator I23. The high frequency oscillations, derived from the crystal, as known. are phase opposed in the tank circuit I04 and balance out. The low frequency oscillations supplied from the oscillator including tube 6d are tuned out and the tuned circuit I04 is adjusted to supply the upper or lower side band. In the embodiment illustrated, the upper side band is used. The selected side band is supplied t the control grid H29 of an amplifier tube I30 and after amplification therein is impressed from the output tuned transformer T4 on to a short line (jack J3 and pin contact P3) and thereby supplied to the amplifiers and frequency multipliers in the transmitter equipment H0. The frequency multipliers are assumed herein to have a multiplication factor of 4 orof 8.

The monitoring oscillator 200 comprises an electron discharge device 205having its first two grids coupled in an oscillation generator circuit including a crystal X3. The tube includes an additional shielding electrode 201 which is grounded to shield'the anode 209 from the 0scillation generating circuits except through the coupling thereto by the electron stream within the tube. The anode of this oscillator tube is coupled to the primary winding I3 of the transformer Tl under certain conditions and for certain purposes as will be described hereinafter. The anode of oscillator tube 205 is also coupled by the primary winding 13 of transformer TI to a positive source of direct current potential through the switch S3 in one of its positions when the crystal controlled monitoring oscillator 200 is put into use. This will be discussed in detail hereinafter in the statement of operation of the system.

At 250 I have shown a switching arrangement (which has been simplified and includes only aportion of the switching arrangement in the embodiment tested) used in regulating, adjusting and tuning the system. The milliammeter Ml may be connected in series with the cathode return circuit of the reactance tube 40, or with the cathode return circuit of the oscillation generator 60, or with the cathode return circuit of the amplifier tube 10; or in series with the oathode return circuit of the tubes of modulator I00, or in series with the direct current screen grid circuits of tubes 82 and 82', or with the grid circult of the amplifier tube I30, or with the anode circuit of the tube I30, or with the resistances RH! and R20 terminating the low pass filter 2 3 depending upon the positions of the switches 249' and 253.

While it is believed that my invention has been made clear to those versed in the art, the purpose and operation of essential and novel features thereof will now be described in detail. The keying voltage from the converter H5 is fed to the voltage regulator tube l 8. As stated above, this voltage varies from zero in a negative direction, say to about 180 volts, but may vary more or less due to variations in the potential used on tubes in the tone generator andkeyer l0. The voltage regulator tube 18 is so poled that when the negative potential at its electrode ll exceeds a selected amount a limiting action takes place. In the embodiment described this tube'is used to limit the negative swing of the keyed pulse to about l volts. The converter it! does not have any limiter and if the power supply voltage therefor varies the keyer output varies and lets the power supply modulate the keying. The tube Hl eliminates this effect. The output of the tube It! is connected to a resistor IS in series with an adjustable part of the potentiometer resistor 29 and ground. The input voltage to the reactance tube 40 is adjusted by means of this potentiom eter 2c and thus the total frequency shift may be set from any value between zero and some upper maximum value say, for example, 1200 cycles by means of this adjustment. The frequency swing is measured and the potentiometer 20 is callbrated to give the proper frequency shift for the correct potentiometer adjustment. This shift should not depart from the calibration unless the reactance tube 40 should change radically or depart from its normal characteristics. By using a monitor receiver and checking the beat note on mark and space keying, the frequency shift may be checked. In the embodiment being described the frequency shift is set at a total of 850 cycles. The output of the potentiometer 20 is fed to switch S2 which is used to include or exclude the resistive pad 28 in or from the input connections to the reactance tube 40. Whether or not this resistive pad 28 is included depends upon the amount of frequency shift to which the output is subjected in the multipliers I'Hl. In the embodiment described, with a total frequency shift of 850 cycles output and a multiplication of 4, the reactance tube and oscillator is adjusted to produce a frequency shift of about 212.5 cycles total. If a frequency multiplication of 8 is used at HG then obviously the frequency shift accomplished at the modulated oscillator 60 must be divided by 2, or be about 106 cycles total. With the resistor attenuating pad 28 out the adjustment is made such that the reactance tube 40 modulates the oscillator 60 through a range of 212.5 cycles total. Then the multipliers at I10 have a factor of 4. If the multipliers at l 19 have a multiplication factor of 8 then the switch S2 is moved to its other position to include the network 28 in the input circuit of the reactance tube 48. The network 28 consists of a 6 db. pad and the attenuation is such as to reduce the maximum negative potential on the control electrode of the reactance tube 40 to reduce the total here to onehalf its previous negative value so that the output total swing will remain essentially fixed at 850 cycles. Thus, once the potentiometer 20 has been set the same does not need to be reset for different multiplication factors unless the total output shift is changed from the assumed 850 cycles.

