US2408791A - Radio communication system - Google Patents

Description

Oct. 8, 1946. H. MAGNUSKI RADIO COMMUNICATION SYSTEM Filed June 21, 1945a 3, Sheets-Sheet 1 INVENTOR. HENRY MAGNUSK] a. L m mun; hmmE B 2 T L 343%. A]? i 2 B m u asdd uddga 55 w o 2 325;

U HMREE ATTORNEY 3, 6- H. MAGNUSK! RADIO COMMUNICATIGN SYSTEM Filed June 21, 1943 s Sheets-Sheet 2 152 324 um oz v o 0 125%: .053 0 v ...v 3 3 v 3 8. =2: 2 9 3. o m EH4 2 :2:

pm mokqziEowa 3 Sum Q @8358 2:5: mm 4 INVENTOR HENRY MAGNUSKI ATTORNEY Oct. 8,1946. MAGNUSKI I 2,408,791

RADIO COMMUNICATION SYSTEM Filed June 21, 1943 3 Sheets-Sheet 5 EGTIFIER 33 B REAGTANCE MODULATOR I3 FIG.5

RELATNE DISCRMINATOR OUTPUT VOLTAGE I INVENTOR.

-|o HENRY MAGNUSKI 2 o l 2 a 4 RELATIVE F- u VOLTAGE mvur m W MIGROVOLTS AT ANTENNA GROUND GIR- I9. 7 ATTORNEY Patented Oct. 8, 1946 UNITED STATES PATENT OFFICE RADIO COMMUNICATION SYSTEM Henry Magnuski, Chicago, 111., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application June 21, 1943, Serial N 0. 491,651

, 9 Claims. 1

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

Complete self-contained combination transmitter and receiver units, both of the portable and fixed position types, are now extensively used in many forms of radio communication work and are'especially useful in two-way police and military communication work. Such units must of necessity be rugged, light inweight, and easily manufactured in production quantities at low cost. They must also be capable of being easily and rapidly conditioned to operate either as a transmitter or as a receiver, and should be easily tunable to transmit or receive at any desired carrier frequency within an allotted frequency band. Other requirements of a unit of this type are that the unit have sufficient signal radiating power and sufficient sensitivity of reception to permit high quality two-wa communication to be held over substantial distances, and that the receiving channel of the unit be capable of maintaining its sensitivity in receiving a signal carrier having a drifting carrier frequency.

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

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

It is a further object of the invention to provide a combined transmitting and receiving system in which porticnsof both the transmission and. receiving channels are used both during signal transmission and signal reception, thereby to minimize the number of component parts of the system without sacrificing. desirable operating features.

According to another object of the. invention, a

combination frequency modulated transmitting mediate frequency section of the receiving channel during signal reception.

According to a further object of the invention, the modulator stage of the transmission channel is controlled by a variable bias derived from the receiving channel in order automatically to control the carrier frequency of asignal carrier traversing the intermediate frequency section of the receiving channel during signal reception. I

It is another object of the invention to provide improved facilities whereby the system may be utilized for'signal transmission and reception at the same carrier frequency without altering the tuning of any of the tunable stages of the system.

It is a further object of the invention to provide an improved radio transmitter wherein two Oscillators arranged in tandem are utilized to produce for radiation a signal modulated carrier.

According to a further object of the invention, the transmission channel is provided with a mixer stage which follows the master carrier producing oscillator and utilizes a crystal oscillator having a resonant frequency equaling the center intermediate frequency of the first intermediate frequency section of the receiving channel to obtain the desired frequency of carrier radiation.

It is a still further object of the invention to provide an improved arrangement for selectively rendering the transmission and receiving channels active and inactive in a manner such that all of the facilities mentioned above are selectively,

automatically and appropriately rendered active and inactive as the two channels are selectively conditioned for signal transmission and signal reception.

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

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

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

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

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

Referring now more particularly to Figs. 1 and 2 of the drawings, there is illustrated, partially in schematic form, a combination frequency modulated radio transmission and receiving system which is well adapted for use as a complete portable unit and includes a transmitting section Ii) and a receiving section II commonly coupled to an antenna ground circuit l9 through a tunable antenna circuit I8. Briefly considered, the transmitting section comprises a combination automatic frequency control and reactance modulator stage I3, a tunable master oscillator M, a tunable frequency doubler network IS, a tunable transmitter mixer I6, a power amplifier l1, and the tunable antenna circuit Hi, connected in tandem in the order named. The receiving section II of the system comprises the tunable antenna circuit I8, a tunable radio frequency amplifier 20, a first mixer or converter stage 2|, a first intermediate frequency amplifier 22, a second mixer or converter stage 23, a second intermediate frequency amplifier 24, a first limiter 25, a second limiter 26, a frequency discriminator 21, an audio frequency amplifier 28, and a loud speaker 29, all connected in cascade in the order named. As pointed out below the system may be selectively controlled to operate either as a transmitter or a receiver and, when conditioned for operation, is set to operate as a receiver. In order to render the audio section of the receiving channel H inoperative to pass noise signals appearing in this channel during intervals when a desired signal is not being received, muting or squelch apparatus is provided which comprises a high pass filter network 33 coupled to the output side of the frequency discriminator 21, a noise amplifier and rectifier 3|, a direct current amplifier 32 and a muting oscillator and rectifier section 33. These stages are connected in tandem in the order named, and respond to noise voltages appearing c explained below.

More specifically considered, the transmitting section of the system comprises a microphone |2 which is arranged to impress audio frequency voltages developed during operation thereof between the input electrodes of the combination frequency control and modulator tube 38 through a. coupling network which comprises a microphone transformer 35, a condenser 36, and a resistor 31. The space current path through the tube 38 is connected in shunt with the space current path through the tube 40 of the master oscillator l4, and also shunts the tunable frequency determining circuit 4| of the master oscillator M. This oscillator is of the conventional tuned plate circuit type, the frequency determining circuit 4| thereof comprising a, fixed inductance element 4|a which is tuned to the desired resonant frequency by means of the shunt connected fixed condenser MI) and an adjustable tuning condenser 4|c, Operating potentials are supplied to the anodes of the tubes 40 and 38 through a resistor 46 and the inductance element 4|, a low impedance direct current blocking condenser 4|d being provided in the tunable circuit 4| in order to isolate this direct current path from ground. The tunable frequency determining circuit 4| of the oscillator I4 is regeneratively coupled to the input electrodes of the tube 40 by means of an inductance element 42 which is inductively coupled to the inductance element 4|. It is also coupled to the input electrodes of the frequency control and modulator tube 38 by means of the inductance element 42 and a phase shifting network which includes the condenser 3'|a and the resistor 311). A suitable grid condenser 44 shunted by a grid leak resistor 43 is serially included in the input circuit of the oscillator tube M for the purpose of maintaining the control grid of this tube at the proper operating potential with respect to the cathode of the tube.

The signal modulated carrier voltage developed across the tunable frequency determining circuit 4| is impressed between the input electrodes of a tube 41 included in the frequency doubler |5 through a network which comprises the coupling condenser 49 and a resistor 48. This tube is provided at its output side with a tunable frequency selective circuit 52, which includes a fixed inductance element 52a shunted by a fixed tuning condenser 52b and an adjustable tuning condenser 520, and is tuned to a center frequency substantially twice the center resonant frequency of the frequency determining network 4| forming a part of the master oscillator stage l4. Anode current is supplied to the tube 4! through a filter resistor 54 and the inductance element 52a, and the usual direct current isolating condenser 5211 is provided in the circuit 52 to isolate the anode current path from ground.

