US2230231A - Phase and frequency modulation - Google Patents

Description

Feb. 4, 1941. m. @..cRosBY PHASE AND FREQUENCY MODULATION Fnad'oct. 24. 193s INVENTOR MURRAY G CROSBY E" 'KZ w/DE BAND.

PHASE MDl/[ATR MUDl/LTING MIEI/NAIS WAVE ENERGY ATTQRNEY Feb'4, 1941 M. G. cRosBY 2,230,231

PHASE AND FREQUEHGY MDULTIN File@ oci. 24. 1956 2 sheets-sheet 2 w/aE BAND WA "E EREauE//cy ENERGY mouz/fraz 1 Manuf/NE PvrE/vr/ALS MURRAY G. CROSBY ATTORNEY Patented Feb. 1941 PATENT oFFlcE PHASE AND FREQUENCY MODULATION Murray G.

Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 24,1936, Serial No. A107,333

9 Claims.

This invention concerns a method of and means for signalling by transmitting and receiving wave energy modulated in frequency or phase wherein high frequency or phase deviation is accoms plished in accordance with the modulating D- tentials. The subject matter disclosed herein is related to the subject matter disclosed in my United States application #317,617 led February 7, 1940, said latter application being a. division of 10 the present application.

In my system at the transmitter I may use any of the known means for producing high frequency or phase deviations ofthe wave energy in accordance with signals and for propagating the 15. same eitherl by way of natural mediums or by of wide-baud phase modulation on certain types.

of phase modulation receivers which are normally limited to the reception of phase modulation 30 using amaidmumvphase deviation 'of less than In the prior art of phase modulation reception on the filtered or synchronized carrier type of receiver, maximum phase deviation has been limited to approximately 90. Since this type of reception was accomplished by combining a lo-v cally synchronized carrier, or .carrier from which the signal-'modulations have been stripped, with the incoming modulated carrier to detectv the amplitude changes of the resultant, a phase deviation of 90 would vary the amplitude of the re.- sulant between zero and twice its unmodulated f amplitude.

My method of overcoming these limitations as to the phase deviation of phase modulation consists in the application of frequencydlvision at the receiver. 'This permits the transmission or propagation of wave energy of extremely high phase deviation at signal frequencies. Means for so producing a high degree of phase modulation has been shown in my Unitedv States application #588,309 filed January 23,1932, Patent #2,081,577 datedl May 25, 1-937. As to the means for produclng the division at the receiver, it is well known o5 that an'oscillato preferably a multivibrator oscillator, may be held in step with an external alternating voltage of a frequency an integral multiple lower than the frequency of the multivibrator, as illustrated by the constants of its circuits. Thus, a 100 kc. multivibrator may be 5 held in synchronism with a 300 kc. voltage. This method of frequency division has properties in common with frequency multiplication in that the phase or frequency deviation of the wave which is frequency divided is divided by the order of frequency division. Hence, a 300 kc olt-` age with a phase deviation of 180, when ded to a 100 kc. voltage, will have a phase deviation of 60. Thus, the original phase deviation has been reduced to one-third the original deviation. Similarly the 300 kc. voltage with a frequency deviation of 3 kc. when frequency divided to 100 kc., will have a frequency deviation of 1 kc. This can be seen when it is considered that the process of frequency division consists of the holding in step of the harmonic of the multivibrator wave with the fundamental of the wave to be frequency divided. Thus, the third harmonic of the 100 kc. wave from the multivibrator is held in st ep with the 300 kc. to be divided. Consequently, if the third harmonic of the multivibrator were shifted 3 kc.. the fundamental would have to be shifted 1 kc. to remain in harmonic relation. The same is true with respect to phase deviation when the phase deviated wave is dividedl 80 as to frequencies.