The output of the potentiometer 2i! feeds directly or by pad 28 into a low pass filter 24 composed of condenser 23 and inductance 25, and a section of the condenser C3 (at the lower end of 2?). These connections include the switch S3 in the position shown so that the voltage 'passed by the low pass filter is supplied to the inductance 2'! and thence to the control grid 3!! of the reactance tube 40. The filter 24, as stated hereinbefore, shapes the signal wave and controls the rate of buildup of the keying characters, thus narrowing the spread of the radio frequency side bands. The filter is terminated by resistors RH! and R211.

The switch S3 has three positions, one designated Frequenc shift, another designated Frequency calibrate, and the third Phone. In the Frequency shift position it will be noted that the regulated direct current potential'is supplied from the positive terminal of the source and through one of the voltage regulator tubes 22!] by lead 3! to the screen grid 32, and by lead 33 to the anode 34 of the reactance tube 40, and simultaneously this regulated direct current voltage is supplied to the anode of the oscillator tube 60 and the anode of the amplifier tube l0. Then the frequency of the oscillations generated in oscillation generator tube an is swung by the keying voltage including the direct current component through a first range if switch S2 is in the position shown, or through a range half as J the resistor Bit and Wide as said first range if the switch S2 is put in the other position. The plate and screen voltages are regulated by means of voltage regulator tubes Vt and V9. The 200 kc. oscillator including tube 8%? is stabilized for ambient temperature variations and also for voltage varialions. The other half of this tube, 1. e., section ill, is used as an amplifier. The amplifier 10 is loosely coupled to .the oscillator 68 through condenser C8 and the output is taken from the cathode impedance L3 through coupling condenser C5.

In the Calibrate position of switch S3, direct current voltages are supplied as described above and also by leads 35 to the anode and screen grid of the crystal controlled monitoring oscillator 290. In the Calibrate position then the direct current modulated oscillator 86, the ampliher it, and the monitor crystal controlled oscillator 268 are all operating. However, in this position of the switch the control grid of the reactance tube ill is connected by resistor RI and inductor 2i to a point between resistors RI9 and RZ J so that only half of the keying voltage on steady mark (1. e., one-half of the highest negative voltage) is supplied to the reactance tube. If the tube 4E3 characteristic is about linear, now the frequency at which the oscillator til operates should be the desired center frequency, in the example given, 200 kc. To check this frequenc the 290 kc. frequency shift oscillator is adjusted or tuned by means of trimmer condensers C5 and CM! if necessary to establish zero beat with the crystal oscillator 200. For this adjustment the transmitter need not be on the air. The intensity of the current flow shown by the milliammeter MI, included by lead 89 and switches 259 and 253 in the cathode return circuits of the balance modulator tubes 82 and 82, indicates zero beat. This centers the carrier frequency of the frequency shift half way (if the reactance tube is linear) between mark and space frequency.

