US2064106A

US2064106A - Receiver

Info

Publication number: US2064106A
Application number: US704122A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1933-12-27
Filing date: 1933-12-27
Publication date: 1936-12-15
Anticipated expiration: 1953-12-15
Also published as: DE662107C; GB448448A

Description

Patented Dec. is 1936.

RECEIVER Murray G. Crosby, River-head, N. Y., minor to.

:Radio Corporation of America, a corporation of Delaware Application December 2'1, 193:, Serial No. 704,122

This invention discloses a receiver for receiving a phase modulated wave or a. frequency modulatedwave or an amplitude modulated wave. In this receiver frequency division of the signal modulated carrier is utilized to obtain a new carrier with a reduced degree of modulation, which new carrier is synchronized with the origi- "nal modulated carrier wave. Thus, the phase {modulation of the carrier maybe converted to amplitude modulation for subsequent detection. Adjustments may also be made to make the receiver receptive to frequency modulation.

In the known phase modulation art the phase modulations on the wave are converted to an amplitude modulation by modulated wave with a locally generated and synchronized carrier in time quadrature with the signal modulated wave or with energy filtered from the phase modulated wave carrier or by receiving the phase modulated wave on a frequency modulation receiver followed by an audio frequency correction circuit.

Of these three receivers the receiver of the present invention. has characteristics in common with the first.

f-Iowever the locally synchronized carrier wave type becomes more unstable as the frequency of the received wave is increased due to the difficulty of maintaining exact synchronism between the local carrier and the received wave with'the' desired space separation. The receiver of the present disclosure has the advantage over prior receivers that the modulation may be removed in various degrees, depending upon the number of frequency dividers used quency division employed. As a result, the sta bility of thereceiver with regard to synchronism tween the local carrier and received wave, and,

the depth of theresulting amplitude modulation are adjustable to suit the frequency of the received wave and otherconditions of reception or transmission.

The novel features of my invention have been pointed out with particularity in the claims at. the end of the specification.

The nature of my invention, circuit arrangement for carrying out the same, .and the mode of oreration of said circuits following'detailed description thereof and therefrom when read in connection with the attached drawings in which:- Figures 1 and-4 are wiring diagrams of the essential components of a receiver arranged in accordance with the present invention;

. Figures 2 and 3are vector diagrams illustrating including the essential combining the phase and the order of fre- Unit 6 contains an -to vary the phase wave;

will be clear from the 22 Claims.- '(01. 250-20) sult desired; while Figures and'6illustrat details of portions of the circuits of Figures 1" and 4.-

Figure 1 shows a circuit diagram of a receiver features of Jny invention; In this figure certain of the elements which are well known to the art are represented by'rectangles for simplicity. The signal picked up by the antenna is amplified by radio frequency am- 10 plifler I and passed to unit 2. Unit '2 contains the first detector and is coupled to the high frequency oscillator 0 of the'superheterodyne receiver. The frequencyof the high frequency oscillations generated by the local oscillator in 2 is partly dependent upon thevoltage received from line 22. In this manner. automatic frequency control is applied to the heterodyne reoeiver to maintain the same properly tuned.

I This automatic tuning of the localosciilator may 2 .be accomplished as disclosed in my U. S. Application Serial No. 616,803, filed June 13, 1932.

After being converted to intermediate frequency in unit' 2 the signal is passed through intermediate frequency amplifier 3 to amplitude limiter orv automatic volume control device land may also-be passed by way of switch 81 in the 'upper position to the" automatic frequency control detector input transformer-i0 and by way of switch S in the left hand position to the audio detector input transformer ii. The amplitude of the signal is maintained constant in unit 4- and the signal of constant amplitude is "fed to the multi-vibrator in unit 5. The multi-vibrator' is. adjusted to normally oscillate at a frequencyl/n times the intermediate frequency where n is the order of. frequency division." Unit 5 also contains an output filter to remove all frequencies except the desired divided frequency intermediate frequency oscillator and detector which heterodynes-the divided frequency back to the original intermediate frequency, 1. e. to the frequency ofthe signal in unit 3. Unit 1 contains a phase adjuster con-Q 'sisting of an artificial line or any other means The output of iinit 'l is fed differentially by transformer ilfto the control grids of the automatlcfrequency control-dc; tectors 8 and 9 and,'via phase shifter ll, to

transformer ii and thence in phase opposition to the control gridsof the audio detectors [1 and ll. Tubes 8 and 8 are differential detectors with resistors l2 and I! in their plate circuits. The differential potentials appearing at the terminals of i2 and "are used as described in. detail in my U. Sfapplication Serial No. 616,803, filed June 13, 1932 to control the frequency of the oscillations produced in 2. Tubes l1 and I8, transformer 20, switch l9, and phones 2| comprise the audio detectors for converting the modulated wave to its signal wave.