'It might be argued that frequency division at the receiver has no advantage since in the division process the carrier has not beenraised with respect to the noise coming in on the antenna, $5 or other receiving circuit. However, while the phase modulated wave with the higher phase deviation has not been changed with respect to the incoming signal-to-noise amplitude ratio, the ratio of the phase deviation of'v the modulated o wave to the phase deviation produced by the noise has been changed. That is to say. the signaltonoise ratio of a wave, the phase deviations fromthe signal frequency of which is high, is l 'higher than the signal-to-noise ratio of a wave 45 of small phase deviation at signal frequency. Since the receiver receives only Phase 'modulations, the effective phase deviation producedby -the combination of the signal and the noise must be considered.- Thus, the signal-to-noise ratio at the receiver output depends upon the signal phase deviation to noise phase deviation ratio. For a given signal-to-noise amplitude ratio, the effective phase deviation produced by the noise is constant.V Consequently, the higher the phase I I will be the signal-.to-noise ratio at the receiver output terminals. Hence, in applying a high phase deviation at the transmitter, when this phase deviation is reduced to the proper value for detection, the noise is reduced in the Same ratio and the, signal phase deviation to noise the signal-noise ratio N times as great or would eiiect a power improvement of N squared.

Although the above explanation is specific to phase modulation, it will be understood that due to the similarities of frequency and phase modulation, the words phase modulation and frequency modulation may be inter-changed. For purposes of convenience the words wavelength modulation will be used and when used, will denote modulation in phase or frequency even though phase modulation and frequency modulation, as is well recognized, Y,each have separate and distinct characteristics, as well as characteristics in common. Thus, frequency modulation may be transmitted with a high frequency deviation and frequency division may be applied at the receiver to reduce the frequency deviation of the modulated wave and the frequency deviation of the noiseat the same time while preserving the high ratio of their respective frequency deviations. For instance, a frequency deviation of 100 irc. at. signal frequency may be applied to the wave energy to be propagated and-a frequency division of 10 to 1 may be applied at lthe receiver to the-received wave to reduce the deviation at signal frequency 'to 10 kc. The selectivity channels preceding the frequency divider may then be 200 kc. wide and 'those following the divider need be only 20 kc. wide. The sloping lter under these circumstances would be required to convert a 10 kc. frequency deviation at signal frequency into amplitude modulation instead of a 100 kc. frequency deviation.

This type of reception may also take advantage i of the inherent limiting properties of a. multivibrator wherein its output amplitude is constant even though the input amplitude varies over a rather wide range. Hence, the multivibrator may take the place of the amplitude limiter normally required in a frequency or phase modulation receiver. The use of the multivibrator for a limiter also has the advantage that when the signal fades, the noise does not rise to prohibitively high values as it does in the case of the over-loading type of amplitude limiter and in the automatic volume control type oi limiter. This is due to the fact that when the signal fades it merely loses control of the multivibrator whereas with the overloading type of limiter and automatic volume control type of limiter, when the signal fades the noise is amplified to extremely high values.

In describing specific embodiments of my invention reference will be made to the attached drawings wherein;

Figure 1 illustrates diagrammatically the .essential elements of a receiver arranged in accordance with my invention. ceiver units have been shown as rectangles since the circuits Iper se of some of the units form no part of the present invention; n

Figure 2 illustrates the details of a multivibrator The various re- 2,280,231 deviation of the signal wave is made, the higher suitable for use vas one of the units of the receiver of `Figure l.:

Figure 3 illustrates a wide band phase modulater;

Figure -illustrates a Wide band frequency modulator. Here again, as in Figure l, units of the transmitter are shown v.as rectangles since the details of the'said unit per se form no part ci the ypresent invention; l

` Figure 5 illustrates a modification of 4the receiver of Figure 1;

e c illustrates a; mouincauonof the circuits of Figures 1 and 5. Figure 6 shows how to increase the amount of division possible. The divided wave energy is hetercdynedf to a higher frequency and again divided.

Figure 1' illustrates by block diagram, how frequency division may/be applied to a phase or frequency modulation receiver. The signalto be demodulated is fed from the antenna to a radio frequency amplifier i where it is amplled;` The amplied signal is then fed to a rst detector and high frequency oscillator into which it is beat down to an intermediate frequency and amplied if desired. The intermediate frequency energy is then impressedon band pass intermediate frequency amplier 3, which amplies and selects the intermediate frequency energy.' 9 is 'a frequency divider which, as stated, may be a multivibrator' and has its input connected to the output of the intermediate frequency amplifier 3. The unit in d may take the form of an unstable oscillator which is unstable enough to follow the frequency or phase deviations of the modulated Wave, or it may take the form of a multivibrator as shown in Figure 2 which will be described in detail later, The output of t, e. g.,' the divided weve energy, is impressed on the input of a unit which contains any form of frequency modulation or phasemodulation receiver' mown in lthe art. If frequency modulated waves are being received, the unit 5 may employ principles of receiving frequency modulation such as described in In general, theunit 5 may contain a limiter, sloping iilter, and detector. The limiter may be eliminated if suilicient limiting is obtained in.

herently in the -multivlbrator in the unit d. 'If phase modulation is being received, unit 5 may employ principles of receiving phase modulation as are described in L Crosby U. s appnaun #588,309 Jan.

l 2a, 1932, Patent #2.081.577 dated Mayes, 1937.