In accordance with the present invention, the carrier voltage developed through operation of the tunable master oscillator l4 and the tunable frequency doubler I5 is utilized as a heterodyning frequency source for converting a received frequency modulated radio frequency carrier into a correspondingly modulated intermediate frequency carrier in the first mixer stage 2| of the receiver channel ll. When, therefore, it is desired to utilize the system to transmit and receive signals at a fixed and preestablished carrier frequency without altering the tuning of the tunable circuits in the system incident to a change from transmission to reception, or vice versa, it is necessary to increase or decrease the output frequency of the frequency doubler |5 by an amount equal to the value of the intermediate frequency utilized in the first intermediate frequency amplifier section 22 of the receiver channel Whether or not the output frequency of the doubler I5 is raised or lowered to provide the desired frequency of carrier transmission, will of course depend upon whether the doubler output frequency is above or below the particular carrier frequency at which transmission is to be effected. In the particular arrangement illustrated, a piezoelectric crystal 58 having a resonant frequency equal to the intermediate frequency utilized in the first intermediate amplifier 22 of the receiver is utilized to increase the frequency of a transmitted signal carrier above the carrier frequency appearing across the frequency selective circuit 52 by an amount equal to the intermediate frequency utilized in the first intermediate frequency amplifier 22. More specifically considered, the tunable circuit 52 is coupled to the input electrodes of the transmitter mixer tube 63 through the shunt connected crystal 58 and condenser 59. A grid leak and condenser network comprising the two resistors 55 and 56 and a condenser 51 is provided for maintaining the proper bias potential between the input electrodes of the mixer tube 63. For the purpose of driving the crystals 58 to maintain oscillation of the crystal at its resonant frequency, a tuned circuit 60 is provided which is suitably designed to resonate at the same frequency as the crystal 58 and comprises a fixed condenser 60a shunted by an adjustable inductance element 691). This network is included in the screen electrode circuit of the mixer tube 63 and also in the path comprising the resistor 62 over which the required. operating potential is positively applied to the screen electrode of the tube 63.

The mixer tube 63 is provided with a tunable frequency selective output circuit 64, which comprises a fixed inductance element 64a shunted by the fixed condenser 64b and the adjustable tuning condenser 640 through the low impedance direct current isolating condensers 64d and- Me. This circuit is normally maintained tuned to a frequency'which is equal to twice the output frequency of the oscillator I4 plus the resonant fre.

quency of the crystal 58, which latter frequency equals the first intermediate frequency used in the receiver channel I l-. The output voltage appearing across the circuit 64 is impressed across the input circuit of the power amplifier I] through a coupling network which includes the condenser 65. 7

Referring now more in detail to the signal receiving channel H of the system, the first mixer stage 2| is illustrated as being resistance-capacitance coupled to the output circuit of the tunable radio frequency amplifier 20 through a network which includes the coupling condenser 68 and resister 69. As indicated above, when the system is conditioned for reception, the tunable master oscillator l4 and the tunable frequency doubler I5 are utilized as a heterodyning frequency source required to effect the desired carrier frequency conversion in the first mixer stage 2|; To this end, an inductance element 53 which is inductively coupled to the inductance element 52a of the frequency selecting circuit 52 is included in the cathode-ground circuit of the mixer tube 19.

The output electrodes of this tube: are coupled to a fixed tuned frequency selecting circuit 1 I which comprises a fixed condenser 'Hb shunted by an adjustable inductance element Ha andis tuned to the desired first intermediate frequency of 4.3 megacycles, for example. Anode potential is supplied to the tube over a path which includes the inductance element 'Hu' and a. filter resistor l3 which is shunted by a by-pass condenser 12. The tuned output circuit H of the tube 19 is coupled to the input electrodes of the first tube in the intermediate frequency amplifier 22 through a network which comprises the coupling condenser 14 and resistor 15.

The output side of the first intermediate frequency amplifier 22 is coupled to the input electrodes of the mixer tube 16 provided at the second mixer or converter stage 23 in an obvious manner. This tube is provided with output electrodes which are bridged by a frequency selective circuit 8'! tuned to the second intermediate frequency of 2.515 megacycles, for example, and

comprising a condenser 81b shunted by an adjustable inductance element 8H1. Anode potential is supplied to the tube 16 over a path which includes the inductance element 8m and a filter resistor 83 shunted by a by-pass condenser 82. The voltage appearing across the frequency'selective circuit 8| is impressed across the input side of the second intermediate frequency amplifier 24 through a network which comprises the a coupling condenser 84 and a resistor 85. For the purpose of effecting the required carrier frequency conversion at the second mixer stage 23, the miXer tube 19 is provided with an oscillator section of the Pierce type which includesa piezoelectric crystal 71 connected. between the control andscreen electrodes. of the tube. This crystal has a resonant frequency of 6.815 megacycles which is greater than the intermediate frequency utilized inthe first intermediate frequency amplifier ZZ-by'an amount equal to the intermediate frequency utilized in the second intermediatefrequency amplifier 23; A suitable biasing network comprising the series connected resistors 18a and 18b shunted by thegrid condenser 19 is provided between the input electrodes of the tube Hiv for maintaining the proper bias voltage between these electrodes.

Noise and signal voltages appearing at the output side of the second. limiter 2B are introduced into the frequency discriminator 21. Briefly considered, this discriminator comprises a tuned circuit 8-1,. a pair of diode rectifier tubes 88 and 89, the space current. paths of which are respectively shunted by load resistors 90 and 9|, a radio frequency by-pass condenser 93 having substantially negligible impedance to frequencies of the order of. the second intermediate frequency, and a stabilizing condenser 92. More specifically, the resonant circuit 81 serves to tune the frequency discriminator network to a center frequency equal to the second intermediate frequency and comprises a pair of series connected condensers 81b and 810 which are shunted by an adjustable inductance element 81a. Preferably the last mentioned element is of the variable permeability type being provided with an adj ustable powdered ferrous metal core, the position of which may be changed. to alter the inductance of the element within the desired limits; The circuit constants of the resonant circuit '81 are so chosen that the discriminator network is provided with a band pass characteristic such that all desired signal components of a. frequency modulated carrier appearing in the second intermediate frequency channel 24, 25, 26 may be detected and impressed upon the input circuit of the audio amplifier 28. The voltage appearing across the. output side of the second limiter 26 is impressed upon the discriminator network 21 through a coupling condenser'86 which is connected' at one. side thereof to the junction point between the two condensers 81b and 81c. Audio frequency voltages detected through operation of the discriminator 21' appear across the condenser 9:3: and are impressed: upon the input side of the audio. frequency amplifier 28 through a coupling circuit which includes radio frequency decoupling resistor 94, an audio frequency filter comprising the resistor 95 and condenser 96', an audio frequencycoupling condenser 91, and a volume control voltage dividing network comprising the two resistors '99 and H10 and a direct current blocking condenser HH. It will be understood in this regard that the proportion of the available audio frequency voltage appearing across the series connected; resistors 98* and I90 which is impressed upon the input circut of the audio frequency amplifier 28, is determined by the setting of the wiper 99 'aiong the resistor 98.

As will be explained more fully below, noise signal's. appearing in the signal transmission channel of the receiver in the absence of a received signal modulatedcarrier are passed through the discriminator 21- and appear as detected audio voltages across the condenser 93'. Such detected voltages are impressed across the high pass filter network 30, and. those components thereof having. frequencies above the cutoff frequency of the plifier and rectifier 3I. More specifically considered, the high pass filter 30 comprises a pair of series condensers I01 and I02 and a pair of shunt resistors I03 and I00, and is designed to pass those components of noise voltages which have frequencies above the normal signal reproducing band of the receiver. The noise amplifier section of the tube I works into a noise rectifier circuit which comprises the diode section of the tube and a. load resistor IIO. This rectifier circuit is coupled to the anode of the tube I06 through a coupling condenser I08 which is of appropriate impedance to pass any noise currents which may be transmitted through the high pass filter 30. Anode and screen potentials are supplied to the tube I06 through the resistors I01 and I09, the second of which is by-passed to ground through a condenser III.