, The operation of this receiver has for its novelty the fact that frequency division is used to remove or decrease the degree or amount of frequency or phase modulation. Thus, the ,intermediate frequency which is phase modulated is arranged to hold a multi-vibrator in step to obtain a frequency division. The divided frequency is then heterodyned back to the intermediate frequency. In the frequency di-' vision the degree of phase modulation is divided by the order of frequency division, but when the divided frequency is brought back to the intermediate frequency by heterodyning there is no change in degree of phase modulation.

Assuming a specific case of an intermediate frequency of 100 kilocycles, the intermediate frequency energy; maintained constant by unit 4 may be fed to the multi-vibrator in unit 5 which of kilocycles for the case of a division order of 10. In this case the multi-vibr'ator is held in step" at {3th the intermediate frequency.-

Thus, a frequency shift from 100 ,to 101 kilocycles changes the divided frequency from 10 to 10.1 kilocycles. Consequently, a frequency modulation having a deviation frequency of 1 kilocycle will have its deviation reduced to .1 kilocycle or 100 cycles.

In the same manner phase modulation may have its phase deviation or degree of phase modulation reduced. This is apparent from a consideration of the fact that phase modulation is equivalent to a frequency 'modulation'with its modulation ratio jd/jm (fd=deviation frequency; fm=modulation frequency) equal to the phase deviation 41. If in the frequency division of frequency modulation fd.is multiplied by 1/n, the modulation ratiois also multiplied by l/n so that the divided modulation ratio would be equal to l/n gfd/jmll Since a phase modulated wave is equal to a frequency modulated wave with o substituted for (fd/fm) the divided phase modulation deviation would be equal to l/n By this type of frequency division the depth of The divided frequency is then returned to the intermediate frequency by means .of a heterodyne so that the wave with its phase modulation reduced can be combined in time quadrapiitude modulation. The combination is made in -thediiferential detector input transformers l6 and IS in the same manner as the energies in the filtered carrier type of receiver are combined; See Crosby JJ. S. application 558,309, filed January 23,. 1932 and U. Snapplication 616,803 filed June 13, 1932. Thus, in Figure 2, U1 and U2 represent the undivided intermediate detectors l'land H! by way of winding |6. D1 and-D: represent the divided (and heterodyned) intermediate frequency voltages fed by way of winding I! to detectors'l] and IS. The resultants, for this assumed instant of zero modulationfare represented by vectors R1 and R2, .which are equal in amplitude. As the phase shifts'with modulation the amount o, the volta'ges areas shown iniFigure 3. The undivided a I D is normally adjusted to oscillate at a frequency ture with the undivided intermediate frequency wave to convert the phase modulation to aim-- voltages designated by vectors U1 and U2 have shifted the amount inphase and the divided voltages indicated by vectors;D1 and D2, the amount /n, where n is the order of frequency division. The resultants have changed to R1 and R2 in a manner producing a differential modulation, that is, a positive amplitude modulation on one detector grid as for example I! and a negative modulation on the. grid of i8. The detector outputs are combined in the proper phase in-transformer by'means of reversing switch I9.

The automatic frequency control detectors 8 1 reactance of a frequency determining element there in the same manner as shown in the automatic frequency control circuits of my United States application No. 616,803, filed June 13, 1932. For example, the reactance L may be coupled to a reactance L in the oscillator cir-' cuit. The reactance L' may determine in' part the frequency of the oscillations produced in and by the oscillator O. In. other words, the.

potential variations produced in l2 and 13 due to a shift in the intermediate frequency may be used to control the value of a reactance in a frequency determining circuit of 'the oscillator of 2 to correct for said shift. Condenser Ci-and resistors l2 and I3 form a time constant circuit to prevent the signal wave'from appearing in line 22. This prevents the automatic frequency control from tending tov remove the modulation. If it is desired to receive frequency modulation condenserGr may be removed to allowthe detected signal wave appearing across resistors I 2 and 13 to change the value of the frequency determining element in said oscillator suflicient to modulate the high frequency oscillator. This modulation demodulates the signal in such a manner as to convert the frequency modulation to phase modulation suitable fordetection in the audio detectors. phase modulations resulting from said conversion is accomplished in the same manner normal phase modulated .waves are demodulated as described above.

