US2426996A - Frequency modulation - Google Patents

Description

Sept. 9, 1947. w. M. GOODALL 2,425,995

' FREQUENCY MODULATION Filed Aug. 17, 1944 2 Sheets-Sheet 2 PFC - 34 1% U8 2, F 1' J T a 4 r 22 2/ AMP. 33 30 AND "1 ER :5 FILTER T l T SMELL qsc. 2

A 7' TOR/VEY characteristics respectively produce phase shifts in opposite directions.

.duced for quite small from the following detailed Patented Sept. 9, 1947 FREQUENCY MoDU'LATionj v I 1 William M; Good-all, oakhurstl-Nul, assignorjto- I Bell Telephone Laboratories, 7 Incorporated;

New York, N. Y., a corporation of New York Application August 17, 1944, SeriaI No. 549.853 I 6 Claims. (01. Iva-#1715) This invention relates to an improved arrangement for generating high frequency oscillators and modulating their frequency by speech or othersignal waves.

Among the objects of the invention are to increase the'range or degree of modulation and to minimize amplitude variations of the frequency modulated Wave.

In accordance with the invention, two vacuum tube amplifying stages, each provided with local negative feedback, are connected in tandem, the output circuit of each being coupled to the input circuit of the other so as to form a closed loop in which oscillations are generated. The local feedback circuits for the two tubes areso arranged;

by means of appropriate impedance elements, thatthe two stages have rising and falling gain and, consequently, The fre uencyof oscillation of the system is determined by the characteristics of the'local feedback circuits and by themutual conductances of the tWo tubes. .Variation of the frequency is accomplished by applying signal voltages of opposite phases to the tubes so that the conductance of .one is increased and that of the other is simultaneously reduced by a corresponding amount. By this means large frequency swings can be pro- Values, of the modulating signal voltages. Further, the loop gain of the system remains constant at the different oscillation frequencies, with the result that factors tendtially eliminated. I I 1 The invention will be more fully understood description and by reference to the accompanying drawings, of which:

Fig. 1 is a simplified schematic of the-invention; Fig.2 is a more detailed circuit of one form of the invention; and

- Fig. 3 illustrates a modified tion.-

The circuit shown in Fig. 1 represents an ele- .ing to produce amplitude variations are substan form of the invenrnentary form of the invention and is intended primarily to illustrate the general principles by virtue of which large degrees of frequency modulation are obtained with negligibly small amplitude variations. For this reason only those elements necessary for the explanation of the principles are shown. Energizing circuits and blocking and 'by-pass impedances may be included oscillation path.

where necessary in accordance with familiar practice'.-

Theoscillation generatorcomprises two vacuum tubes V1 and'Vz, preferably screen. grid tubes or pentodes', which are coupled together serially by equal resistors R1 and Rz'to form a closed feedback loop. Reactive impedances Z1 and Z2 consisting of an inductance and a capacity respectively provide local degenerative feedback around each tube and together establish a definite frequency of oscillation. For'the purpose of modulation a source Ill of speechor other signals is connected through a transformer H-havingtwo secondary windings to the grids of the two tubes in sucha way as to apply thereto signal voltage substantially equal in magnitude and opposite in phase. "High frequency chokes l2 isolate the signal l2" serve to circuits from th high frequency The properties of the circuit readily appear fromaconsideration of the loop gain around the oscillatorifeedback.path. This quantity, which will be denoted by pd has the value for the, circuit'shown, j

, 1'+g.z. (1+g.z2)

where 91 and gz are the. transconductances of tubes V; and V2 respectively and R is the resistance of equalresistors R1 and R2. For-the case illustrated where Z1 and Zz'consist of an inductance L and a capacity C respectively, Equation The frequency at whichthe systemoscillates is that for which the phase angle of as is zero or for which'the imaginary termin Equation 2 disappears. -'Its value, denotedbywq+121| is given by a I V91 LC" The magnitude of th m g at this frequency hasth i gm r 7 7 great as unit to enquency'is variedand hence that frequency modulation is effected Without concurrent amplitude modulation.

Assuming the tubes to be similar and equally biased so that in the absence of signal voltages their transconductances are substantially equal, then the application of equal signal voltages in phase opposition to the two grids will change the transconductances by about equal amounts in opposite senses. So long as the variation-is relatively small, for example per cent or less, the product of the transconductances will remain very nearly constant.

The frequency variation is closely proportioned to the applied signal voltage and, as follows from Equation 3, is substantially proportional to the percentage change in the transconductance of either tube. Thus a signal voltage which produces a five per cent increase in the transconductance of one tube and a decrease of five per cent in the other will produce a frequency swing of substantially five per cent. Since variations as great as ten per cent or even more can be readily accomplished in commercial vacuum tubes, very large degrees of frequency modulation may be achieved.

To insure an-adequate amount of over-all feedback so that the loop gain as expressed by Equation 4 will be greater thanunity at the oscillation frequency, it is desirable to make the local feedback impedances Z1 and Z2 small relatively .to the coupling impedances R1 and R2 and to make the latter fairly high.