Rectified noise voltages appearing across the load resistor IIO are utilized to control the bias between the input electrodes of the tube I I3 provided in the direct current amplifier 32. Theinitial or threshold bias established between the electrodes of this tube is derived from a voltage dividing network, which comprises the series connected resistors IIBa, H61), and H00 bridged across the available source of anode potential, and is provided with a tap III adjustable along the resistor I IE to impress a variable positive potential upon the control electrode of the tube I I3 through the filter resistor H5. The biasing circuits connected between the input electrodes of the tube II3 are by-passed for audio frequency currents by means of a condenser I I4. Screen and anode potentials are applied to the amplifier tube I I3 through the resistor H8 and the resistors H6 and I I9, in series, respectively.

The direct current amplifier 32 as controlled by the variable bias voltage derived from the load resistor I I 0, is utilized to control the starting and stopping of the muting oscillator and rectifier 33. This stage of the muting or squelch apparatus comprises a dual purpose tube I20 having an oscillator section which includes a tuned frequency determining circuit I2I connected between the output electrodes of the tube through a by-pass condenser I22. The resonant circuit 'I2I is fixed tuned to a particular frequency of from 200 to 300 kilocycles and comprises an inductance element I2 Ia shunted by a tuning condenser I2 I12. It is regeneratively coupled to the input electrodes of the tube I20 by means of a feed back circuit which comprises an inductance element I23 inductively coupled to the inductance element I2 Ia and connected in series with a parallel connected grid leak resistor I24 and condenser I25 between the control grid and cathode of the tube I20. Anode potential is supplied to the tube I20 over a path which includesthe inductance element I2Ia and a resistor I26. The oscillator section of the tube I20 is coupled to the rectifying circuit of the tube through a coupling condenser I29, and the indicated rectifying circuit serially includes the diode rectifier section of the tube and the resistors I28, I32 and 102;. Any bias voltage appearing across the load resistors I28, I32 and 'I8b during operation of the oscillator and rectifier stage 33 is negatively applied to the control grid of the first tube in the audio frequency amplifier 28 over a path which comprises the resistor I21, the resistor I00 and the lower portionof the resistor 98.

In order to insure that the system will operate efficiently with low battery current drain, all of the tubes, with the exception of the discriminator diode 88, are of the filamentary cathode type. The diode rectifier 88 must of necessity be of the indirectly heated cathode type since the cathode thereof is, during operation of the discriminator 2I, maintained at potentials substantially above the reference ground potential present upon the filamentary cathodes of the remaining tubes provided in the system. More specifically, the circuit arrangement of the cathodes provided in the various electron discharge tubes referred to above and also provided in the diagrammatically illustrated sections of the system, is shown in Fig. 3 of the drawings. In this circuit, reference characters corresponding to those used in Figs. 1 and 2, but having the differentiating subscripts a and b, are used to identify the relationship between the cathodes and the respective associated circuit sections, as shown in Figs. 1 and 2. From a consideration of the circuit arrangement shown in Fig. 3, it will be noted that the various cathodes are effectively isolated at radio and audio frequencies by means of the separating filter networks comprising the illustrated high impedance choke coils and the low impedance by-pass condensers. It will also be noted that current for energizing the various cathodes in the seriesparallel circuit is supplied by a direct current source I35 through the contacts of a manually operable on and off switch I36. The cathodes IBa and Ho of the electron discharge tubes respectively provided in the mixer I6 and the power amplifier H are arranged to be energized in series with each other and with a suitable current limiting resistor I31 through the contacts of a manually operable press-to-talk switch I38. This switch is normally spring biased to its open circuit position and may be utilized in the manner explained below selectively to condition the system for signal transmission or signal reception, as desired. It is provided with a pair of normally open contacts I38a which are closed to connect the microphone I2 across the primary winding of the transformer 35 only when the switch is operated to condition the system for signal transmission. From an inspection of the cathode circuit arrangement, it will be apparent that this circuit has been carefully arranged to utilize the voltage drops across certain of the cathodes as bias voltages between the input electrodes of certain of the other tubes provided in the system. For example, the voltage drop appearing across the cathode 25a of the tube provided in the first limiter stage 25 is impressed between the filamentary cathode I5a of the tube 4'! in the proper direction to bias this cathode positively with respect to the control grid of the tube. These bias voltages, as derived from the circuit network shown in Fig. 3, are appropriately indicated in Figs. 1 and 2 of the drawings by the illustrated battery symbols, and the relationship between the respective battery symbols and the voltage drops across certain of the cathodes shown in Fig. 3 will be readily apparent from a careful comparison of the circuit shown in Fig. 3 with that shown in Figs. 1 and 2.

Preferably, the transmitter mixer tube 03 is a pentode of the well known commercial 3A4. type, the frequency doubler tube 4'! and the master oscillator tube 40 are commercial type 1T4 pentodes, the automatic frequency control and reactance modulator tube 38 and the first mixer tube I0 are commercial type 1L4 pentodes, the second mixer tube I5 is a commercial type 1R5 pentagrid converter, the two diodes and 89 are of the commercial type 1A3 and 155, respectively, the noise amplifier and rectifier tube I and the muting oscillator and rectifier tube I20 are commercial type 135 pentodes, and the direct current amplifier tube H3 is a commercial type 1L4 pentode. Suitable screen potentials are applied to the tubes 63, 4'1, 40', 38 and 16 over direct current paths which respectively include the filter resistors 52, 50, 45, 39 and 80, respectively. The potential applied to the screen electrode of the muting oscillator and rectifier tube I20 is controlled in the manner more fully explained below to effect the desired starting and stopping of the oscillator section of this tube. It will be understood that the tuning elements of the various tunable circuits provided in the system are gang controlled to be operated in unison, so that frequency alignment between the various resonant frequencie thereof is maintained during each tuning operation. More specifically, the tuning element of the antenna circuit I8, the tuning element of the radio frequency amplifier 20, and the adjustable condensers 64c, 52c and Mo, respectively provided at the tunable stages I6, i5 and I4, are mechanically connected in the manner indicated by the dash line U, so that all of the enumerated tuning elements may be operated in unison.

Briefly to consider the operation of the system, it will be understood that when the switch I36 is operated to its closed circuit position, the cathodes of all tubes provided in the system, with the exception of the cathodes I50: and I-"Ea of the tubes provided in the mixer I6 and the power amplifier H, are energized from the current source I35. If now the push-to-talk switch I33 is operated to its closed circuit position, the cathodes 16a, and I-Ta are also energized. Due to the filamentary character of the energized cathodes, they are rapidly heated to electron emitting temperatures following the energization thereof.

