US1624498A - Modulating system - Google Patents

Description

F. MOHRv April 12 1927.

MODULATING VSYS TEM Filed Jan. l5. 1925 Patented pr. 12, 1927.

EEaNxLm nona, or NEw- YoEx, N. Y.,

'assIGNoE `BY' M EsNE ASSIGNMENTS, 'ro

WTESTERNl ELECTRIC COMPANY, INCORPORATED, CORPORATION E NEW YORK.

MoDULaTmG SYSTEM.

Application led January v125, 1925. Serial No. 2,486.

This v'invention ,relates to translating circuits, and more particularly to the circuits Y o'f space discharge inodulatlng devices.

A11 object of the invention isA to increase the out ut of useful energy of amodulator, preferahlyof the space discharge type.

A feature of the invention is the combination with the modulating device of load or output circuit having an impedance which varies with the wave frequency according toy cial mode. ther objects of the invention and the principle of its "o erationwill be clearlyv understood from t e following detailed description when read in connection with the A accompanying drawings, ofv which Fig. 1 shows in schematic form a modulating system embodying the invention; l

K Fig. 2 shows attenuation characteristics relating to the system of Fig., 1; and

Fig. 3 shows impedance characteristics corresponding to the attenuation vcharacteristics of Fig. 2.

' The system shown in schematic form in Fig. 1 includes the necessary apparatus for f preparing' and transmitting a single side bandof high frequency waves for the purpose of radio tele hone communication by the so-called homodyne method. In accordance withthis method,a high frequency carrier wave is modulated by speechV waves and fromthe modulation products one sideband corresponding to the speech waves is selected for transmission, the other side band and the carrier wave bothAbeing suppressed by suitable selective circuits; At the receiving point, a local generator supplies an auxiliary Vwave of the proper frequency, usually that of the suppressed carrier, which is impressed along with the received Waves upon a demodulator, thereby enabling the transmitted signals to be properly detected.

The advantages accruing from the method are principally an economy of power due to the restriction of the number of waves trans` Y mitted, a more eieient use of the frequency spectrum due to the elimination of one side band, and, in the case ofradio signaling, a certain degree of secrecy vdue tothe absence of the carrier wave. Y f Y As illustrated, the system/ comprises a balanced space discharge modulator 1, of

the type disclosed inU. S. patent to J. R.

Carson, No. 1,343,307, issued June 15, 1920, upon the input'terminals of which are impressed s'peech frequency waves from microphone 2 and a carrier'wave from a source 3.

` The products of `modulation are impressed .through `a balanced out ut transformer 4 upon a high-pass wave lter' 5, the. cut-off frequency of which is so adjusted that only those waves correspondin to the sum of the two input fre uencies, t at is, theI higher frequency sideand waves, are freely transmitted. This filter maybe of the well known typ'e disclosed vin U. S. patent to G. VA.. Campbell, No. 1,227 ,113, issued May 22, 1917,

Y. but 1s preferably a composite filter, having "a very great 'attenuation at frequencies close tothe cutfotl frequency. Filters of this general type are described by O. J. Zobel,

Theory and Design of Composite Electric Wave Filters, Bell System Technical Journal, Vol. II No. 1,-January, 1923. The-balancedl modulator in accordance withthe VCarson patent mentioned above delivers to the` filter only the side-band modulation products` vand those amplified waves corresponding to the speechinput, the amplified 'carrier waveV being balanced out by thewind- -ings of transformer 4. Filter 5 is thereforerequired to suppress only the speech waves,

products, and` such smallfraction of the Vcarrierwave as-may resultfrom imperfect y tions of the output transformer balance.`

The selected side-band isimpressed upon asecond space discharge modulator 7 through transformer Gand is modulatedtherein with a second carrier Wave supplied froml source 8 through transformer 9. A band-pass filter 10 selects from the outputV of modulator 7 one bandof side frequencies and transmits seA 'the lower side-band of ythev modulation them to' an amplifier 11, which may be of anyV well known type, lfrom the output of which wavesv of amplified energy are de-.

livered to a radiating' antenna 12. Filter 10, as illustrated, consists of a single mid'-shunt. terminatedsection` but additional sections may be used if desired.

