US2397157A - Wave length modulation system - Google Patents

Description

March 26 1946. w VAN O E 2,397,157

' WAVELENGTH MODULATION SYSTEM I Filed Feb. 12, 1945 a s Sheets-sheaf. 1

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March 2 1946' w. VAN RoBE T 7,

WAVELEI JGT H MODULATION SYSTEM Filed Feb. 12, 1943 s Sheets-Sheet 2 *1 I I Hill 6 6 I 53/ g gang 51 QQE ME a: M '13 a i s 633 E $3 44 m g Q5; ktjflq E t K) v v v I .k Q Q I k. f a! I:\ k

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WAVELENGTH MODULATION SYSTEM Filed Feb. 12, 1945 3 sheets-Sheet 3 M/VE/vT R WM BEETS Patented Mar. 26, 1946 wave LENGTH, MODULATION srsrrnm.

Walter van B. Roberts,

to Radio Corporation of Delaware Princeton, J1, assignor f America, a corporation Application February 12, 1943; Serial: No.; 4!7- 5,60- 1 '7 11 Claims.

This application; discloses improvements in transmission from a. phase: modulation. transmit.- ter and in receivers of said transmission where the allowable; signal channel width issufiicient to permit. large. phase swing.

The general. object of; the present invention; is toreduce noise by utilizing as fullyas possible the full available channel width whether or: not

- the original modulating. voltage is of low orv high audio. frequency and whether or not it: is of. larger or smaller amplitude Other objects attained in thepresent invention and advantages derived by" the use thereof will appear in. the detailed description which follows and in which reference is made to the attached drawings wherein;

Fig, 1 illustrates a preemphasis circuit used to reform or. modify the modulating potentials used to modulate the: transmitted carrier. This preemphasis circuit is arranged-arid operated; inaccordance with my invention to modify the modulating potentials in a particular manner appropriate to the use to which I put them,

Fig. 2a is an amplifier or relay wherein the r-nodiulati ng potentials are further modified in accordance with their amplitude in such a manner that the output voltage varies substantially as the logarithm of the input voltage,

Fig". 2b-is a modified ci rcint using the principle or the circuit of Fig. 2ain a full waveamplifier or reIay of the modulating potential,

Fig. 2c shows qualitatively the characteristic of the circuit of Fig. 219,

Fig. 3- illustrates partly by block diagram and chiefly schematically a phase modulation transmitter including the novel features of my present invention,

Fig. 4 illustrates in like manner a modified phase modulation transmission system arranged in accordance with my invention,

Fig. 5 shows a tube relay: and voltage modifying circuit to beused at" a receiver adapted to receive. the transmission: from a transmitter such as the type illustrated in. Figs; 3- and 4 while,

Fig, fi'illustrates a receiver arranged and adapt-- ed to receive waves transmitted in accordance witlr my invention;

Phase modulators have the advantage over: frequency modulators. inthat crystal. control circuits maybe-employed to stabilize the transmitted carrier. However, inzthe. case of a phase: modulatingr transmitter it is also known that for a. given amplitude of modulating voltage; the radiated spe'ctmm: is the wider; the: higher the; frequency of: the: modulating: voltage; If the? modulating:

voltage were to reach. the: same maximum. ampl-ia tudeatallfrequencies a simple voltage "pre' emphasis of. the various modulating. frequencies inversely inaccordance their frequencies would result in a full utilization; of the available channel orfre uency spectrum regardless of: the frequency r. the modulating, voltage. However,. in ordinary speechor music the. low firequenciea are likely to reach. maximum amplitudes considerably in; excess oilthe maximum! amplitude efx any -of the'higher frequenciesp Hence a; simple preemphasissuch asmentioned: above: would; re suit the. low audio frequency components oi speech or music creating a-wider than allowable spectrum if. the transmitter were adjusted to cause the higher-frequency components towuti-lize the allowable. channel; substantially fully.