When the switches are in the position shown R29 at the terminal of low pass filter 2d are connected to ground through lead 2? and meter M l. Other switching means (for the sake of simplicity not shown) opens the ground connection to R59 and R283 at the point 29. The meter Ml may be connected in series with resistors Bit and R26 and ground to indicate the keying current flowing in R19 and R28, thus giving the keyin voltage applied to the grid 36 of the resistance tube to. In fact this may be used as an indication of frequency shift for setting the potentiometer 28, so that the frequency shift could be calibrated against keying current on mark condition.

The reactance simulated by tube s8 depends on the current therethrough. The current therethrough depends on the grid potential. In order to facilitate adjustment of the circuits to cause the tube to operate at the middle of the linear portion of its characteristic curve, its

cathode may be connected to ground through lead 53 and switches 2&9 and 259 instead of directly as shown at 35. In the embodiment used additional switching means, not shown, opens the ground connection at at when the connections through lead 43 and switches 249' and 253 are completed to put the meter MI in the cathode return circuit of the reactance tube 40. Now the adjustments at 2Q can be made to get the desired maximum currents through the tube (observations made at M!) to get the desired reance cha e and fre uen y swine. The additi nal switching means ha be n omi t n rder to simplify the drawin s. he c hodes 53 and 53 of tubes I50 and III are also arranged to be onnected. throu h meter M by l a 55 and 65 respectively, and switches 249 and 253. When s on ected. the direct ound nne tions for these cathodes are broken at 51 and 51 by the swit h ng means (not shown). escri d a ove, which is controlled simultaneou ly with switches 249 and 253. In the shown po-. s tion of hese switche the ground connectio for cathodes 53 and 53 are completed at 51 and 61.

Similar means switches 249 and including lead 89 between 253 and-the cathodes of the balanced modulator tubes 82 and 82 and the switch (not shown) contact 84, and leads I03 and 105 between switches 249 and 253 and the screen grids of balanced modulator tubes 82 and 82 and the switch (not shown) contacts I01 and I99, and lead lI'I between the grid I29 and switches 249 and 253, and switch (not shown) contact H and lead I41 between the anode and tuned circuit I53 and switches 249 and 253, and switch (not shown) contact I49 and lead 2IlI between'the cathode of the monitor oscillator 200 and switches 249- and 253 and switch (not shown) contact 293 and lead 21' between the resistance R20 and the meters 249 and 253 and switch (not shown) contact29, are provided for tuning and adjustment of the balanoed modulator I00, the amplifier I90, the monitor oscillator 200, and for adjustment of the potentiometer 29" and. 28.

On Phone position of switch S3 only the crystal controlledoscillator 290 is excited and the transe mitter frequency control is supplied by this crystal oscillator and the crystal oscillator at I20 or I40. This position may be used if amplitude phone or amplitude keying is desired. The trans.- mitters may be equipped with amplitude tele: phony or in some cases amplitude (oneoff) keying may be desired. The amplitude keying is usually accomplished in a later stage by methods well known to the art. The amplitude telephony may be accomplished in the last (output stage) or one of the other stages of the transmitter.

When the extent of frequency swing is changed by say adjustment of potentiometer 29 or by including network 23 in the reactance tube input the center frequency of operation of the system is changed. In this respect it may be kept in mind that the minimum negative voltage (zero potential) applied to the input is fixed. To further improve systems of this type means for automatically centering the carrier frequency is provided.