When the two switches I35 and I38 are thus operated, the system is conditioned for signal transmission at the particular carrier frequency established by the tuning of the five tunable stages I4, l5, l6, i1 and I18 of the transmission channel. In this regard it will be understood that when space current flow through the: tube 45' is initiated; the master oscillator I4starts to oscillate at a carrier frequen'cy"which is primarily determined by the setting of the tuning condenser 4'I:c and is secondarily determined by the magnitude of the bias voltage between the control rid and cathode of the tube '38. More generally considered, if the receiving channel II of the system is designed to operate 'with a first inter-- mediatefrequency of" 4:3 megacycles and signals areto be transmitted and received at a carrier frequency: of 44.3 megacycles, the condenser 4'Ic is so adjusted that with zero bias upon the control grid of the tube: 38, the master oscillator I4 will produce a carrier voltagehaving a frequency of" 20 megacycles and the other tunable circuits of the transmissionchannel. I'Ilare adjusted'am cordingly. With the frequency of the signal carrier thus determined, an audio frequency voltage developed through operation ofthe microphone I2 i impressed through the microphone transformer 3 5' and the coupling condenser 36' between the. control grid and cathode of the modulator tube 38. The resulting audio frequency variation 4 of the voltage between thecontrol grid and cathode of the tube 38' effectively changes thereactance of the tunable. frequency determining circuitJ4I' of'the master oscillator I4' at a correspond.- ing rate. In other words, varying the voltage ap- 10 plied between the input electrodes of the tube 38 effectively serves to vary the tuning of the network 4| in like manner, whereby the carrier output of the oscillator I4 is reactance modulated in accordance with the audio signal voltage impressed between the input electrodes of the tube 38. This modulated carrier voltage is impressed between the input electrodes of the tube 41- of the frequency doubler I5- through the coupling condenser 49. Due to the action of the tube 41 in distorting the signal modulated carrier voltage andthe action of the tunable frequency selecting circuit '52 in selecting only signal modulated carrier components having twice the frequency of the carrier voltage developed at the output side of the oscillator I 4, the modulated carrier voltage appearing across the output circuit of the-doubler I5 has a carrier frequency which is twice that of the oscillator carrier output frequency, 1. e. 40 megacycles in the case assumed above. The signal modulated carrier voltage appearing across the frequency selecting circuit 52 is impressed between the input electrodes of the transmitter mixer tube 63 over a path which includes the coupling condenser 59 and the heterodyning piezo-electric crystal 58. As previously explained, this crystal has a resonant frequency which is equal to the first intermediate frequency used in the receiving channel II of the system. Accordingly, this crystal, acting in conjunction with the tuned circuit 60, functions to produce a carrier voltage which is electronically mixed in the tube 63' with the carrier frequencyoutput across the tuned circuit 52, so that a carrier is produced at the output side of the mixer tube 53 having a frequency equal to twice the output frequency of the oscillator I4 plus the first. intermediate frequency. This carrier voltage is. frequency modulated in accordance with the audio frequency voltage applied to. the. input electrodes of the modulator tube 38. At the output side of the tube 63,. this particular signal modulated carrier voltage is selected throughthe action of the tuned frequency selecting circuit 64 and is impressed across the input circuit of the power amplifier I! through the coupling condenser 65. After being amplified by the amplifier I1, the voltage is transmitted through the tunable antenna circuit I8 and impressed across the antenna ground circuit t9 for radiation.

,Referring now more specifically to the function performed by the tunable transmitter mixer I6, it is pointed out abovethat the tunable frequency selecting circuit -64 is tuned to respond only to a signal. modulated carrier having a carrier frequency which is greater than twice the output carrier frequency of the oscillator I4-by an amount equal to the intermediate frequency utilized in the first intermediate frequency channel 22,0f the receiver. Since the carrier voltage appearing acrossthe tuned output circuit 52 of the frequency doubler I5 is used as a heterodyning frequency source at the'first mixer stage 2iduring reception and this frequency is mixed I with the; frequency produced by the crystal 58 to produce at frequency of carrier radiation which is justed by means of the adjusting element U to' 11 a setting wherein the carrier output frequency of the oscillator I4 is 20 megacycles and the carrier output frequency of the doubler I5 is 40 megacycles, then the tunable stages ll, [8 and 29 are tuned to a carrier frequency of 44.3 megacycles. This of course means that if the tunable stages of two remotely located sets of the character illustrated are tuned for transmission and reception at the same carrier frequency, it is unnecessary to alter the settings of the tuning elements of either set when the direction of transmission between the two systems is changed. Thus, the systems of the two sets may rapidly be altered for transmission in either direction with a minimum number of manual operations on the part of the persons using the respective sets for two-way communication.

As indicated above, the desired increase in the frequency of the radiated carrier over the carrier frequency appearing at the output side of the frequency doubler I5 is provided through the action of the piezoelectric crystal 58. In considering the manner in which this crystal is driven at its resonant frequency, it is pointed out that at this resonant frequency, the upper terminal of the tuned circuit 52 is efiectively at ground potential due to the low impedance of this circuit at the particular frequency in question. The resonant circuit 39 which is coupled between the cathode and screen electrode of the tube 63 is precisely tuned to the resonant frequency of the crystal 58. Due to the electronic and capacitance coupling between the upper terminal of the tuned circuit 60 and the lower terminal of the crystal 58, a sufiicient driving voltage is applied across the crystal 58 through the tuned circuit 52 to maintain the oscillation of the crystal; This coupling also serves to maintain the tuned circuit 60 oscillating at its resonant frequency.

In order to condition the system for signal reception after signal transmission has been effected in the manner explained above, the pushto-talk switch I38 is released. Incident to the restoration of this spring biased switch to its normal position, the cathodes lSa and Ila of the tube 83 and the tube provided in the power amplifier H are deenergized in an obvious manner. Thus, the transmitter mixer stage IB and the power amplifier stage I! of the transmitter channel it! are rendered inactive without in any way interrupting or otherwise affecting the operation of the preceding stages I3, l4 and I5. In this regard it is pointed out that when space current fiow through the mixer tube 63 is interrupted, the operation of the oscillator section of this tube, i. e. that portion of the tube input circuit which comprises the intercoupled crystal 58 and resonant circuit 60, is arrested. Thus, no carrier voltage is produced in the transmission channel l having a frequency approaching the intermediate frequency used at either the first or second intermediate frequency sections of the receiving channel ll. Accordingly, the continued operation of the three stages l3, l4 and I5 of the transmission channel H can in no way interfere with the reception of the selected signal modulated carrier.

Assuming that the system is conditioned for signal reception in the manner explained above, and that the tunable stages of the system are appropriately tuned to the center frequency of a desired frequency modulated signal carrier, the signal carrier voltage appearing across the antenna ground circuit 19 is transmitted through the tunable circuit l8 and the coupling condenser 6! to the input side of the tunable radio frequency amplifier 2D. This voltage, as amplified by the amplifier 20, is mixed with the carrier output voltage of the frequency doubler l5, which output voltage is impressed between the cathode and control grid of the tube 10 over a coupling path including the inductance element 53. It is thus converted into a signal modulated intermediate frequency carrier which is amplified through the first intermediate amplifier 22 and impressedbetween the input electrodes of the tube 16 provided in the secondmixer stage 23.

In the second mixer stage, the intermediate frequency carrier output from the amplifier 22 is mixed with the carrier frequency produced through operation of the crystal Tl so that a beat frequency carrier, modulated with the signal voltage and of the desired second intermediate frequency, appears across the tuned output circuit 8|. This modulated carrier, as selected through the action of the tuned circuit 8|, is transmitted through the condenser 84 to the second intermediate frequency amplifier 24 where it is amplified and transmitted successively through the limiter stages 25 and 26 to the input side of the discriminator 21. In this discriminator the modulation components of the second intermediate frequency carrier, as represented by deviations in the carrier frequency from the established center frequency, are detected in the manner pointed out below. The detected signal voltage appears across the condenser 93, which condenser is possessed of exceedingly low impedance at the center carrier frequency and exceedingly high impedance at audio frequencies. This voltage is impressed across the voltage dividing network comprisin the resistors 98 and I00 through the carrier frequency decoupling resistor 94 and the audio frequency coupling condenser 91. The portion of this voltage which appears between the wiper 99 and ground is impressed across the input circuit of the audio frequency amplifier 28 in an obvious manner. The audio frequency signal voltage as impressed across the input side of the audio frequency amplifier 28 is amplified in this amplifier and transmitted to the loud speaker 29 for reproduction.