Inaccordance with one aspect. of the: present invention. I. therefore provide a pre mpt-resizing; networleior the voioe or music,,. which network; transmits the: higher frequencies output am:- plitudes substantially inversely proportionai. to: their jrequencies but. which. provides lessening preemphasis as. the frequencybecomes lower so that only a. limited amountof. preemphasis is imposed on. the lowest. modulation frequencies Theexact constitution; of. this network may be varied somewhat inaccordance with the nature ot the signals to be'transmitted hut for the.

poseof illustrating as; simply as possible a. net-- work; which lSi capable of realizingto a large extentthe advantages ot the present invention Fig. l may be discussed.

Fig; 1, i0 is the: signal or. control potential source. such as. current or voltages. representing voice, music or pictures originating in a studio;- R, and.v C respectively. represent. circuit. resistance and capacity which modify the voltages from the studio m accordancewith their frequency but as will be. seen l'at'er'not necessarily in inverse pro porti'onali'ty to. their frequency;

It is easily shown that the output voltage of Fig.- 1 for a giveniinput amplitude. to. R, C, is proportional to f where W equals 21: times the rrequencyor: input. In1 2;CCQ1-dal1e with. the present inverttioni. R and; (dare sm chosen that at the frequencies ismuchigreater thamr. for: high frequenciesl a: simple preemphasis sub? stantially: inversely; proportional: .to' frequency re? sults. However, R and C are further so: chosen that at very lowaudible frequencies: is: less r of transformer l 8 of in Fig. 20.

than 1 so that at very low frequencies there is a substantially lessened preemphasis. This choice of R and C distinguishes the network of Fig. 1 from previously 'used networks of similar diagrammatic compositions wherein R and C were so chosen as to make WRC less than 1 throughout the entire audible range, thus producing in these known networks a simple preemphasis,

namely inversely proportional to frequency,

throughout the whole signal range.

To recapitulate the foregoing, the preemphasizing network of Fig. 1 alters the relative am: plitudes of the various component frequencies of speech or music etc. in such a fashion-that the radiated spectrum of a wide phase deviation transmitter (modulated by the output of Fig. 1) more fully and uniformly utilizes theavailable channel at all modulation frequencies than would be the case in the absence of Fig. 1.

Quite distinct from the foregoing considerations there is another reason why full utilization of 'the allowable channel in either wide phase or wide-frequency modulation has not been attained in-the past. This reason is the great variation of tensity modulating voltage relative .to that resulting from high intensity voltage. This means provided may be illustrated by one suitable device shown in Fig..- 2a although it will be understood that otherdevices having similar characteristics maybe employed alternatively;

iIn Fig. 2a is'shown a relay or amplifying tube l2 having-a resistance r in its grid circuit to which the modulating potentials are applied and a resistance rp in its plate circuit from which the modified modulating potentials are derived. -Referring now to 2a it is known that if a positivevoltage of variable amplitude is impressed through 1' on the grid of tube l2 by way of the input terminals and, if r is very large while the 1 plate resistance rp is small, then the output voltage will vary substantially as the logarithm of thisinput voltage. This is because the large value of r makes the grid current substantially proportionalto the input, in combination with the fact that for small positive grid potentials the grid current varies exponentially With respect to v grid potential. Hence, the grid voltage varies as j the'logarithm of the grid current and thereby as j the logarithm of the input voltage. The plate voltage to a phase modulator current varies linearly with grid voltage for small variations, so that finally we have a plate current which varies as the logarithm of the inputvoltage.

If now I employ a pair of tubes 12 and I2 each arranged 'in a full wave circuit as shown in Fig. 2a,.I..obtain the push-pull arrangement as illustrated. in Fig. 2b. The .overall relation derived between secondary of transformer 16 and the in-' stantaneous input voltage applied to the primary Fig. 2b is shown qualitatively Imagine {now that modulating voltages are applied to a phase or frequency modulating transmitter through the arrangement of Fig.'2b. It

will be seen from Fig. 20 that small amplitudes of modulating voltage will produce relatively much larger phase or frequency deviation, than large amplitudes. Thus even during weak passages of music for example, the radiated. spectrum may utilize a; substantial portion of the allowable channel while strong passages are prevented from exceeding this channel by virtue of the saturating or limiting property of the device of-Fig. 2b. Fig. 3 shows schematically a complete transmitter arranged to utilize the allowable channel more fully than has been the case in prior transmitters, both with respect to varying frequency and varying amplitude of the components of speech or music to be transmitted.