Figure 4 shows an alternative arrangement wherein detection takes place at the divided frequency. Thus, instead of heterodyning the divided frequency back to the intermediate -frequency for combination with the undivided wave as in Figure 1, the undivided intermediate fre- This detection of the quency is heterodyned down in a second heterodyne arrangement in 6 to the divided frequency.

The arrangement of Figure 4 may be adapted very easily to a triple detection superheterodyne receiver wherein

units

3 and 23 are the two band pass intermediate frequency amplifiers. A receiver as described above may be used in a space diversity receiving system of the automatic phasecontrolled type as described in my United States application No. 727,931 filed May 281 1934. The apparatus in units! and 6 .of Figure 1 may be substituted for thecrystal filter or intermediate frequency oscillator of said application. 1

In the previous description, 8 multivibrator is given as the means for obtaining frequency division of a wave to'obtain corresponding energy.

be used. Thus, a vacuum tube-oscillator of the tuned circuit type might also be used to-an advantage; Likewise a mechanical frequency divider may be substituted.

A circuit for'dividing. of reducing the frequency in accordance with signals has been shown in Figure 5 of thedrawings. In this ,figure the leads marked in may be connected-to the" limiter or atuomatic volume control in unit 4 of.

Figures 1 and 4,- while the leads marked out i l5 may be connected with the intermediate frequency oscillator and detector .6 of Figure -1 or- Y with the phase adjuster 1 of Figure 4. The signal modulated intermediate frequency oscilla tions from 4 appear inthe line .marked"in and areapplied to an inductance between the control grid g of a tube T and the anode q of tube TI. The oscillations of reduced frequency appear in a circ'uit connected between the anode a of tube. '1'. Y

and the control grid 9' of tube TI and are trans ferred from said circuitto the line marked ou The anodes a and a are interconnected by resistances r and r and a common source of potential.

The'electrodes of the tubes being interconnected as shown byjvariable condensers and resistancesproduce in the-circuit marked out signal modulated oscillations of a frequency which maybe made a sub-multiple of the frequency of the os-'- cillations impressed on the circuit markedfiin. I

' The condensers connected with the control grids of the interconnected tubes charge and discharge alternately at a rate determined in part by the size of the condensers, in part by'the value of the resistances, and in part by the frequency and amplitude of the waves fed to the line marked in. Theextent to-which the frequency and degree of modulation of the impressed wave is re- 1 duced by the multi-vibrator is accordingly determined in part by the constants of the circuit which may be varied to obtain therefrom. a har- 4,=5 monic frequency. 1

Any type of phase adjusting circuit known in the .artrnay be used. For example, a circuit as yrecombining said oscillations of reduced vire-- illustrated in Figure 6 may be used. In this figure .the incoming signal modulatedcarrier wave of 50 intermediate or reduced frequency may be applied'to the leads marked input" from the in-.

termediate frequency oscillatorand detector; of

Figure l or from th frequency reducerjrof Figure.4. The phase adjuster may cOmpriseseries 55 reactance X and parallel reactances Z and may be terminated by a resistance. -W equal to the surge impedancejof the line so that traveling waves only appear in the line. Waves at the- Y thesinput transformer connected with the ireesw quency controlled detectors and with the phase- T'ad1uster H of Figures-1 and 4; The phase adjuster of unit I may be the same as the phase adjuster of unit 1.. as describedabove. I,

Although the receivers, have been-described m above asbeing primarily adapted to the reception of phase or frequency modulated waves 12 contemplate the reception of amplitude waves also e 1 n the frequencyQto which the derived energy is reduced without altering the degree of modulai on said receivers when desired. This may be acwithout increasing quency v I H without increasing theflegree of modulation} so that unlimited energy from the unit.- 3 is fed to the windings l6 and ID for signal indicating purposes in detectors l1 and 18 and frequency control purposes indetectors 8 and I0. The-acmodulated waves are received is substantially as" indicated above. The potentials on the grids of tubes l'l and is the vectoral sum of the potenq encyof the wave modulated as to phase or ire- "depend on the phase of the oscillations applied to thecontrol grids and consequently on the frequency ofthe oscillator in first. detector 2. If the frequeneyof the oscillator in I -is too high one of 15 the detectors draws more plate current than the other and a correcting potential appears across l2 and I3 and acts on the frequency determining element in 2 to correct the frequency.