The first step of modulation in modulain the frequency'spectrum close to the carrier wave. The selection of a single sidetor 1 produces side-bands of waves which'lie band, while at the same time the other is substantially sup ressed, requires a. sharply discriminating se ectivity in the filter 5. In practice, ithas been found that such selection cannot be efiiciently accomplished if the frequency of the carrier wave is much above 40 kilocycles er second;

The use o a carrier wave of about 30 kilocycles per second frequency rovides, as the result of the first step of moiiiilation and selection, a band of waves of frequencies ranging from 30 kilocycles per second up to about 35 kilocyclcs per second, and these waves when modulated in the second modu' later 7 with the second carrier wave produce sidebands which are separated from the carrier wave by a frequency difference of at least 30 kilocycles. The second wave may be of comparatively high frequency, for example 10() kilocycles per second, without imposing upon the filter 10 any severe requirements as to selectivity.

The problem of preparing a single sideband, or a single band of pure modulated waves, involves not only the problem of selection as outlined in the foregoing, but also that of securing the maximum power output from the modulating devices.

A mathematical theory of modulators, having particular reference to simple space discharge modulators of the three-electrode type7 is contained in U. S. patent to J. R. Carson, No. 1,448,702, issued March 13, 1923.

The following formulae are given in the above noted patent for the side frequency curi-cnt eomponents of the products of the modulation in a space discharge amplifier and E -E 2 .2 IUQHn-MRO) Znfzruml.) (2) the various factors ofwhich are defined as follows:

E1 and lil2 are respectively the amplitudes of the carrier wave and the signal wave elec tromotive forces impressed upon the modulator input terminals, both waves being assumed to be sinusoidal;

`Zf, and Zr, are the impedances of the modulator output circuit, including the internal resistance of the modulator, corresponding to the frequencies f, and f2 of the carrier wave and the ysignal wave respectively; Zmm) and Z( are the corre spending impedances at the frequencies of the modulation product components, that is, at the upper and lower side frequencies;

n is the amplificationrfactor and Ro is the internal resistance ofthe modulator. The factor M is a constant the value of which depends upon the rate of variation of the resistance Ro with the anode potential of the modulator in accordance with the following equation The power output of the modulator in a given side-band, for example the lower frequency band, is equal to the square of the current component of that frequency multiplied by the resistance of the connected output circuit at that frequency. From equation (1) it is evident that the side-band powerv output may be expressed by Ruz-h).

in which P is the power output, and Rukh, is the resistance of the connected output circuit at the frequency (ff-f1). The factor' K involves the coefficients, p, R0, and M, and the voltage E, and is a constant so lon as the voltage is constant.

rlghe power output being determined in part by the load resistance, the impedance conditions necessary to produce the maxi mum power may be determined by computing the partial derivative of the foregoing expression for the sideband power with respect to the load resistance, and equating the result to zero in the usual manner. In this Way, the following equation is found which states a condition for maximum,

power:

ZUn-f) A RUa-Ii) Z( are each equal to that is, to the internal resistance of the modulator plus the constant resistance ofthe connected output circuit.

Substitution of this value inequation (5) gives the solution 1 R(f,m=5Ry V(7) the partial derivatives beingr all equal to unity.

From equation (4) by means of equations l quency that accords with .the requirementsequations (6) and (7 the maximumL powerisy found Uaae 2. The load impedance is zero `to wavesof frequency f1 and is uniformly rel sistive to waves of the other frequencles.

The impedancesare v Zn' Rd Za Zon-n) Re l'lRua-m (9) `The partial derivative of Z', in equation (4) is zero and the other-partial derivatives.

are each unity. The solution of equation (5) obtained when the values of equatlons (9) are isubstituted therein is which corresponds to a maximum power Oase 3. The load impedance is zero tpwaves of both frequencies 'f1 and f2 and 1s resistive at the -side band frequency.

The impedances are zl--a-.R- t Q 0 Zas-h) R0 fi' BH2-h) l The substitution' of the values defined by for which condition 'i the given by equation (4.)v is' The maximum powers obtainable yi n 4 the three cases investigated are the p1 oporindicating that a very great increase inside band power may be obtained if th connected output impedanceis so designedv as to have' a characteristic variation with frestated in connection with the thirdcase. The modulating systems-of the present invention lembody 'features whereby this increased power out ut is obtained.