Referring now more particularly to the operation of the discriminator 21, it will be noted that this circuit is essentially a fou terminal bridge circuit two arms of which respectively include the condensers 87b and 810 of equal capacitances. A third arm of the bridge comprises the capacitive impedance of the diode 88. l The fourth arm of the bridge comprises the combined capacitive impedance of the diode 89 and the condenser 92. The inductance element 81a is bridged between two terminals of the bridge circuit and the frequency modulated. signal voltage is applied to the circuit across the other two terminals thereof. Since the load resistors 99 and 9| have impedances far in excess of the capacitive impedances of the diode legs of the bridge circuit at the frequencies involved, they may be neglected in analyzing the circuit. Again, the capacitance of the condenser 93 is so much greater than that of either diode leg-Of the circuit, that this condenser may also be neglected in analyzing the circuit. With this bridge circuit arrangement the voltage appearing at the output side of the discriminator is the difference between the absolute values of the voltages to ground at the upper and lower terminals of the inductance element 81a. From an examination of the bridge, it will be understood that if the capacitance of the condenser 81b 13 equals that of the condenser 810, which it does, and the capacitances of the two diode legs of the circuit are equal, such that the bridge is balanced, the currents respectively traversing the condensers 81b and 810 are equal so that equal voltage dropsappear across these condensers. Accordingly, no difference between the voltages to ground is developed at the upper and lower terminals of the inductance element 81a, regard-' less of the frequency of the exciting voltage ap-- plied to the circuit. In the actual circuit, however, the capacitance of the leg which includes the diode '89 is greater than the capacitance of the leg including thediode-88 by an amount equal to the capacitance value of the condenser 92, such that the bridge is unbalanced. Accordingly during excitation of the circuit, the current traversing the condenser 81c exceeds the current traversing the condenser 81b so that a current is caused to flow through the inductance element The magnitude of this current obviously depends uponthe reactive impedance of the inductance element 81a at the particular frequency of excitation, and the direction of current flow is such that the voltage drop across the condenser 81b is enhanced and that across the condenser 810 is decreased. It will be understood, therefore, that by suitably proportioning the impedance of the inductance element 81a relative to the reactive impedance of the condensers 81b and 8T0. at a particular center frequency to establish a given relationship between the currents traversingfthe circuit elements 81a, 81b and 8-10, the absolute voltages between the upper and lower terminals of the inductance element'illa and, ground become equal. In their relationship to each other,,however, these voltages are out of phase so that a difference voltage actually exists between the upper and lower terminals of the I circuit 81. Ihis difference voltage is, of course, equal to the vector sum of the absolute voltages from the upper and lower terminals of the inductance element 870. to ground. The particular frequency at which these absolute voltages become equal to balancethe bridge represents the center frequency at which the 'voltage appearing at the output side of the discriminator between the cathode'of the diode .88 and ground becomes zero. In'this regard it is pointed out that when the bridge is balanced so that the voltages from the upper and lower terminals of the inductance element 26a to ground are equal, equal direct voltages are produced across the load resistors 99 and '91. These voltages'are opposingly'combined in a direct current path through the inductance element 81 so that when equal, no direct voltage appears between the cathode of the diode 88 and ground.

As the exciting voltage forthe resonant circuit 81 is increased above the center frequency, due to the signal 'modulation'thereof at an audio rate, the reactive impedances of the circuit constants change to alter the relative magnitudes of the currents traversing the circuit. elements 81a, Bl'b' and. 810, so that the voltage from the upper terminal of the inductance element 81a to ground exceeds that between the lower terminal of the inductance element fll a'and ground." Accordingly, a voltage which is positive with respect to ground is produced between the cathode of the diode 88 and ground. If, on the other hand, the exciting frequency for the circuit 81 is decreased below the center frequency, the reactive impedances'of the circuit constants change 'to alterthe 14 relativemagnitudes of the currents traversing the circuit-elements 8.la, 8-lb and 810 so that the voltageb'etween the lower terminal of the inductance element 87a and ground exceeds that between the upper terminal of the inductance element 81a and ground. As a result, an output voltage which negative with respect to ground is produced between the cathode of the diode 88 and ground. It has been found that the extent or magnitude of the discriminator output voltage varies in accordance with the departure of the exciting frequency from the center intermediate frequency to which the discriminator network 21 is center tuned. It will be understood, therefore,

that if'the. frequency of the carrierappearing at the output side of the limiter 26 is frequency modulated in accordance with a given audio sig-v nal, a corresponding audio frequency .voltageis accurately reproduced across the condenser -93 at the output side. of the discriminator 2.1.

To consider somewhat more fully the action of the condenser 92in stabilizing the operation of the discriminator network 21, it ma be pointed out that if the impedances of the four legs of the bridge circuit are perfectly balanced, changes in the exciting frequency will not produce the desired differences of potential between the upper and lower terminals of the inductance element 81a and ground. By providing the condenser 92 connected in the manner illustrated, however, thereby to insure that the over-all capacitance between the lower terminal of the inductance element. Bid and ground exceeds that between the upper terminal of this element and ground, the desired circulating current within the resonant circuit 81 will always be produced to insure stability of circuit operation. In this regard it is pointed out that the unbalancing or stabilizing condenser .92 may be connected either between the lower terminal of the resonant circuit '81 and ground or between the upper terminal of this circuit and ground. In either case, the desired operation of the network is produced. It is noted, however, that when a condenser 92 of appropriate capacitance value is connected between the upper terminal of the circuit 81 and ground, the direction of circulating current flow within the circuit is reversed. Accordingly, the polarity of the output voltage produced across the condenser 93 incident to a given departure of the exciting frequency from the center intermediate frequency is the reverse of that which is obtained for the same frequency departure when the condenser 92 is connected between the lowerterminal of the resonant circuit and ground.

If desired, one rectifying section of the improved discriminator 21. may be combined with the audio frequency amplifier 28 in the manner illustrated in Fig. 5 of the drawings, wherein reference characters corresponding to those ,used in Fig. 2 identify the same circuit elements. From an examination of the Fig. 5 arrangement, it will be seen that the diode section of the tube 89 is utilized as one of the rectifying paths of the discriminator, and that the cathode, anode and three grids of the tube are used to amplify the audio'frequency voltage which is developed.'be-' mitted to the loud speaker 29 for reproduction through a coupling transformer I39. The manner in which the audio section of the tube is blocked under the control of the muting oscillator '33 and mode of operation of the discriminator 21 are exactly the same as explained herein with 15 reference to the system shown in Figs. 1 and 2. In fact, the circuit of Fig. may be directly substituted for the discriminator 21 and the audio frequency amplifier 28 in the system of Figs. 1 and 2 to perform in the same manner, when the indicated connections are made between this circuit and the limiter 28, the high pass filter 38,

the transmitter mixer [8, the muting oscillator and rectifier 33, and the modulator stage I3.

Automatic frequency control As previously indicated, provisions including the discriminator 21 and the modulator stage [3 of the transmission channel ID, are made in accordance with the present invention for automatically adjusting the output frequency of the frequency doubler [5 so that the difference between this frequency and the center frequency of a selected carrier is held at a substantialy constant value which substantially equals the center intermediate frequency to which the resonant circuits of the first intermediate frequency section of the receiving chanel II are tuned. The purpose of this arrangement is to correct for any drift in the output frequency of the oscillator M or in the center frequency of the received signal carrier. In this regard, it is noted that regardless of the settings of the tuning elements provided in the tunable stages [8 and 28 of the receiving channel I I, these stages are broadly tuned to the center carrier frequency which corresponds to the settings of the tuning elements, so that irrespective of any drift in the center frequency of the received carrier all modulation components of the received signal are passed through these stages of the receiving channel. In a similar manner, the fixed tuned stages of the first and second intermediate frequency sections of the channel I I are somewhat broadly tuned in order to-permit, within limits, deviations in the center carrier frequencies appearing therein without cutting off the modulation components of the frequency modulated carriers which are transmitted therethrough. It will be understood, therefore, that by providing the improved automatic frequency control arrangement described below, any drift in the output frequency of the oscillator M or in the center frequency of a received signal carrier is substantially corrected in so far as the intermediate frequency sections of the receiving channel and the discriminator 21 are concerned.