YCrosby U. S.A application #565.005 Sept. 25, 1931,

Patent #2,114,335 ted April 19, 1938.

, Crosby U. S. applicat on #618,154 Junev 20, 1932. Crosby U. S." application #616,803 June 13, i932,

Patent #2,065,565 dated December 29, 1936. Crosby U. S. application #704,257 Dec. 28, 1933.

Patent #2,112,881 dated Apr. 5, 1938. Crosby U. S. application #46,285 Oct'. 23, 1935,

Patent #2,076,175 `dated Apr. 6, 1937.

Crosby U. S. application #47,933 Nov, 2, 1935,

Ptent #2.085.008 dated June 29, 1937*.

if, for instance. the principes v4of' demodulauon disclosedl in my United States application #47,933

filed November 2, 1935, Patent #2,085,008 dated a triple detection superheterodyne receiver.

` R1 and the capacity C. Due tothe fact that ,these oscillations are'quite unstable ,as to frequency, an alternating voltage fed to the control The multivibrator of Figure 2 is of the negative transconductance pentode type. The values of resistance R and condenser C determine the time constants of the circuit and fconsequently the frequency at which the multivibrator oscillates. R1 isa grid leak and C1 is a bypass condenser. The energy to be frequency divided is fed to the grid of-the pentode 8 by way of transformer 1. The. divided energy is taken from the plate circuit of the Ipentode by way of transformer 9. Y

l This type of multivibrator. which is of the negative transconductance type will -now be described. Briefly, the oscillations take place` by virtue of' the negative resistance characteristic existing between the suppressor grid I2 and screen grid I4 of the tube 8 when proper element voltages are applied. With a resistance, R, in the screen-grid circuit and a condenser, C, coupling the screen grid to the suppressor, approximately square wave form oscillations will take place at a frequency depending upon the time constant of the combination of resistors R and grid I6 of the tube, for instance by means of y transformer 1 of Figure 2, will cause the oscillations to lock in step with the applied alternating voltage at-a frequency which is an integral multiple or sub-multiple of the applied alternating voltage frequency. By using the .sub-multiple lock-in, frequency division is obtained. The frequency divided energy may be taken directly from the resistor and condenser circuits R, R1 and C, but may be conveniently taken from the plate circuit by taking advantage of electron coupling.

Since the electron stream within theftube may be modulated by the oscillations of the multivibrator circuit, the pla'te current willvary 'in accordance with these oscillations. Consequently, the output may be taken from the plate. circuit by inserting an output circuit such as the transformer 'l .in Figure 2. 1t will be noted that since the anode is coupled to the` other electrodesin thetube andin particular, the auxiliary electrodes in the circuit RC, by way of "the electron stream only of the tube, changes in load cannot affect the input and auxiliary electrodes or the circuits `connected therewith.

75 vibrator of Figure 2 is included in the unit 4 as' The primary and/or secondary winding of transformer 1 may be timed to a frequency f e. g.. the frequency of the wide band phase or -frequency modulated energy to 'be demodulated. The primary and/or secondary winding of the transformer 9 may be tuned .to a frequency where N equals the number of times it is desired to reduce the incoming frequency. The 'multishown by the dotted lines enclosing said unit the oscillations which are produced therein of a "frequency determined by R and C. By adjusting R and C to the proper values the desired order of frequency division may be obtained. 'I'he signal carryingenergy of reduced frequency appears in 9 and is impressed on 5. This type of pentode negative trans-conductance multivibrator is particularly suited for suclnpperation since the control grid is available to apply the energy and the electron coupled output may be taken from the plate circuit. However, any other type of multivibrator may serve the purpose equally well.