Automatic centering of the carrier frequencymay be accomplished if abias'voltage is applied to the reactance tubev through a potentiometer which is mechanically coupled to frequency shift adjustment 29. When the frequency shift cone trol is varied or when the selector switch (XL-X8) is changed age is applied to the grid of the reactance tube to keep the carrier essentially centered. In Fig. 2 I illustrate a method of applying a bias to the reactance tube control grid in the proper relation, to keep the carrier frequency essentially centered. In this embodiment reference nu merals corresponding to those used in Fig. 1 are used in so far as possible. Resistors RIM and RIOI of which RI D0 is a potentiometer ganged with potentiometer 20 have been added with BI 00 connected by switch S I 02 and part of source the proper correcting volt-i 3.90 to ground. or by way of source 300 to ground. The tap on EH10 i conne ted by swi ch W3 to a po nt between res stances R162 and R103 or to a tap on EH13. B492, and BN3 are in series with RIB. The contacts of S3 in the direct current supply circuits or the Various tube electrodes etc, other than that used between the reactance tubeand ign l inpu nd the meter M nect ons by switches 249 and. 2 .3 h ve been omitted for the s ke of simplicity- When the switches are in the positions shown the grid circuit of tube 48 includes RI, 2?, RIB, EH12 and all of RIM and also part of RIOS shunted b an adjustable portion of RIIlI'l in series with BiIiI andsource 309. The resistor RIIJI and variable resistor RIUO are in series of a value sufliciently high, so that the current through R I 93 is independent of the value of BI 93. The potentiometer 20 is mechanically connected to R IIIQ so that the bias is varied correctly as the frequency shift adjustment 29 is changed. As the frequency shift is made smaller, bias is applied to the reactance tube to recenter the carrier. Switch SHJZ is ganged with switch S2, so that when the input signal is twice (the pad .51 out). twi the normal. bias i appl ed to th grid of the reactance tube. It is not necessary to have SW3 as illustrated and this switch may be deleted from the drawing. Then the connection from R499 goes directly to resistor Rlfl3. However, if desired, switch SI93 may be included (as shown) and be ganged with switch S2. Then w t h 92 may he. removed and Source 309 included between RI!!! and ground.

Another method is to have a small trimmer ndenser in para lel with co mechanically coupled to frequency shift adjustment 20, so ar-. ranged that when the frequency shift adjustment is changed the carrier. is shifted to provide a fixed center frequency. A small fixed cone denser may also be switched in and out of the circuit when switch S2 (X I-X8) is changed. This embodiment is illustrated in Fig. 3. When the potentiometer is adjusted to change the frequency shift the condenser CIIIG in shunt to condenser C6 is adjusted to center the carrier frequency. When the switch S2 is thrown to the lower position to put the pad 28 in the input circuit the condenser Cid! heretofore in shunt to condenser C5 is removed from the oscillator circuit to increase the oscillator frequency.

Another method is to use pushpull D. C. input signals, say plus for mark and minus for space. Pushpull reactance tubes may then be used and to center the carrier it is only necessary to remove the input signal and the reactance tubes re en. at their normal carrier and the frequency shift for equal mark and space voltages. Thepushpull reactance tubes also make the frequency shift system less susceptible topower supply variations in the unit.

In operating the system the procedure described above and the following procedure may be followed.

, After the set is turned on the operator will of course know and select which of th high frequency crystal controlled oscillators I29 or I49 he is to use. This selection is made by use of switch S4. Then the switch S3 is placed on the frequency shift position and the crystal tuning dial on condenser CI'5 in circuit 83 is set at the proper position or approximately there. This circuit is tuned for maximum modulation as denoted by maximum cathode or screen grid ourrent onthe meter MI, switches 249 and 253 being reactance tube.

1 l 7 turned to the proper position. The modulator frequency of the oscillatory energy with respect tuning dial on the condenser CH} of circuit I04 to a mean or average frequency, means for adis then placed at the proper setting. Curves supjusting the value of said applied pulse energy plied with the unit may be used to approximately to thereby change the extent of said frequency determine this setting and the dial is then 5 shift, means for recentering the mean or average slightly tuned to obtain maximum output which frequency of operation of said oscillation genmay be denoted by the grid current in the tube erator when the value of the applied pulse en I30, its grid return circuit then being connected ergy is changed comprising an adjustable reby switches 24:) and 253 through the meter Ml. sistance in the bias circuit for said control elec- This circuit, which in the embodiment described trode, and common control means for said last is tuned to the upper side band, must be caretwo means.