Briefly to consider the manner in which the output frequency of the oscillator I4 is automatically controlled, it may be assumed that the center frequency of the received signal carrier starts to drift to a value higher than the center frequency to which the resonant circuits of the tunable stages 18 and 28 are tuned, or that while this center carrier frequency remains constant, the output frequency of the oscillator I 4 starts to drift from an established value to a lower value. As the frequency drift starts and regardless of where it originates, the center frequency of the carrier transmitted through the first intermediate frequency amplifier 22 increases to produce a corresponding decrease in the center frequency of the carrier transmited through the second intermediate frequency stages 24, 25 and 26. -As will be apparent from the above explanation, this departure in the exciting frequency of the tuned circuit 8'! from the center frequency to which this circuit is tuned, causes a bias voltage, which is negative with respect to ground, to be produced between the cathode of the diode 88 and ground. This bias voltage is negatively applied to the control grid of the modulator tube 38 over a path which includes the radio frequency decoupling resistor 94, the audio frequency decoupling resistor and the filter resistors 13! and 31. At this point it is noted that the audio frequency filter comprising the decoupling resistor 95 and the by-pass condenser 96 prevents the'audio frequency components of the voltage appearing at the output side of the discriminator 21 from being impressed between the input electrodes of the modulator tube 38. This filter also prevents audio frequency voltages developed during signal transmission by the microphone l2 from being impressed upon the input side of the audio frequency amplifier 28 through the coupling condenser 91.

As the bias applied to the tube 38 increases, the magnitude of the out of phase current component traversing the space current path of the modulator tube 38 and the tuned circuit 4| changes so that the frequency of the voltage developed by the master oscillator 14 increases. As this frequency increases, that appearing at the output side of the doubler l5 obviously changes in like manner to decrease the center frequency of the signal carrier traversing the first intermediate frequency section and increase the center frequency of the signal carrier traversing the scond intermediate section of the receiving channel II. The resulting increase in the center excitation frequency of the resonant circuit 81 produces a correspondin decrease in the rate of increase of the negative bias voltage applied between the control grid and cathode of the modulator tube 38. As the bias applied to the tube 38 continues to increase at a constantly decreasing rate the center frequencies of the signal carriers traversing the first and second intermediate frequency sections of the receiving channel ll change in like manner until the two factors become balanced. It will be understood, therefore, that when the received carrier frequency is stabilized at a particular above-normal value, the bias voltage applied to the modulator tube 38 is likewise stabilized at a particular value. Moreover, if the circuit constants of the system are properly chosen, this bias voltage will in each instance be stabilized at a value such that the center frequencies of the signal carrier traversing the first and second intermediate sections of the receiving channel II will be held at values which closely approximate the center frequencies at which these sections of the receiving channel and the discriminator 21 are designed to operate.

If the center frequency of a selected signal carrier drifts to a value below the center frequency to which the stages l8 and 20 are tuned, or the output frequency of the oscillator Id drifts from its established value to a higher value, the center frequencies of the signal carriers traversing the first and second intermediate frequency sections of the receiving channel I I are decreased and increased respectively. As a result, a positive bias voltage appears at the output side of the discriminator 21 which is applied to the tube 38 to produce a decrease in the output frequency of the oscillator l4. The center frequencies of the signal carriers traversing the intermediate frequency sections of the receiving channel are increased and decreased accordingly. Thus, the frequency correcting action proceeds in the exact manner explained above until a point of stability is reached at which the center frequency of the signal carrier voltage mpressed upon the discriminator network 81 closely approximates the 17 center frequency to which the resonant circuit 81 is tuned.

Operation 01, the muting apparatus Referring now more particularly to the manner in which the audio section of the receiving channel I I is muted or squelched during periods when the system is conditioned for operation but is not being used either for signal transmission or reception, it may be pointed out that at all times when the system is conditioned for reception but is not receiving a desired signal, noise signal voltages appear in those stages of the receiver channel which precede the discriminator 21. These voltages are transmitted through the interme: diate frequency and mixer stages of the channel II and are detected by the discriminator 21 to appear as audio frequency voltages at the output side of the discriminator. They may be produced as a result of thermal agitation within the tubes provided in the receiving channel, shot effects, extraneous noise voltages appearing across the. antenna-ground circuit I9, or by physical shockto the circuit elements provided in the receiving channel. Regardless of the originthereo f, however, the noise signals are manifested as audio frequency voltages across the output side of the discriminator which, in the absence of the mutin apparatus provided in the system, would be passed through the audio frequency amplifier -23 to the d speak 291 p duction. 1

More specifically considered, the noise response of the receiver is graphically illustrated in Fig. 4 of the drawings, wherein the noise voltage appearing across the condenser 93 is plotted as a function of the selected signal carrier input voltage appearing across the antenna-ground circuit L9. From a consideration of this curve, it will be noted that when no signal carrier is being received, the noise voltage appearing at the output side of the discriminator 21 is high and that the magnitude of this voltage is sharply reduced in response to the application of a selected signal carrie to the antenna-ground circuit I9. The decrease in the level ofthe noise voltage which accompanies the transmission of a selected signal through the receiving channel I l, is largely effected in the amplitude limiters 25 and 26 To consider the action of the muting apparatus, it-is pointed out that the noise voltage appearing between the cathode of the diode 8,8 and ground at the output side of the discriminator 2! is impressed upon the input side of the high pass filter .39. This filter acts to pass only those components of the noise voltage having frequencies above the normal signal reproducing band of the receiver. For example, this filter may be designed to pass frequencies above 2;) kilocycles. The noise voltage appearing across the output side of the filter 30 is impressed between the input electrodes of the noise amplifier and rectifier tube I06 and appears in amplified form across the coupling condenser I08 and the diode section of the tube I06 in series. Due to the rectifying action of the diode section of the tube I06, 2. direct voltage is produced across the load resistor III] which varies inv magnitude in accordance with the magnitude of the noise voltage impressed between the input electrodes'of the tube I06, This direct voltage i. e. that across theresistor no, is negatively applied to the control grid of the direct voltage amplifier tube ,I-I;3 through the resistor H2 in opposition to the fixed bias voltage normally positively applied to the control 18 grid of the tube II3 through the resistor H5. The negative voltage appearing across the resistorfl In so greatly predominates over that positively applied to the control grid of the tube I I3 that this tube is biased beyond its space current cutoff point. Accordingly, the voltage drops across the two resistors I18 and H9 are sharply decreased to very low values, with the result that the fullvoltage of theavailable source of anode current is positively applied to the screen electrode of the oscillator and rectifier tube 33. The application of this voltage to the screen electrode of the tube I'Zil initiates the operation of the I oscillator section of this tube, so that an oscillatory voltage is developed across the series connected coupling condenser I29 and the space current path between the diode electrodes of the tube. Due to the action of the diode section of the tube I20 in rectifying the oscillatory voltage, a direct bias voltage is produced across the diode load circuit comprising the series resistors I23, I32 and 181). This bias voltage is negatively applied to the control grid of the first tube provided in the audio frequency amplifier 2.8, over a path which includesithe resistors I21 and IE0 and the encircuited portion of the resistor 98. The magnitude thereof is sufficient to bias the first audio frequency amplifier tube beyond cutoif, whereby theno-ise signals are prevented from being transmitted through the audio channel of the receiver to the loud speaker 29 for reproduction.

