US2284402A - Modulation circuit - Google Patents

Description

y- J. M. MANLEY an. 2,284,402 I a MODULATION CIRCUIT Filed Aug. 27 1940 h/ms mom P FIG. 2 I

I MODl/L/PT/IVG; 1 W/ME I 1 a Y mwuwss J. u. MANLEY WVENTORS L.R. WRATHALL ATTORNEY Patented May 26, 1942 UNlTED srar MODULATION CIRCUIT Jack M. Manley, East Orange, and Leishman R. Wrathall, River Edge, N. J., assignors to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application August 27, 1940, Serial No. 354,373

9 Claims.

This invention relates to modulation of carrier frequencies for radio or other signal transmission purposes and, more particularly, to phase modulation of a carrier frequency. The invention is applicable to such phase modulation circuits as are shown in the copending application of Wrathall, Serial No. 354,357, filed August 27, 1940, and its purpose is to improve phase modulation by providing means to neutralize certain distortion effects present in some forms of such modulation circuits. The invention will be better understood by reference to the following specification and the accompanying drawing, in which:

Fig. 1 shows a circuit adapted to operate in accordance with our invention;

Figs. 2 to 5 are curves explanatory of our invention;

Fig.6 shows a modification of a portion of the circuit of Fig. 1; and

Figs. 7 to 9 show further modifications of Fig. 1.

Referring more particularly to Fig; 1, there is I shown a source P of carrier frequency which is associated with a circuit containing condenser.

C1, linear inductance L1 and non-linear inductance L. Across the non-linear inductance L are connected in series the condenser C3 and resistance R3 and as thus described the circuit constitutes the circuit for the magnetic harmonic generator described in the copending application referred to. I

When the magnetizing force of the input wave is P cos (pt+0) as in curve a of Fig. 2, the output of the coil consists of aperiodic succession of pulses, as indicated by the vertical solid lines. Analysis shows that this group .of sharp impulses consists of a large number of harmonic terms, the frequency of which can be represented by zip/2w where p/21r is the frequency of the source P and n takes on successive integral values. now a biasing magnetizing force of magnitude 1), such as-curve b of Fig. 2, is added to the nonlinear coil L, analysis shows that theforms of these pulses are unchanged and that to areasonable approximation the heights of the pulses are unchanged. The principal effect of the bias is to Where Q is the amplitude of the signal and (1/271' is .its frequency. The extent of the shift of sin qt pulses in Fig. 2 is approximately proportional to the instantaneous value of the modulating signal wave so long as this value is kept small compared with P. Analysis of this succession of variably displaced pulses shows that it consists of harmonics of q and p and side-band frequencies (npimq) /21r but only those frequencies are present for which (m-l-n) is odd, this being so because of the odd function property of the magnetic characteristic.

As pointed out in the copending application re? ferred to above, any one of the carrier freline and the other branch as a dotted line.

quencies, corresponding to different values of 11., with its side-bands takes on some of the characteristics of a phase modulated wave but is defective because of the absence of certain sidebands. Also as 'pointed out'in the copending application, this defect can be overcome by suppressing one set of peaks, such as the lower peaks of Fig. 2, which suppression serves the purpose of introducing the otherwise absent sidebands. One method of suppression of the undesired peaks is by the insertion of a rectifier R across the load circuit, as shown in Fig. 1.

In our investigations we have found thatcere tain distortion effects appear in the output of the modulating device. One of theseis due to the hysteresis which is characteristic of the iron forming the core of the coil L. The hysteresis loop of such a .core is illustrated in the B-H curve of Fig. 3. We find that, whereas it is desirable that the peaks should occur at times cordisplacement of the pulses is not quite in accordance with the simple relationships given above.