' 'quency of 2v istoo low-the reverse action takes '20 place.

Of course in some cases the deteetors 8 and IO may-serve also to render the signals or the detectors l1 and Ii may furnish'the frequency ntroll ng energy as well as renderingthe signal. 25

Having thusdescribed my invention and the operation thereof, what I claim is: 1. The method of demodulating a carrier wave varied'as to phase or frequency in'accordance with si al potentials which includes the steps of, di- {0 ver ing .energy from..said phase hr frequency varied carrier wave, reducing the mean frequency of thedivertedenergy and simultaneously the degree of modulation of said energy, increasing the .meanfrequency of the energy of reduced 35 frequency to the frequency of the carrier wave the degree of modulation thereof, and combining said last namedenergy with carrier wave energy to render the signal.

2. The method of demodulating high frequency 4 oscillations which are modulated as to phase or frequency atsignal frequency which includes the steps of, reducing the frequency of the o'scilla tions and the degree ofmodulation thereof, beating downthe frequency of the oscillations to the frequency, of said reduced oscillations without altering th -degree of m' ulation thereof, and

quency and oscillationsthefrequency of which has been beat down in phase quadrature to render the signal.

' 3; The method of demodulating a carrier wave varied as to phase or frequencyin accordance with signal potentials which ineludes the steps.

'cf, reducing the frequency of. said carrier, deriving energy from said phase or frequency varied carrier wave of reduced frequency, reducing the..-

frequency-of said derived energy and simu1tane ously the degree of' modulation increasing the frequency of said last named energy to the to which said carriercombining said energies'to rendenthe signa gscillationswhichare-modulated as to phase or equencyatsignalflequency which includes the stem or. remade reducing-att c meanfr quency; of the derived energyand-the degree 0 modulation fth of: beating down the frequency of the first oseillations of reduced frequency 'to tion-of the detectors l1. and t8 when amplitude 5f If the fre-.

4. Themethcd of-demodulatinghigh mqney' 351 the mean frequency of said oscillations, derivi g-energy fromsaid oscillations of reduced frequeney,

ergies in phase quadrature to render the signal.

5. The method of demodulating a carrier wave varied as to phase in accordance with signal potentials which includes the steps of,'-diverting energy from said phase modulated carrier wave, limiting the amplitude and reducing the mean frequency of said diverted energy and simultaneously the degree of phase modulation thereof,

increasing the mean frequency of said limited and reduced energy to the frequency of the car- Y limiting the amplitude of the diverted energy, reducing the frequencyof oscillationsin said diverted and limitedenergy and simultaneously reducing the degree of phase modulation, changing j the frequency of the original phase modulated oscillations to a frequency equalto the frequency of the oscillations of reduced frequency, and

combining said oscillations to'produce indications.

7. The method of receiving phase modulated oscillatory energy which includes the steps of, Jdiverting energy from said oscillatory energy, limiting the amplitude of thediverted energy,reducing the frequency of the oscillations in said diverted and limited energy and simultaneously reducing the degree of modulation, adjusting the mean phase of the oscillations in said last named energy, changing the frequency of the original phase modulated oscillations to a frequency equal to the frequency of theoscillations of reduced frequency, and combining said. oscillations in phase quadrature to. produce indications. 8. A device for demodulating oscillations modulated as to phase or frequency in accordance with signals comprising, in combination, signal wave receiving means having an output, a frequency dividing device having an'input and-'an output,

said device being connected atitsinput with v the output of 'saidreceiving means, demodulating means having an input, andla coupling between the output of said receiving means and the input of said demodulating means and be-' J tween the output of said frequency dividing device 'and the input of said demodulating means.