In Fig. 3, It] represents the signal source. 20 is atransformer feeding the signal'voltage to a preemphasis circuit comp-rising L and R and. to an amplifier tube 24, the anode of which is coupled by a condenser 26 and resistance 39 to the input electrodes of a second amplifiertube,

34, the anode of which-is coupled to the primary winding of transformer I8 the secondary winding of which feedsthe pair of tubes IZ- and l2 connected as 'illustrated'i n Fig. 2b to modify the voltages as illustrated in Fig. 2c. The output transformer of the tubes [2 and of any approv'ed type'in 38 to modulate therein carrier wave oscillations from a source 40. The source 40 may comprise'a stabilized oscillator and/or frequency multiplier. These modulated waves are fedto' a unit 44 wherein they are frequency multiplied,

converted in frequency and. amplified as desired. In the arrangement-of Fig. 3 the combination of L and R is shown as an alternative to the net-' work of Fig. 1. The ratio v plays thesame role as the The network of Fig. 2b; is, as is shown incorporated without alteration in Fig. 3.; The signals from lb are modified in L, R as described hereinbefore, amplified in tubes 24 -andfj34 and further. modified in accordance with thelr amplitude in tubes l2 and i2; amplified in 36' andfinally used to phase modulatethe carrier in 38 the high frequency outputof; which'is increased in amplitude and frequency as desired in for utilization.

'In the modification illustratedin as. 4 a push:

pull preemphasis network of the type of Fig.1 is

combined with thefull wave or pushpull intensity preemphasis circuit of Fig. 2b. This circuit, it is believed, needs no further discussion at; this point.

Having now shown how to improve the chiciency of channel utilization ina phase modulating transmitter, :there remains the questionof reception of signalsfrom sucha transmitter, 1

and it is a further object/of. the present invention to provide a receiver capableof producing, from such transmission, the priginal studio signal without the amplitude and frequency distortions imposed'on it during transmission. 1:

Let us suppose :we start with asimple FM're ceiver and that the amplitude; distorting device of Fig. 2 is absent from the transmitter.. the output of an FM receiver is proportional. 'to' the rate V of change of phase. of the received signal; 7

and the.latter is equal to.- W'where W is defined l2 feeds an amplifier 36 which in turn supplies the modulating product OR of Fig. 1.

aeer, reenetw rk. must have an ou put to inp t. rat o g ven. hr

1 ej-WRQ so as to offset the undesired frequency dependence just mentioned. Such a correction networkmay be constituted as shown schematically in' Fig. 5, in which a choke feed for D. C. is assumed. The transmission ratio in Fig. is proportional to an hence o 1 +jW'R C Ther ore, if RIC. is. made equa to R0,, he ou put o the sim e FM receiv r wil be c rr ed.

or frequency i tor ion Ne we may consider the co .reetiehv of ampli ude dis ortion. This may est be e p a ed y re eren e, to. t e pa tly schema c. iagram Fig, 6.

In. the r ce er n. 6,, tillv s a rad o freq ency amplifier and. m xe such a o a. h t r dyne reei er .2 ep ese s the so r e. f osc l ions o b at g with the eceived wave i t e mixer er onve te in 11 5 is an in erme i e frequeher ampl fieran am l tude. imiter. 5 he freq ency di c minator an ect fier sy tem. so. isa i au iov frequency am lifier w l amp e tube 64 has in its output a resistance Rf and a onde s r C. he. p rpos of. w ic i be e scribed ere na er fhe. utput ro he nt rmediat fr e cy amplif er 541s supplied to a rectifier 111 wherein,

pote als fo automa c vo me ont ol. PuiIpQ s. a ede eeed a d ed ee e or mo e f. e a s. n Audie, f qu ncy and direct current. i taken, from the output of 5,8 and f back to, a ac ant t b modulator in .4. to stab lize, he local oscillator in 5,2 in a manner described here: inafter. equen y co trol pote ial a filt red o y esist s 8. and ondenser 0.-