The phase displacement would, as a matter of fact, be given by f) 5- P u for the pulse occurring at f. In this Fig. 3 one branch of the hysteresis loop is shown as a full The interest is confined to one of these, say the full line, inasmuch as the peaks due to the other branch are to be suppressed by the rectifier R;

We have found that we can correct for the dis corresponding to f to that corresponding to zero magnetizing force at 0. The direction of this current will. depend primarily on the poling of the rectifier which. suppresses the unwanted by the rectifierlR.

peaks. Such a direct current will also, of course,

shift the time of the pulse corresponding tothe point 9: in the same direction but this has no' deleterious effect for the reason that the rectifind present, due to the timing of the pulses with respect to the exciting current. This will be better understood by reference to Fig. 4, which is for somewhat idealized conditions butis qualitatively descriptive of what occurs. If, in this figure, the full line 1 represents the current from coming from a battery l5. A choke coil I6 is included in the circuit of suflicient magnitude to prevent appreciable flow therethrough of the signal current,'the carrier frequency and its barthe exciting source flowing through the coil Ii,

then the period ofmaximum reactance of the coil is in the region 2 where the current is near its zero value but has its maximum rate of change. The induced electromotive force in the coil may at that time be represented 'by'the pulse wave .3 and it is for the duration of this voltage peak that the condenser C3 is charged. At the curve, that isat f or g of Fig.v 3. It will be noted that this delay in timing of the peak, re-

sulting from the discharge of the condenser, is

in the same direction as the delay due to the hysteresis. Thus the accompanying distortion maybe corrected by an additional direct current biasing effect such as might be provided by an additional winding l2 on the coil. L. Again it will be observed that the shift of one of the 'pulses toy is of no significance inasmuch as the pulse corresponding to this point is suppressed A further explanation of the invention will be apparent by reference to Fig. 5 in which the curve gives .the relationship between the phase displacement aand the signal biasing voltage b,

being for ideal conditions where the distortions referred to above are not present. it is desirable in the operation of this modulator that 6 shall be linearly proportional to b, it is evident that, operation should be limited to the Since monics.-

Fig. '8 shows a further modification of Fig. 1, the biasing current in this case being obtained by rectifying a small portion of the carrier frequency; To this end a rectifier 2| in series with resistances 22 and 23 is bridged across the terminals of the generator. A choke coil 24 is connected from one side of the rectifier in one terminal of the coil L. This will give rise to a small direct current through the coil L in the proper direction to correct distortions of thewanted peaks. 7 V V Fig. 9 shows a further modification of Fig. 1 in which the bias is obtained from the direct current due to the rectifier which is insertedfor removing the one set of pulses. In Fig. 9 there is shown in series with the suppressing rectifier R a resistance 21 and condenser 28 in parallel. The condenser 28'is of sufficient capacity to offer low impedance to the carrier frequency and its harmonics which make up one of the peaks. is noteworthy thatwith this circuit connection the direct current component will flow in the right direction through L to give the desired biasing effect.

It should be mentioned that we find the amount of distortion increases as the amplitude of the exciting base frequency increases. Ac-

- cordingly, the correcting biasing current should linear portion of the curve of Fig. 5. In view of the distortions referred to above, the timing of the pulses is so delayed that one effectively operatesabout the point 0' instead of the point 0 on the curve'of- Fig. 5. Thus the range over which 6 is proportional to b is decreased on the one side and increased on the other, so limiting the permissible amplitude of signal wave which can be used without producing further distortion.

In order to operate over as wide a linearrange 'the biasing currents of Fig. 1.

While in Fig. 1 the coils H and 12 have been shown as separate, for a better understanding of the invention, actually in practice onewould 'usefbut one coil such as coil I3 of Fig. 6, the magnitude of the current flowing through that lcoil being adjusted to give the desired bias.

Still other expedients for obtaining this bias also increase. The circuits of Fig. 8 and Fig. ,9

automatically make this provision in that as the amplitude of the source P increases, the rectified current also increases.

7 In the description above, reference has been scribed in the Wrathall application, C3 is made very largeand R): is also made large. In this case the current flowing through R3 is verysmall may be desirable, 'such as shown in Figs. 'Ito 9.