In the system o Fig. 1 the conditions giving rise to the optimum power output front second modulator 7 are obtained by the cooperation' of two features. The first is the use of a mid-shunt termination of the band-pass filter 10 in theoutput-circuit of the modulator for the' selection of the proper side band and the suppression of the other waves.

(12) inequation (5) leads tothe power output nsare indicated bythe frequencierf. and fb.

Between these limits the im' edancc 1s resistive; infinitely great at the imits but falling rapidly to al fairly uniform value throughout the greater part of the range..

Outside the limits the impedance is reactive. and falls rapidly .toa low value as the frequency decreases; above the upper limit it is capacitive and falls rapidly to zero as the frequency increases. By well :known methods the filter may'be so proportioned that the resistance between the band limits is equal to the modulator internal resistance for a substantial range of frequencies. A possible relationship between the two resistances is illustrated in Fig. 3 by the curved line B, representing the filter resistance and the straight line DB1 representing the modulator resistance. t

Figs. 2 and 3 show also the characteristics It is inductivebelow the lower limit o fv thefilter 5, the' attenuation characteristic being represented in Fig. 2 by the discontinuous curve G,=the discontinuity of 'which corresponds to t e resonance of the antiresonant Acircuit 13 in the series terminal branch of the filter, and the impedance characteristic being shown 'in Fig. 3 by the discontinuous -curve H1 H2 H3. The various wave frequencies involved in the production vof the inalside band are also indicated.

In modulator 1 a wave of frequncy fs -which may be considered as one component of a complex speech vwave modulates a carrier wave from source 3 having a frequency less than the cut-off frequency of filter 5. f Amongst the products `of modulation is a wave of frequency f1(=fc|fs), which is transmitted through the lter to the input of modulator Z in which it is applied to modulate the second carrier )2, supplied from source 8.

As may be seen from Figs. 2 and 3 itis necessary that the second modulation product of frequency faQ-f1 should fall between lic , f. which is preferably equal to or slightly the limits f. and b `of the band filter` 10,in

consequence of w ich .f2 must lie well outside these limits. lhe relative posit-ions of these waves in the' frequency spectrum are clearly indicated in the diagrams.

In the load circuit the waves of frequencies f1 and f2 encounter practically zero im'- pedanc'e, whereas the side-band waveof frequency ffl-f1 encounters a resistance practically equal to that of the modulator tube. 'The conditions are therefore correct to obta'in the greatest possible amount of power from the modulator. It should be noted also that the second carrier wave bein well outside the transmission range of t e filter is strongly. attenuated thereby.

The action in the output circuit of modulator 1 is of a similar character but is slightly different in detail. Filter 5 is terminated mid-series instead of-midsl1unt in consequence of which the impedance is zero at the cut-off frequency, instead of being infinite as in the case for mid-shunt termination. By setting the frequency of the first carrier wave equal to or slightly lower than the cut-off frequency the desired condition that the load impedance be zero to t-he carrier wave is attained. If an ordinary Campbell type high-pass filter were used the impedance would, on account of .the series capacity, increase towards infinity at low frequency, and the condition of low impedance to the second input wave f, could be attained only by making the impedance of transformer 4 sufficiently low at speech frequencies. However, the use of the special termination shown in the ligure produces an impedance characteristic which falls to zero at very l'ow frequencies and thereby secures the desired condition of zero impedance in the load circuit to both of the input waves.

The use of a balanced type modulator for the first stage of modulationl secures by the neutralizing action of windings 'of the out`-' put transformer 4, an output impedance which is effectively z ero at the frequency `of the carrier wave, and at the same time substantially suppresses the carrier wave. `In a modulator of this sort the ellect of the load impedance characteristic is therefore principally to supplement the action of the output transformer at the carrier wave frequenc and to provide the lowest possible impe ance for the Waves of speech frequency. l

If instead of a balanced modulator as shown, a si'm le modulator, such as is used for the secon step, were employed the filter would' still provide the desired impedance characteristic by which the maximum power outputis attained. The carrier wave, however, would not be completely suppressed but would be partiallytransmitted with the side of a smaller frequency range for the com munication channel.

The system shown in Fig. 1 may be made to operate in accordance with this method by opening switch 14 in the output circuit of one modulator thereby rendering that modulator inoperative.