As will be apparent from further consideration of the curve shown in Fig. .4 of the drawings, when a selected signal carrier of substantial magnitude appears across the antenna ground circuit I9, the limiters 25 and 26 function sharply to decrease the noise voltage developed at the output side of the discriminator 21. This produces a corresponding decrease in the bias voltage developed across the load resistor III When the negative bias applied to the control grid of the tube I I3 is thus reduced to a low value, the current fiow through the resistors H8 and lie the space current path of the tube H3 is sharply increased to produce a corresponding increase in the voltage drops across the two identified resistors. As .a result, the voltage which is positively applied to the screen electrode of the oscillator and rectifier tube 3t through the two resistors M8 and H9 is sharply decreased to a value such that operation of the oscillator section ofthi tube cannot continue. When the production of an oscillatory voltage across the space current path of the tube I23 is thus arrested,the negative bias voltage across the rectifier load circuit resistors 128,132 and 18b is reduced to zero, permitting the normal negative bi voltage s developed across the resistor 18b to be impressed upon the control electrode of the first tube provided in the audio frequency amplifier 28. When this amplifier tube is thusunblocked or biased to a normal value, the audio sectionof the receiving channel is rendered opera ve to ampl y the u o re e y om onents of the received signal and to transmit the same to the loud speaker .2 f r e o uct on,-

From the fore oin x anation i wi b 1. 1 derstood that normally, i. e. when the system is conditioned for signal reception, the noise signals appearing in the receiving channel II are utilized to completely block the audio section of the receiving channel against the transmission or noise signals to the loud speaker 29. More pecifically, 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 528 is approximately volts. To this end, from to 50 volts must be positively applied to the screen electrode of the oscillator and rectifier tube 33 when a tube of the commercial 1S5 type is employed in the oscillator and rectifier stage 33. Further, the component circuit elements of the muting apparatus should be such that when a selected frequency modulated carrier is received having a magnitude exceeding a predetermined low value, the voltage positively applied to the screen electrode of the tube 26 is dropped to approximately 20 volts such that operation of the oscillator section of the tube I23 is arrested. In the absence of an oscillatory voltage between the anode and cathode of this tube, the only negative bias voltage applied to the control grid of the first tube in the audio frequency amplifier 28 is that developed across the grid leak resistor 1819, which voltage is of the order of one volt.

When the apparatus is designed to have the characteristics just described, the audio channel of the receiver will at all times remain blocked during periods when a selected signal is not being received and will be automatically unblocked when a selected signal is transmitted through the receiving channel of the system to the discriminator 21 for detection. In this regard it will be understood that since the high pass filter 30 will only pass frequencies outside of the normal signal reproducing frequency band of the receiving channel, the muting apparatus is not responsive to the audio frequency components of a received signal carrier and thus this apparatus is prevented from blocking the audio section of the receiving channel against the transmission of detected signal voltages to the loud speaker 29.

Blocking the receiving channel during transmission In considering the manner in which the receiving channel II is blocked against reproduction of the signal components of the modulated carrier radiated during operation of the transmission channel II, it is pointed out that the equipment is deliberately designed and is physically so arranged that a relatively large amount of stray capacitance coupling exists between the circuit elements provided in the input and output circuits of the first mixer 2|, and the electrodes of th crystal 5S and the circuit conductors connecting these electrodes with the input electrodes of the tube 63 and the terminals of the tuned circuit 52. More specifically, the electrodes of the crystal 58, the elements of the tuned circuit 60 and the circuit elements forming the input circuit for the mixer tube ID are unshielded; and the control grid of the tube I0 is spaced approximately one inch from th circuit conductor which connects the control grid of the tube 63, the lower electrode of the condenser 59 and the lower electrode of the crystal 58. With this arrangement and during signal transmission, when the crystal 58 and the tuned circuit 60 are oscillating, a strong unmodulated carrier voltage appears at the output side of the first mixer 2I, having a frequency equal to the center frequency to which the resonant circuits of the first intermediate frequency section of the receiving channel are tuned. This strong carrier voltage as transmitted through the first intermediate frequency amplifier 22, the second mixer 23, the second intermediate frequency amplifier 24, and the two limiter stages 25 and 26, to the input side of the discriminator 21, effectively blocks the enumerated stages of the receiver against the transmission of the signal modulated carrier which is impressed upon the input side of the tunable radio amplifier 20 through the condenser 61. More particularly, the blocking carrier voltage which appears across the output side of the first'mixer 2I in the receiver, a a result of the stray capacitance coupling between the circuit elements of this mixer and the circuit elements associated with the crystal 58, exceeds by several times the modulated signal carrier which appears at the input side of the mixer 2I due to the coupling between the transmitting and receiving channels through the condenser 61. Since the carrier voltage as derived from the crystal 58 so greatly predominates over that transmitted through the tunable radio frequency amplifier 20 to the input side of the mixer 2|, those stages of the receiver which follow the mixer 2I are effectively blocked against the transmission of the signal modulated carrier to the discriminator 21. Thus, the loud speaker 29 is prevented from reproducing the audio frequency voltage developed through operation of the microphone I2 when the system is conditioned for transmission.

The crystal 58 also acts in conjunction with the fixed tuned stages of the receiver to set the radiated carrier center frequency so that this frequency cannot be changed by the discriminator 21 and is maintained at substantially the exact desired value. Thus, due to the capacitance coupling between the circuit elements of the first mixer 2i and the circuit elements of the fixed tuned crystal 58 and resonant circuit 60, a carrier having a frequency exactly equaling the first intermediate frequency is injected into the first intermediate frequency section of the receiving channel' This carrier is mixed with the frequency produced by the oscillator section of the second mixer tube I6 to produce a carrier in the second intermediate frequency section of th receiving channel which exactly equals the center frequency to which the resonant circuit 8! is tuned. When this carrier voltag is applied to the discriminator 27, the bias voltage appearing at the output side of the discriminator between the lower terminal of the resistor and ground is reduced to a negligible or zero value. Moreover, since the crystal 58 and the crystal I? which controls the oscillator section of the second mixer tube 76 are invariably fixed to oscillate at set frequencies, the negligible bias voltage appearing at the output side of the discriminator 21 cannot be changed or altered in the slightest degree during transmission. Accordingly, the bias applied to the input electrodes, of the modulator tube 38 through the resistors I3I and 31 is held at a fixed negligibl value during transmission with the result that the modulator oscillator I3, I4 are infiexibly set to produce a signal modulated carrier voltage having a fixed center frequency. Thus, the crystal 58, acting in conjunction with the stages 2|, 22, 23, 24, 25, 2'5 and 21 of the receiving channel II, functions to stabilize the center frequency of the radiated signal carrier at the definite and fixed value desired. This center frequency value can, however, be altered by adjustment of the adjusting element U to alter the settings of the tuning elements-provided in the tunable stages I4, I5, I6, I! and I8 of the transmission channel, but-once the desired value is established it is maintained by th crystal 58 in the manner just explained. a

The strong carrier injected by the'crystal 58 and its associated circuit elements into the first intermediate frequency channel of: the receiving channel ll through the capacitance couplingbe tween the circuit elements associated with the crystal 58v and th circuit elementsof the first mixer 2|,also serves to control the muting apparatus so that the audio section 28 of the receiving channel I l is unblocked or rendered active during signal transmission. More specifically, this carrier has the same effect, in so far as the reduction of noise voltages at the output side of the discriminator 21 is concerned,- as does the application of a strong signal of a selected center carrier frequency to the antennaground circuit I9. Accordingly, and as will be apparent by reconsidering the curve shown in Fig. 4 of the drawings, when the strong carrier is injected'into the first intermediate frequency section of the receiving channel, the noise voltage developed between the cathode of the tube 88 and ground at the output side of the discriminator 21 drops to a negligible value. As a result, the oscillator section of the tube I23 stops oscillating, for reasons explained above, and the negative blocking bias isremovedfrom the control grid of the'first tube in the audio frequency amplifier 28. Thus, this amplifier is rendered operative, and may, if desired, be used to amplify and transmit to the loud speaker 29 for reproduction, any side tone voltage suitably derived from the audio channel of the transmission channel I;