The system shown in Fig. 2 is similar in .general to that of Fig. l, but'the circuit is shown in more complete detail and includes certain modifications for better operation at highfrequencies. Tubes V1 and V2 are similar pentodes and are coupled serially to form a closed feedback loop by means of similar tuned impedances Z, Z blocking condensers l3, I4, and grid leak resistors l4, [4. Local feedback impedances Z1 and Z2 consist of an inductance and a capacity respectively as in Fig. 1. A resistance shunted by a by-p'ass condenser is included in series with inductive impedance Z1 and a similar by-passed resistance IS in series with a high frequency choke coil 15 is shunted around the capacitive impedance Z2. Thesecircuits provide bias voltages for the control grids of the two tubes in a well-known manner. Plate potentials are supplied to the tubes through the coupling impedances Z, Z, as indicated. The screen grids are energized through resistors l1 and I1 and are by-passed to ground through condensers l8, l8. Modulating potentials are supplied to the control grids in phase opposition through audio transformer H and high frequency chokes l2, l2 as in Fig. l. The modulated output may be taken from the system by a circuit connected to the plate of either tube, for example, between conductor l9 and the negativ terminal, -B, in the figure. v

The substitution of the tuned coupling impedances Z, Z for resistors R1 and R2 of Fig. 1 has the advantage that the magnitude of the impedance may be kept high at high oscillation frequencies in the presence of parasitic tube capacities. It is desirable that the impedance should have a broad-band characteristic in the sense that it should have very little reactance and should have substantially constant resistance at all frequencies in the operating range. There are many ways of realizing such a characteristic, for example, by the use of resistance terminated band-pass filters as two-terminal impedances. The arrangement illustrated comprises a series resonant circuit and an antiresonant circuit connected in parallel and both tuned to the mean oscillation frequency of the system. A high resistance added in parallel with the resonant circuit as shown serves to smooth out the frequency characteristic of the impedance and to reduce its reactance. If desired these resistors may be omitted and their smoothing effect may be obtained by giving the grid leak resistors l4, l4 appropriate resistance values.

In Fig. 3 is shown a modified form of the invention in which the modulating signals are applied to the suppressor grids of the vacuum tubes instead of to the control grids. The figure shows also how the central frequency of the oscillator may beheld substantially constant.

The oscillator circuit is similar to that of Fig. 2 in most respects and need not be described further. The output from the plate of V2 is taken by conductor l9 to cathode follower tube V3 in the cathode lead of which is included a tuned impedance Z0 which may be similar to the coupling impedances Z, Z. The output of the system is taken from the terminals of this impedance between conductor 20 and ground.

For the purpose of frequency control part of the output is taken by conductor 2i to a mixer tube 22 which is also supplied with oscillations from a stable frequency oscillator 23. The difierence frequency oscillations resulting from the mixing operation are supplied to intermediate frequency amplifier 24 and from thence to a discriminator 25. This discriminator may be of any of a number of Well-known types, but it is desirable that the detectors and their output circuits be balanced with respect to ground so that two equal control voltages of opposite sign may be obtained. In the arrangement shown the discriminator comprises two slope circuits 26 and 21, preferablyof very steep slope, whose characteristics intersect at the desired intermediate frequency, rectifiers 28 and 29, and equal output resistances 30 and 3! together with suitable filtering condensers. The balanced control voltages, which appear when the oscillator mean frequency departs from its assigned normal value, are-applied through conductors and 33 and high frequency chokes 34, 35 to the control grids of the tubes so'as to produce symmetrical changes in the transconductances tending to restore the frequency to its proper value.

What is claimed is:

l. A generator of frequency modulated oscillation comprising two amplifying vacuum tubes having input terminals and output terminals, separate circuits coupling the output of each tube to the input of the other to form a closed transmission loop in which high frequency oscillations are generated, reactive impedances coupling the input and the output circuits of each tube and producing degenerative feedback therein, said impedances comprising reactances of unlike signs at the oscillator frequency and jointly constituting the principal frequency-determining elements of the oscillating system, a source of signal oscillations and circuit means for impressing signal voltages from said source upon the tWo tubes in phase opposition whereby their transconductances are varied simultaneously in opposite senses.

2. A generator of frequency modulated oscillations comprising two vacuum tubes each having an anode, a cathode and a, control grid, circuits coupling the anode of each tube to the grid of the other to form a closed loop in which high frequency oscillations are generated, a. reactive impedance connected to the cathode of each tube and coupling the grid and anode circuits thereof, said impedances comprising reactances 0f unlike signs at the oscillation frequency and serving jointly to determine the frequency of oscillation, a source of signal oscillations, and circuits coupling said source to the grids of said tubes in phase opposition.

3. A generator of frequency modulated oscillations in accordance with claim 2 in which each of the said vacuum tubes is provided with an additional control grid isolated electrostatically from the other control grid by a screen grid and in which the signal oscillations are applied to said additional control grids.

4. A generator of frequency modulated oscillations according to claim 2 in which the frequency determining reactances consist respectively of an inductance and a capacity.

5. A generator of frequency modulated oscillation comprising two screen grid vacuum tubes, resistive impedances coupling the anode circuit of each tube to the grid circuit of the other to form a closed oscillation circuit, reactive impedances connected respectively to the cathodes of said tubes and degeneratively coupling the input and output circuits thereof, said impedances having reactances of opposite sign at the oscillation frequency and together determining the frequency of oscillation, a source of signal oscillations, and circuits coupling said source and said tubes for impressing signal voltages in phase opposition thereon whereby their transconductances are varied simultaneously in opposite senses,

6. A system in accordance with claim 5 in which the resistive coupling impedances comprise multiple tuned circuits with damping resistance providing a broad resonance characteristic centered in the operating frequency range and in which the reactances of the local feedback impedances are small relatively to the resonance resistances of the coupling impedances.

WILLIAM M. GOODALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,031,224 Moullin Feb. 18, 1936 2,300,996 Vanderlyn Nov. 3, 1942 2,304,388 Usselman Dec, 8, 1941 2,312,977 Percival Mar. 2, 1943 2.326314 Usselman Aug. 10, 1943

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