The modulation output, of the amplif er in. 6.0 is sup ied o he. pr mar w ndin of. a trans: ormer t h seco dary w ndin o wh ch is connected, to he c ntr l rids. r a air o ube a d. t. the, no es. of a e oupl d by a. tra siermer t an audio. am lifier. n. .0..v

he. mp fier l hili I! and their couplings are lar to he arrangement in Fisahdzeperate in the amem nner T ou put o the mplifier, in S llisalso. fed tothe reactan ce tube and is used along with the direct current automatic frequency control potential supplied from 58 to control the operation of the reactance tube modulator coupled to the oscillator in 52.

Basically this is a superheterodyne receiver whose mixer input system (including R. F. stages The audio variations in this. automatic.

if; any); is broad enough to; admit. the full 51160.1 trum of the transmitter of Fig. 3, for example 10 0 kc. However, intermediate frequency system A is hreferahlv mueh. narrower icr reasons. which aim-ear la er- T e interm d ate. frequency uto t eeds rec fi that which pro uces AVG:

. voltage. the su l way; and the intermediate; fre uency output also xci e limiter'ih. 5i.- Ifhe rre peten 1.5:

limite e iiu impressed, on: a. sharply resnem sive discrimiri 0, er o the wel n unit 8 that i discriminator h ids. heh a mall chan e or the termed ate tre ueher f om, ts nomi al va ue. a. larse .dimet a w ose si h and. amnli udeare ateer ahee wi h the d ection and a ount of said. cha g The a r ge ti -rim hater output. mar be s ara ed it m sud-i. ariatiohsh the R9 lter it nd 8 hown in the eehaeetieh n. 5 and the r aetahce t me in it and used as voltage-to h ar he me n in ermediate f e uency e ual to the zero-output: ire quency of the discriminator. 7

The. audio ir queher outp t: o the discrimi at or is m fie in.v it. .0, and c rec ed f r he quency distortion by the, coupling; network BJQ as escr d in onnec i n w th F 5 and. then im ressed on the loud. speaker or otherut iizatieh dev c Some of the audio frequency output of: the dis,- e hate is a so impressed, ithout. frequ ncy d stortion cor ectio upon a loearith it ou pu device .8., 1 .21am a dtlilike ec r uit of g. 1 he ou pu r thi de ice isth h amp ified i 90. nd i pr s e on the reac ahee tube n 14. Th insertion of the; logar t mi devicea es riloed a comp ish s c rre tion of ar p itude distortion as fo ows:

I a rdanc w h. t nvention, the caverning action of the. Circuit, connecting; the discri inator output to the reactance tube is so strong t the fr q en y departur of the int rmed a frequency from its mean value, is at all times kept very smallcornpared with the variationor instant taneous frequency of the signal received upon the mixer in at. This requires, h w ver, hat th local oscillator frequency from 52 must about, s an f quency n; t e same way and: almost as mu h. as the, instantaneous f u n yefthe received signal, varies. Butv this in requires that the voltage input to. the, logarithmic; devicel2, l2 should differ from the transmitter studio voltage only to. the extent; that the; perfor ance e h m s r vF a- 3. r 4, di ters rom simple inverse variabitm with frequency. This. difference between. discriminator output and, studio voltage iswhat is corrected by. the-network R'CI of Fig.; 6. It should be emphasized thattl'ieC exactness of compensation for frequency and amplitude distortions introduced at, the transt i re z d that the particular. devices, here how f r imi ed low. fr uency. nd low: amplitude preemphasis in the transmitter may be replaced by others which accomplish similar results, and that the correction for these distortions may be accomplished in other than the particular manner shown. However, the present invention is believed to cover all such modificainator and detector excited by currents tlie mod ulations onwhich correspond to the carrier modulations, an amplifie coupled to said detector, for amplifying detected components in accordance'with the logarithm of their amplitude and connections for modulating said currents in accordance with the output of said amplifier.