fully tuned and is quite sharp. The output tun- 5. In a signalling system, in combination, an ing condenser C22 is then properly set and in oscillation generator circuit wherein oscillatory doing so the anode current of tube 30 as indienergy appears, a reactance tube having elecca-ted on the meter MI may be observed in maktrodes in shunt to at least a part of the gening this adjustment. The input keying signal erator circuit to tune the same, said reactance is then placed on mark condition to apply a tube having a biased control grid, the magnisteady potential equivalent to mark potential tude of the energy on which controls its reactive across the resistor 20. The frequency shift adeffect, connections .to said control electrode for justment by potentiometer is then made to applying pulse energyto said electrode the maggive the desired frequency shift as indicated by nitude of which is keyed between first and seccurrent through the meter Ml when the same ond values in accordance with signals to vary is connected in series with th resistors RIB and the frequency of the generated oscillatory energy R213. The switch S2 is then set in the X4 or X8 with respect to a selected value, an adjustable position, X4 indicating that a multiplication resistance for changing the magnitude of the pulse factor of 4 is used, and X8 indicating that a energy to thereby change the extent of variamultiplication factor of 8 is used. The control tion of the oscillatory energy with respect to said switch S3 is now placed in the Calibrate position selected value, means for recentering the freand by adjustment of the condenser CB and/or quency of operation of said oscillation generator C 36 the oscillators 60 and 260 are brought into when the magnitude of the pulse energy is changed zero beat, this condition being indicated by slow comprising an adjustable resistance in the bias variation of current in the meter MI when the circuit of said control grid, and a common consame is in series in the return circuit of the trol for said adjustable resistances. modulator tubes 82 and 82'. Now the switch S3 6. In a signalling system, in combination, anv is put on the frequency shift position and the oscillation generator circuit wherein oscillatory steady mark signal potential is removed and keyenergy appears, a reactance tube having elecin signal put on. trodes in shunt to at least a part of the generator What is claimed is: circuit to tune the same and control the generator 1. In a signal-ling system, in combination, an frequency, said reactance tube having a biased alternating current circuit means for causing 0 control electrode the potential on which controls oscillatory y 0 flow 111 clrcuit, a its reactive efiect, connections to said control ietit pitt titii trftiit iit thi tifie aii elem applying Pulse energy Said c e S reactance tube having a biased control grid, the 33 elgctrode gg g g i fi Value magnitude of the energy on which controls its Oug K V ue a 1rd Va be m accord reactive effect, connections to said control grid 9 Wlth slgnals shlfi the frequency of the for applying pulse energy to Said control grid the oscillatory energy with respect to a mean or avermagnitude of which is keyed between first and age frequency, means r dj s n t e Va ue O second values in accordance with signals to vary said applied pulse energy to thereby change the the tuning of the circuit between two limits extent of said frequenc shift, means for recensymmetrically rel to a n er fr q n y, an tering the mean frequency of operation of said adjustable resistance in said connections for oscillation generator when the Value of the apchanging the extent of the separation between said first and second values of the pulse energy plied pulse energy is changed compnsmg an to thereby change the extent of variation of the justable reactance in said oscillation generator Said tuning of Said circuit, and means for circuit, and common control means for said last multaneously recentering the tuning of said cirtwo meanscuit at said center frequency when the extent of In a Signalling System, in Combination, o separation between said two values of the pulse cuit wherein oscillatory energy flows, a reactance energy is changed. tube having input electrodes and having output 2. A system as recited in claim 1 wherein said 1 t d coupled t 5 last means is a second reactance element in said th tuning th f nd thereby trolli the circuit. timing of the said oscillatory energy, a source of A System as recited in Claim 1 wherein Said direct current potential which varies between two last means is a controllable bias source for said values connected to the input electrodes of said tube to vary the current therethrough and therei. In a signalling system, in combination, an 5 by vary the timing of the oscillatory energy beoscillation generator circuit wherein oscillatory tween two values, frequency increasing means energy appears, a reactance tube having eleccoupled to said circuit for increasin the range trodes in shunt to at least a part of the generator between said last named two values, direct ourcircuit to tune the same, said reactance tube rent and alternating current potential attenuathaving a biased control electrode the potential ing means, and switching means arranged to inon which controls its reactive effect, connections clude said last named means in the connection to said control electrode for applying pulse enbetween said source of direct current potential ergy to said control electrode which varies from and the input electrodes of said reactance tube a. first value through a mean value to a third when the multiplication factor of the frequency value in accordance with signals to shift the increasing meansismade larger.v