v'I'l'ie process of frequency reduction employed in this receiver is not limited to the use of a single frequency dividing oscillator. 1 contemplate the use of several multivibrators cascaded so that each one performs a part' of the frequency dividing. Such a system has been illustrated in Figure 5 wherein the rectangles 4 and 4' designate two frequency dividing umts in cascade. For instance, if a 600 kc. intermediate frequency wave is to be divided to 100 kcs., the 600 kc. energy may be fed to the first frequency dividing oscillator in 4 where it will be divided by a ratio of 3 to 1`or to 200 kc. 'Ihis 200 kc. energy may then be fed to the frequency dividing oscillator in 4', Jwhich may apply a frequency division of 2 to 1 so -as to divide the frequency to 100 kc.l Such multistage dividing .has the advantage of greater stability since the lower ratios of frequency division hold in step more easily. Amplification may also be applied between limiting stages .to further improve the stability. An amplier has been e shown connected between the stages 4 rand 4' of Figure 5.

For high ratios of frequency division the process of `frequency division and heterodyning may be combined. Thus, in the instance of the frequency division of the 600 kc. intermediate frequency energy, if a greater frequency division ratio than 6 to 1 were desired with a nal frequency of 100 kc.,a frequency division of 3 to 1 may be first applied to divide the frequency to 200 kc. 'I'his 200 kc. energy may then be heterodyned to, for instance, 1000 kc. -The 1000 kc. energy may then be applied to another frequency dividing system to obtain a frequency division from 1000 to 100 kc. A total frequency is, in this manner obtained in the whole process lof division and heterodyning. By further heterodyning and division the process of frequency division vmay be carried to any degree desired. As has been described in various copendingapplications, the process of heterodyning phase or fre vquency modulated waves does not alter the degree of phase' o r frequency modulation of the Iwave whereas the process of dividing the phaseA division of 30 to l Inv Figures 3 and 4 I have shown( diagrammatically a wide band phase modulation transmitter and'a wide band fraquency modulation transmitter respectively.

may use the wide band -phase modulation of vCrosby United States vallpliciiin #588,309 filed January 23, 11932, Patent 2,081,577 dated May These modulations may be of any type know'nin the art. For example, I

v of United States application #13,886 led h I claim;

i. The method of improving by increasing the signal-to-noise ratio in phase mocha.

lated wave energy used in signalling which includes the steps of, modulating the phase of said wave energy through a wide range or degree at signal frequency whereby the ratio of signal modulation to noise modulation of said wave energy is high, transmitting said wave energy so modulated to the point oi' reception, producing oscillations with phase variations characteristic of the phase modulations on said wave energy at said point of reception, passingv said produced oscillations over a single path only to c lelnodulating means and dividing the frequency oi.' the said oscillations and simultaneously reducing the amount oi'vphase variations thereof in said single path.

2. A method as ,recited in claim 1 including the v additional step o f. dividing the frequency and simultaneously reducing the phase variation' of the oscillations, of divided frequency and reduced phase variation, in said single path. l

3. A method as recited in claim 1 wherein the said oscillations areV limited in amplitude as they are'being divided as to frequencyin said single 4. A method as recited in claim l wherein said oscillations are limited in amplitude in said single Bambi` f 5. The method of demodulating `:gaveen the length ct which has been modulated through a wide range or degree before transmission to ixnprove the signahto-noise ratio'ln the energy re..

sulting from demodulation thereof which includes the steps of. producing voltages charaeteristicof v lthe -wav'e energy so vmodulated in length, passing said voltages over a singlepath only to wave demodulating means and dividing the frequency of the voltages passed overrsaidsingle .path to reduce the range or degree of modulation thereof; 10

6. A method as recited in claim 5 wherein the voltages are limited in amplitude-in said single.

path.

Al?. A method as recited in claim 5 wherein said voltages .are limited 1n amplitude whne being l divided as to frequency in said single path. i

' 8. In a system for demodulating transmitted Wave energy the length of which has been modu- .lated through a wide range or degree to obtain a favorable l-to-noise ratio'in the wave energy so' trtted. means for picking up saidwave energy. and producing voltages characteristic of the said wave energy so modulated, wave length deuriodulating means, a single path only coupling said demodulating means to said voltage producing m and a frequency ydivider in said single Vh. f yf 95. A system as recited 'in claim wherein ain-t? putuee limiting means' 1s induced in said single ,n

path to t the amplitude lof the voltages passed thereby. I

MURRAY o. CROSBY.

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