From the foregoing explanation it will be apparent that an improved system is provided in which all available tubes and circuit elements are utilized with maximum effectiveness both during reception and transmission. Thus, the master oscillator I4 is not only used as a carrier producing oscillator during signal transmission, but is also coupled to the first mixer stage 2 l to operate as a local oscillator during reception. Again the modulator stage I3 is not only used'ito reac tively modulate the carrier output of the oscillator 54 during transmission but, in addition, functions to provide automatic frequency control in the intermediate frequency sections of the receiving channel during signal reception. The use of the transmitter stages l3, !4 and [5 in the dual capacities mentioned may be directly attributed .to the provision of the mixer stage V5 in the transmission channel in. Thus by providing at this stage a mixing oscillator having an output frequency equaling the center frequency of the first intermediate frequency. section of the receiving channel, it becomes practical to utilize the three stages 13, 14' and [5 of the system in the dual capacitie indicated. This is true for the reason that by providing the mixing oscillator at the mixer stage I6, transmission and reception may be effected at the same center carrier frequency without in any way altering the tuning of any of the tunabl stages of the system as it is selectively conditioned for signal transmission or reception. Moreover, by providing the described novel system arrangement, only the transmission mixer l6 and the power amplifier II are required to be controlled in order selectively to condition the system for signal transmission or reception.

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: 7

1. In a frequency modulated radio transmitting and. receivingsystem which-includes means for selectively conditioning the system for signal transmission orreception, a receiving channel including a mixer stage followed by an intermediate frequency section, a transmission channel-including a carrier producing oscillator 'coupled to said mixer stage to operate as a local oscillator during signal reception, and inversely acting means coupled between said intermediate frequency channel and said oscillator for automatically controlling the output frequency ofsaid oscillator to hold substantially constant the center frequency of a frequency modulated carrier transmitted through the intermediate frequency section of said receiving channel during signal reception.

2. In a combined radio transmitting and receiving system which includes means for selectively conditioning the system for signal trans-' mission or reception, a receiving channel including a mixer stage followed by an intermediate frequency section, a transmission channel including a carrier producing oscillator coupled to said mixer stage to operate as a'local oscillator during signal reception, and inversely acting means coupled between said intermediate frequency channel and said oscillator for automatically controlling the output frequencyof said oscillator to hold substantially constant the carrier frequency of a signal modulated carrier transmitted through the intermediate frequency section of saidreceiving channel during signal reception.

3. In a frequency modulated radio transmitting and receiving system which includes means for selectively conditioning the system for signal transmission or reception, a receiving channel including a mixer stage followedby an intermediate frequency section and a frequency discriminator in the order named, a transmission'channel including a modulator and an oscillator controlled by said modulator to roduce a frequency modulated carrier during signal transmission, means coupling said oscillator to said mixer stage to operate as a local oscillator during signal reception, and means coupling the output side of said discriminator to said modulator'to govern the output frequency of said oscilator so that-the center frequency of a frequency modulated carrier transmitted through the intermediate frequency section of said receiving channel during signal reception is held substantially constant. e

4. In a frequency modulated radio transmitting and receiving system which includes means for selectively conditioning the system for'signal transmission or reception; a receivingchannel including a mixer stage followed by an intermediate frequency section and a frequency discriminator in the order named, a transmission channel including an oscillator coupled to said mixer stage to operate as a local oscillator during signal reception, and means controlled by said discriminator for automatically controlling the output frequency of said oscillator to hold substantially constant the center frequency of a frequency modulated carrier transmitted through the in- 7 termediate frequency section of said receiving channel during signal reception.

5. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, transmission and. receiving channels respectively including high frequency carrier and intermediate frequency carrier sections, two like sections of said channels including frequency selective stages which are tuned to the same resonant frequencies, frequency converters respectively provided in said channels at the junctions between the high and intermediate frequency sections thereof, means coupling the intermediate frequency section of said transmission channel to the frequency converter of said receiving channel so that a frequency beating carrier is supplied to said last-named frequency converter from said transmission channel during signal reception, and means for rendering the high frequency section of said transmission channel inactive when said system is conditioned for signal reception.

6. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a transmission channel provided with an audio frequency section, a tunable intermediate frequency section and a tunable radio frequency section in the order named, a receiving channel provided with a tunable radio frequency section, an intermediate frequency section and an audio frequency section in the order named, a frequency converter provided between the radio and intermediate frequency sections of said receiving channel, tuning elements respectively provided in said tunable stages, and unicontrol means for controlling said tuning elements so that regardless of the settings of said tuning elements the center resonant frequencies of said radio frequency sections are substantially equal and the center resonant frequency of at least one stage of the intermediate frequency section of said transmission channel is equal to the difference between the center resonant frequencies of the radio and intermediate frequency sections of said receiving channel, and a coupling path for injecting a carrier voltage into said converter from said one stage of said transmission channel during signal reception.

. '7. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception, a transmission channel provided with an audio frequency section, a tunable intermediate frequency section and a tunable radio frequency section in the order named, a receiving channel provided with a tunable radio frequency section, an intermediate frequency section and an audio frequency section in the order named, a frequency converter provided between the radio and intermediate frequency sections of said receiving channel, tuning elements respectively provided in said tunable stages, and unicontrol means for controlling said tuning elements so that regardless of the settings of said tuning elements the center resonant frequencies of said radio frequency sections are substantially equal and the center resonant frequency of at least one stage of the intermediate frequency section of said transmission channel is equal to the difference between the center resonant frequencies of the radio and intermediate frequency sections of said receiving channel, a coupling path for injecting a carrier voltage into said converter from said one stage of said transmission channel during signal reception, and means for rendering the radio frequency section of said transmission channel inactive when said system is conditioned for ignal reception.

8. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmissionor signal reception; a transmission channel provided with a modulator, a carrier producing oscilator, a tunable frequency doubler, a tunable mixer stage and a tunable radio frequency section connected in tandem in the order named; a receiving channel provided with a tunable radio frequency sec tion, a tunable mixer stage, an intermediate frequency section and a frequency discriminator connected in tandem in the order named; means for utilizing the carrier voltage developed at the output side of said doubler as a local oscillator voltage for frequency conversion at the mixer stage of said receiving channel during signal reception, tuning elements respectively provided in said tunable stages, and unicontrol means for so controlling said tuning elements that regardless of the settings of said tuning elements the center resonant frequencies of said radio frequency sections are substantially equal.

9. In a combined radio transmitting and receiving system which is adapted to be selectively conditioned for signal transmission or signal reception; a transmission channel provided with a. modulator, a carrier producing oscillator, a tunable frequency doubler, a tunable mixer stage and a tunable radio frequency section connected in tandem in the order named; a receiving channel provided with a tunable radio frequency section, a tunable mixer stage, an intermediate frequency section and a frequency discriminator connected in tandem in the order named; means for utilizing the carrier voltage developed at the output side of said doubler as a local oscillator voltage for frequency conversion at the mixer stage of said receiving channel during signal reception, tuning elements respectively provided in said tunable stages, unicontrol means for so controlling said tuning elements that regardless of the settings of said tuning elements the center resonant frequencies of said radio frequency sections are substantially equal, and means coupling the output side of said discriminator to said modulator to govern the output frequency of said oscillator so that the carrier frequency of a signal modulated carrier transmitted through the intermediate frequency section of said receiving channel is held substantially constant.

HENRY MAGNUSKI.

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