In each of the modulation systems described a unique maximum power output is obtained by so arranging the connected load circuit that it provides a short circuit or zero impedance path'to the amplified waves corresponding to each of the input waves, and that it provides a resistive path for the modulation products, equal in resistance to the modulator space path. By the use of a broad band filter structure the equality of the modulator and the load resistance is maintained over a substantial range of flequencies thereby enabling a band of signal waves to be efficiently transmitted.

The operation of a space discharge modulator in producing the maximum slde band power may be described very briefly as follows z--when the amplified currents corresponding to the impressed waves are permitted to attain their maximum intensity in the space discharge path, the maximum side band electromotive force is generated therein, and the maximum side band power output is delivered to a load resistance when that resistance has the same magnitude as the internal resistance of the modulator, or side band enerator.

The m of operation is not peculiar'to space discharge modulation but is broadl applicable to modulating systems in genera In the space discharge modulator the fea.- ture that causes the generation of the modulation products is the variability of the Space path resistance according to the intensity of the impressed forces. In modulators generally, it is the variability of the impedance of some particular element in accordance with the intensityl of the waves by which it is traversed that results in the conditions of the familiar generator law arefrequencies to the second modulator and/ fulfilled.

would appear in the output of the antenna 12 in conjunction with tln` final side band. l The radiated wave would then no longer consist of pure modulated waves but would be accompanied by a carrier wave component, enabling the signals to be detected by simple detectors. The advantages of single side band transmission are, however, retained,

namely an economy of power and the use What is claimed is:

1. A modulating system comprising in combination, a space discharge modulating device having an anode, a cathode and a control electrode, means for impressing simultaneously upon said control electrode a carrier wave and a band of signal waves, whereby,there are generated in the space path of said device side bands of waves en. eration of the modulation products, an in iso having frequencies differing from the carrier wave requency by the fre uencies of .the signal waves, and a broad and wave filter connected to said anode and said cathode adapted to transmit. selectively one of said side bandsto a loadimpedance, said Wave vfilter being so arranged as to presentdevice having an anode, a cathode, and a control electrode, means for impressing simultaneously upon said, control electrode a carrier wave and a band lof signal waves, whereby there are @generated in the space path of said device side bands of waves having frequencies differing from the carrier wave frequency by the frequencies of the signal waves, and a broad band wave filter connected to said anode and said cathode adapted to transmit selectively one of said side bands to a load impedance, said wave filter being terminated at the junction with said device by a shunt impedance branch comprising an induetance and capacity connected in parallel and resonant at substantiall the frequency of said selected side ban s.

3. In a wave transmission System, asource ofvspeech waves, a source of intermediate frequency carrierwaves, means for modulating said carrier waves, by said speech waves, means for selecting from the output r10i said 'modulation means a single inter- 40 mediate frequency side bandV corresponding to said speech waves, a source of high frequenc. carrier waves, a space discharge modu ating device; means for im ressing thereon' to be ii'iodulated said high frequeiicy and said selected lintermediate frequenc side band, whereby there are gen-v erate in said device side band waves of the summation and of the diereuce frequencies of said impressed waves, and an output circuit connectedl to said modulating device including a broad-band wave filter, said filter being adapted to transmit with maximum power waves of one of saidcgenerated side bands and to present to waves of the frequencies impressed upon the space' discharge modulator an impedance that is substantially zero.

4. AIn a wave transmission system, a source `of speech waves, a source of intermediate frequency carrier waves, means for modulating said carrier waves by said speech waves means for selecting from the products o said modulation a single intermediate frequency' sidefband corresponding to said speech waves, a :source of high frequency carrier Waves, alspace discharge modulating device, means for impressing thereon to be modulated said high frequency and said selected intermediate frequencyside-band whereby there are enerated in said ,device side band vwaves o the summation and of the difference' frequencies of said impressed waves, and an outputcircuit connected to said modulating device including a broadband wave filter, said filter bein ranged asto provide substantially s ort-circuit pafhs for waves of the impressed frequencies repeated by said spacedischarge modulator and to selectively transmit waves of said difference frequenc side hand, the impedance thereto being su stantially equal to that of said modulating device.

5. In combination with a space discharge wavemodulator an output circuit including a broad band wave filter, said filter being so arrangedA as to provide short circuit paths for waves repeated by said modulator and to transmit lectivel wave modulation products of predetermined frequencies, offering 1925. y FRANIQIN MOHR.

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