aid circuit for controlling 8. In a signalling system, in combination, a circuit wherein oscillatory energy flows, a reactance tube having input electrodes and having output electrodes coupled to said circuit for controlling the tuning thereof and thereby controlling the timing of the said oscillatory energy, a source of direct current potential which varies between two values, one of which is fixed and the other of which may vary in an undesired manner, coupled to electrodes of said tube to vary the current therethrough and thereb vary the timing of the oscillatory energy between two values, a voltage regulator tube in shunt to said source and poled so as to limit variations of said other potential so that the timing of said oscillatory energy is varied through a fixed range, frequency converting and frequency multiplying means coupled to said circuit for multiplying the range through which said oscillatory energy is varied, a direct current potential attenuator, and a switch to include said attenuator in the connection between said source of direct current potential and the input electrodes of said reactance tube when the multiplication factor of the frequency increasing means is made larger.

9. In a signalling system, in combination, a circuit wherein oscillatory energy flows, a reactance tube having input electrodes and havin output electrodes coupled to said circuit for controlling the tuning thereof and thereby controlling the timing of the said oscillatory energy, a source of direct current potential which varies between two values, one of which is fixed, coupled to the input electrodes of said tube to vary the current therethrough and thereby var the timing of the oscillatory energy between two values, a voltage limiter coupled across said source and poled to fix the other value between which said potential varies, to thereby fix said two values between which the oscillatory energy is timing modulated, a low pass filter in the coupling between said source and the input electrodes of the reactance tube to reduce side frequencies produced by variations of said potential, frequency increasing means coupled to said circuit for increasing the frequency range through which the oscillatory energy is modulated, a control potential attenuator, and means to introduce said attenuator in the connection between said source of direct current potential and the input electrodes of said reactance tube when the multiplication factor of the frequency increasing means is made larger.

10. In a signalling system, an oscillation generating circuit, a reactance tube having output electrodes associated therewith for controlling the frequency thereof, a source of modulating potentials of varying value coupled to the input electrodes of said reactance tube to modulate the frequency of said circuit, means for multiplying the frequency of the output of the said modulated circuit, a potential attenuator, and a switch for introducing said potential attenuator in the connection between said source of modulating potentials and the input of said reactance tube when the multiplication factor is changed.

11. In a signalling system, in combination, an oscillation generator circuit wherein oscillatory energy appears, a reactance tube having electrodes in shunt to at least a part of said circuit to tune the same and control the generator frequency, said reactance tube having a control grid the potential on which controls its reactive effect, connections to said control grid for applying pulse energy to said control grid the magnitude of which varies from a first to a second value in accordance with signals to vary the effective reactance and the frequency of the oscillatory energy, and means for centering the frequency of operation of said oscillation generator comprising a crystal controlled oscillator operating at the desired center frequency, means for applying a potential equal to one-half said second value to said control grid of said reactance tube, and means for then adjusting the frequency of the modulated oscillation generator until it equals the frequency of the crystal controlled oscillator.

HALLAN E. GOLDSTINE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,054,883 Schlesinger Sept. 22, 1936 2,176,168 Bellescize Oct. 17, 1939 2,279,659 Crosby Apr. 14, 1942 2,285,044 Morris June 2, 1942 2,380,959 Frankel Aug. '7, 1945 2,339,608 Alvira Jan. 18, 1944 2,351,193 Crosby June 13, 1944 2,379,614 Tunick July 3, 1945 2,398,793 Magnuski Apr. 23, 1946 2,401,629 Finch June 4, 1946 2,403,358 Gerhard et a1. July 2, 1946

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