2. The method of modifying the form of signals to be used in a system wherein the instantaneous frequency of a carrier is to be modulated in accordance with' the said modified signals Which includes these steps, modifying the am plitude of currents representing the signals, as a function of their frequency to derive resultant amplitude modified currents and modifying the resultant currents as a function of their modified amplitude. v

3. The-method of modifying the form of 'Signals to be used in a system wherein the instantae neousfrequency of a carrier is'to be modulated 1 in accordance with the said modified signals 1 which includes these steps, modifying the amplitude of voltages representing the signals sub- 1 stantially inversely in accordance with their frequency to derive resultant amplitude modified voltages and modifying the resultant voltages substantially in accordance with the logarithm of their amplitudes. r

'4; The method of signalling by Wave length modulated carrier energy and improving the signal to noise ratio during the signallingprocess which includes these steps, modifying modulating potentials as a function of their frequency and also as a function of their amplitudes as 3 modified in accordance with their frequencies, modulating the carrier in accordance with the j modified voltages, transmitting the carrier, de-

modulating the transmitted carrier and restor- 3 ing the voltages to their original form.

5; The method of signalling by wave. length modulated carrier energy and improving the signal to noise ratio during the signalling process which includes these steps, modifying modulatance with their frequency and substantially in accordance with the logarithm of their amplitudes as modified in accordance with their fre-' quencies, modulating the carrier in accordance with the modified voltages, transmitting the carrler, demodulating the transmitted carrier and restoring: the voltages to their original form.

'6. The method of increasing thesignal to noise ratio ofsignals wherein a carrier has its instan-' -ta'neous frequency'modulated'in accordance with voice which includes these steps, modifying the signals substantially in accordance with their frequency and substantially in accordance with I their" voltages as modified in accordance with their "frequency, imodulating. the instantaneous frequency of carrier energy in accordance with the modified voltages, transmitting the modulated carrier, demodulatingthe transmitted carrier to derive the signal voltages, modifyingthe signal voltages in accordance with their frequencies for utilization purposes, modifying the signal voltages in accordance with their amplitude as demodulated and controlling the carrier being demodulated voltages.

7. The method of increasing the signal to noise, ratio of signals wherein a 'ca'rrier has its instan-i taneous frequency modulated in accordancewith voice which includes these steps, modifying the in accordance with the modified:

signals substantially inversely in accordance with I r their frequency and substantially in accordance with the logarithm of their voltages as modified i in accordance with their frequency, modulating the instantaneous frequencyof carrier energy in accordance with the modified voltages, transmitting the modulated carrier, demodulating the transmitted carrier to derive th signal voltages, modifying the signal voltages in accordance with thei frequencies for utilization purposes, modi fying the signal voltages in accordance with the logarithm of their amplitude as demodulated and controlling the carrier being demodulated in accordance with the last mentioned modified voltages.

for use in angle modulation system which in clude these steps, modifying the amplitudes of signalling currents substantially inversely in accordance with their'frequen'cy andalso modifying the signalling currents substantially in'ac 7 cordance With'the logarithm of their amplitudes;

9. The method of signalling including these steps, modifying the amplitude of modulation current as a function vof their amplitudeffur ther modifying the amplitude of the said modulation currents as a function of their frequency, and

' modulating the instantaneous frequencyof wave energy of carrier wave frequency with the modified currents. i

10. In a wide band phase modulation system, the method of improving signal to'noise ratio in in accordance transmitted energy which comprisesimoreland more increasing the phase deviation of 'ca'rrier current per volt of studio signal both as signal amplitude and signal frequency diminish tra'n's l ing potentials substantially inversely in accordmitting the resultant'energy," receiving the resultant energy for deriving therefrom the sig nal, and correcting both'theamplitudeand frequency distortion introduced by said method of 7 transmission.

11. In a signalling system, a sourceof oscilla- 5 tions of carrier wave frequency, a source ofm'od ulating currents, a coupling between said last named source and said first named source to mod ulate the instantaneous frequency of the oscilla tions by the modulating currents, elements in said coupling for modifying the modulating cur-l rents substantially inversely in accordance with their frequency,"and elements in sai'd lcoupling for modifying'the modulating currents substantially in accordance with their amplitude;

WALTER. Wm B. ROBERTS i 8. The method of correcting signalling currents

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