9. A device for demodulatingoscillations modulated as to phase in accordance with signals comprising, in combination, signal wave receiving means having an output, an .amp'litude'limiter havingan inputand an output,-said limiter being connected at its input with the output of said I receiving means, a frequency dividing device having input and output,-said device being-connected at its input with the output of said amplitude limiter, a phase adjuster having an input and an output, said adjuster being connected at its input with the output of said frequency dividing device, demodulating means having an input and 5 a. coupling between the output of saidphase adjuster and the input of said demodulating means and between the output of, said signal receiving means and the input of' said demodulating means.

. 10. Means for demodulating phase modulated waves comprising, a heterodyne receiver including an intermediate frequency amplifier having anroutput, signal amplitude limiting means having an i put and an output, and being connected at its input with the outputof said intermediate frequency amplifier, signal frequency reducing means having an input and an output, said means being coupled at its input with the output of.

' said amplitude limiting means, a demodulator having an input, phase adjust ing means coupling 5 the output of said signal reducing means tov the input of said demodulator, and means coupling 'the output of said intermediate frequencyamplifier to the input of said demodulator.

11. Means for demodulating phase modulated l0 waves comprising, a heterodyne receiver including an intermediate'frequency amplifier having an output, signal amplitude limiting means having an input and an output, and being connected at its input with the output of said intermediate .15

frequency amplifier, signal frequency reducing means having an input and an output. said means being coupledat its input with the out- -put'='of said amplitude limiting means, a demodulator havin an input, phase adjusting means 20 coupling the output of said signal frequency reducing means to 'thei'nput of said demodulator,

and means coupling the output of said intermediate frequency amplifier .to the input of said demodulator, said phase adjusting means insur- 25 ing that the wave energies reaching the input of said demodulator are in phase quadrature. i 12. A signalling device comprising, a signal absorption circuit, a heterodyne receiver including a localoscillato'r and an intermediate frequency 30 amplifier having an output, an amplitude limit ing device having an input and an output and being connected at. its input with the output of said intermediate frequency amplifier, a frequency reducing device having-input and output, 35 said device being connected at its input with the output of said amplitude limiter, detecting means connecting the output of said frequency reducing device and the output of said intermediate frequency amplifier to said local oscll-Ao later, a signal translating device and a coupling between said signal translating device and the output of said intermediate frequency amplifier and the output of said frequency reducing device.

-13. A signalling device comprising, a signal ab- 45 I sorption' circuit, a heterodyne receiver including a local oscillator and an intermediate frequency amplifier having an output, an amplitude limitingdevice having an input and an output, said device being connected at its inputwith the outso put of said intermediate frequency amplifier, a frequency reducing device ha'ving an input and an output and being connected at its input with the'output of said amplitude limiter, detecting means connecting the output of said frequency 55 reducing deviceand the output of said intermediate frequency amplifier to said local oscillator,

a signal translating'device, a coupling =including a heterodyning circuit between said signal translating device and the outputof saidintermediate frequency amplifier, and a coupling between said signal translating device and the output of said frequency reducing device.

14. A phase modulated wave receiver comprising, wave receiving and amplifying means, said 55 amplifying means including a local oscillator having frequency determining means and an intermediate frequency amplifier, a pair of thermionic detectors each having a control grid and an output electrode, a circuitfor applying signal 70 3 energy from said intermediate frequency amplifier in phase opposition to the control grids of said detectors including amplitude limiting means, frequency reducing means and phase adjusting means in series, a circuit for applying 75 energy from said intermediate frequencyamplifier in phase to the control grids of said detectors, indicating means connected with the output electrodes of said detectors, a rectifier comprisinga pair of thermionic tubes each having an output electrode'and a control grid; said control grids being connected to said first circuit in such a manner that the control grids are energized in phase opposition by energy from saidcircuit' including said amplitude limiting frequency reducing and phase adjusting means,

-means for applying energy from said interme-,

diate frequency amplifier in phase to the control grids of said tubes of said rectifier, and a circuit including time element control devices inter- '-posed between the output electrodes of the tubes in said rectifier and the local oscillator in said signal amplifying means.