For example, in the circuits of Figs. 1 and 6 and with the high frequencies contemplated, the adand the voltage across it is substantially the same as that acrosslth'e coil L corresponding ,tothe .curve 3 of Fig. 4. At no time is there any appreciable: voltage presentin the condenserCa; In this=mo,de of operation, whichwe have called open circuit operation, the distortion arising from the shift from jto f is not present and the correction is then mainly confined to that due to hysteresis. Y

vWhat is claimed is:

1.' In a phase modulating frequency multiplying system, 'a source of base frequency waves, a harmonic generator excited thereby, a source of signal waves, means to-impress the signal waves upon said generator to produce phase modulated waves of high frequencies 'relative to said base frequency, said generator possessing inherent tendency toward unsymmetrical operation with consequent tendency toward production of distortion in the modulated waves, direct current biasing means for applying a bias to said generator that is small in comparison with the maximum amplitude of the signal wave and of a magnitude and polarity such as to compensate said distortion, and means to utilize the phase modulated waves for signal transmission.

2. In a phase modulating system, a source of neutralize distortion effects in the harmonic gen-.

erator and a circuit for utilizing the phase modulated waves.

3. In a phase modulating system, a source of base frequency waves, a magnetic harmonic generator comprising a non-linear coil excited there-- total range of magnetization covered by the ama plitude range of the signal by such amount. and

in such direction as to compensate said distortion.

4. In a phase modulating system, a source of base frequency waves, a magnetic harmonic generator comprising a non-linear coil excited thereby, a source of signal waves, means to apply said base frequency waves and said signal waves to said generator to produce output impulses, means to derive from said output impulses carrier frequency waves whose phase is modulated in accordance with said signal waves, said base frequency waves being of su'fiicient amplitude to swing past the knee of the magnetization curve of said generator, whereby the system tendsto shift the timing of the output impulses and cause distortion of the modulated waves, and means to apply a relatively small unidirectional bias in said coil of such magnitude and direction as to compensate said distortion.

' 5. In a phase modulating system, a source of base frequency, a magnetic harmonic generator comprising a non-linear coil excitedthereby, a source of signal waves, means to impress said signal waves upon the generator to produce phase modulated waves of carrier frequencies, a load coil to suppress impulses of one polarity, and means for applying a magnetic bias to the core of the coil to neutralize distortion due to hysteresis of the coil core and due to the delay of the condenser discharge in the load circuit of the harmonic generator because'of its charging time.

6. In a system for obtaining phase modulated carrier frequency waves comprising a source of base frequency, a magnetic harmonic generator comprising a non-linear coil excited thereby, a load circuit associated with the coil including a series condenser and resistance, a source of signal waves associated with'the coil in parallel to the source of base frequency whereby there are generated positive and negative pulses of a frequency corresponding to the base frequency and displaced in phase in accordance with the amplitude of the signal wave, means associated with the coil for suppressing peaks of one sign whereby there is produced in the circuit a plurality of harmonic carrier frequencies each of which is phase modulated in accordance with the signal wave, and means associated with the circuit to neutralize distortion effects in the coil arisingfrom the hysteresis of the coil core and the'delay of the condenser discharge in the output circuit associated with the non-linear coil because of its charging time.

'7. The combination of claim 6, characterized in this that the means forneutralizing the distortion comprises a rectifier circuit associated with the source of base frequency, the magnitude of the direct current through the non linear coil being adjusted to compensate for the said dis tortions.

8. In aphase modulating system, a source of base frequency waves, a sourceof signalwaves, a saturable core inductance, means to impress both said base frequency waves and said signal suppressing impulses of one sign, a load circuit circuit coupled to said coil and including a reconnected to said rectifier and coil for utilizing the resulting phase modulated waves, means producing a steady biasing magnetism in the core of said coil to compensate distortion arising from hysteresis of said core.

9. A system according to claim 8 comprising meansderiving said bias magnetism from a direct current and connections for supplying said direct current from said rectifier.

JACK M. MANLEY. LEISHMAN R. WRATHALL.

sistance and a capacity of such magnitudes that v the capacity has appreciable charging time, a unilateral impedance element shunted across the

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