15. A phase modulated receiver comprising,

signal receiving and amplifyingmeans, said amplifying means including a local ing frequency determining means and an intengiediate frequency amplifier, ap'air of thermionic etectors each having a control grid and an anode, a circuit for applying signal energy from said intermediate frequency amplifier in phase opposition to the control grids of said detectors including amplitude limiting means, frequency and modulation degree reducingmeans; phase adjusting means and, a frequency increasihg means in series, a circuit for applying energy from said intermediate frequency amplifier in.

thelcontrol grids of said detectors, in-' phase to dicating means connected with the anode electrodes of said detectors, a second-detector comprising a pair of thermionic devices each having control gridand anode and a circuit coupling ,the control grids of said devices to said first,

clrcuit in such a manner. that the control grids are energized in phase opposition by energy from said circuit including said amplitude limiting,

Y frequency and modulaton degree reducing, phase adjusting and frequency increasing-means in series, means for applying energy from said intermediate frequencyamplifier in phase to-the contrpl grids of said thermionic devices, and a circuit includingtime element control devices interposed between the. anodes of said thermionic device and the local oscillator in said signal amplifying means.

16. A phase modulated receiver comprising,

signal receiving and amplifying means, said a!n -'v plifying means including a local oscillator having variableituning means and an intermediate frequency amplifier having an output, a pair of V thermionic detectors each having control grid and anode, a first circuit for applying signal energy from the output of said intermediate frequency amplifier in phase opposition to the control grids of said detectors, saidcircuit including -amplitude limiting means, frequency dividing.

means and phase adjusting means in series,.a

second circuit for applying energy put. of said intermediate frequency amplifier in phase to the control means, indicating means connected with v the anodes of said detectors, a second detector comprising a pair of a control grid and an anode, the control grids of said tubes being coupledto said first circuit to be energized in phase opposition by energy from said firstcircuit including said amplitude limiting frequency oscillator hav-- limited and reduced energy rier wave energy to render the signal.

- signal.

from the out-'- grids of said detectors said second circuit including 1 frequency reducing receiving means and thermionic devices each having dividing and phase adjusting means, a, coupling between the control grids of said devices and said second circuit including said frequency reducing means for applying energy from said second circuit and said intermediate frequency amplifier in phase to the control grids of said last named devices, and a circuit including time element control devices interposed between the anode electrodes. of' the devices in said last named detector and'the.local oscillator in said. i

-signal amplifying means.

, 17, The method of reducing the degree or amount of phase or frequency modulation on a carrier wave modulated in phase or frequency at signal frequency,- which includes the steps -of setting up oscillatory energy characteristic of said modulated carrier 'wave, and-dividing the mean frequency of said oscillatoryenergy.

, 18. Signal heterodyne receiver frequency amplifier having an output, a pair of thermionic detectors each; having input electrodes, circuits. and switching means connecting the input electrodes of said detectors in'parallel demodulatingmeans comprising a including an intermediate with the output of said intermediate frequency amplifier "directly, in, one position-of the switch and by way of .an amplitude limiter in another position of said switch,v and a circuit connected between said amplitude limiter and the input electrodes of said detectors for applying energy from said limiter in phase opposition to the input 30 electrodes of said detectors in either position of said first named switch- 19. The method of demodulating a carrier wave varied as to phase in accordance with siging, energy from said phase modulated carrier wave, limiting-the amplitude and redut'gingv the nal potentials which includes the steps of, divertv mean frequency of said diverted energy and 51- "multaneously the degree of phase modulation to the frequency of I ducing the frequency of a'portionbf'said oscillatory energy and the degree of phase modulation 'therein,- reducing the frequency of another por- J ticn of the oscillatory-energy to a frequency equal to the frequency to which'said first portion is re.- duced', said last named portion being reduced in frequency without modulation thereof and combining-said oscilla-, tory energies of reduced altering the degree bf. phase r frequency to render the 1 21." In asystem for demodulating oscillatoryl energy-modulated in phase or frequency in accordance-with signals, in combination, phase or' frequency modulated oscillatory energy receiving means having an output, a frequency reducing device having an input-and an output said device being connected at its input with the output of said receiving means, demodulating means having an input, a couplingbetween the output of said heinput of said demodulating means, frequency changing means in said coupling, and a coupling between the output of' 'said frequency reducing-device and the input of said demodulating means. I

224m 'a system for demodulating oscillatory energymodulatedin phase or frequency in accordance with signals, in combination, phase or frequency modulated oscillatory-energy receiving means having an" output, afrequhcy reducing device having an input and an output, said device being connected at its input with the output 01- said receiving means, demodulating means having an input, acoupling between the output of said receivingmeans and the input of said de- MURRAY q. cnoseir;