US2294908A - Electric wave system - Google Patents

Description

P 1942- L. w. HUSSEY 2,294,908

ELECTRIC WAVE SYSTEM Filed April 8, 1941 2 Sheets-Shee t 2 Has SIGNAL 2 C(ARRIER E, /7

SIGNAL SIDES/1ND OUTPUT FIG. 9 FIG/0 Q f! (L2 I I /Nl/ENTOR 6 LIV/V0555) TORNEV Patented Sept. 8, 1942 UNITED STATES PATENT OFFICE 2,294,90 ELECTRIC WAVE SYSTEM Luther W. Hussey, Madison,

Bell Telephone Laboratories,

N. J., assignor to Incorporated,

New York, N. Y., a corporation of New York Application April 8, 1941, Serial No. 387,434 14 Claims. (Cl. 179171.5)

work with the resistance element to compensatefor variation in its impedance with frequency.

Another feature comprises proportioning the circuit or mesh in which a thermally sensitive variable negative resistance element is included,

such that the total effective impedance around the circuit is positive,

' In accordance with the invention a variable reon or varies with its temperature is included in a circuit in which an audio frequency or signal wave is to be modulated on a high frequency or carrier wave. The variable resistance element or thermistor is initially biased to a preassigned point on its voltage-current characteristic, and the signal wave is superposed on the biasing current. As the signal wave varies in amplitude, the current flow through the element, and, consequently, its temperature and resistance vary. For high frequency, the element presents a substantially ohmic and constant resistance, but the signal wave, acting as a, .bias varying means, causes the carrier wave to work into a resistance varying in accordance with the signal wave, thereby modulating the signal wave onto the carrier wave. Such a resistance elementwaries in impedance with frequency. To minimize-distortion and to maintain the impedance substantially constant with frequency,-a compensating network is associated with the variable resistance element: With a variable ne ative resistance element there may be a tendency to singing around the circuit or mesh involving the element. This is compensated for by including in the circuit a suitably proportioned positive or constant resistance element.

. for a typical temperature controlled variable resistance element;

Figs. 3 and 4 show the curves of Fig. 2 to a different scale, and characteristic curves of a network to be associated with the variable resistance element, whereby the latters effective impedance variation with frequency will be substantially constant;

Fig. 5 shows a modulation system embodying a single temperature controlled variable resistance element or thermistor in accordance with the invention;

' sistance element whose resistance is dependent A more complete understanding of the invention will be obtained from the detailed description that follows taken in conjunction with the appended drawings, wherein:

Fig. 1 shows typical static and dynamic characteristic curves for a 'typical temperature controlled variable resistance element or thermistor;

Fig, 2 shows typical reactance versus frequency and resistance versus frequency curves impressed on the resistance Figs. 6 and 7 show modulation systems embodying a bridge arrangementof thermistors;

Fig. 8. shows a modulation system embodying a single resistance element in accordance with the invention, and in which both the carrier wave and the signal wave are amplified before being element;

Fig. 9 shows the equivalent electrical circuit for a thermistor; and

Fig. 10 shows the electrical circuit or network to be associated with the thermistor to compensate for the latters impedance-frequency characteristic at audio frequencies.

Variable resistance elements or devices whose resistance is dependent on or sensitive to the temperature of the element are known. Such an element may have a positive or a negative temperature coeificient of resistance, and may have a voltage-current characteristic that is nonlinear. The temperature change in the element may result from the application of heat thereto from an external source, or the withdrawal of heat therefrom; or result from current flow, or change in current flow; through the element. An element of the character described and whose resistance at any particular moment is markedly dependent onits temperature has come to be known, in recent years, as a thermistor.

Typical characteristic curves for a thermistor having a negative temperature coeflicient of resistance, and comprising a thin layer of a semiconductive material, such as uranium oxide, engaged by a relatively small-area contact, for example, of chromium, are shown in Fig. 1. A thermistor of this type, together with others, is disclosed in the G. L. Pearson Patent No. 2,276,864 of March 17, 1942, If the thermistor is subjected to a direct current of increasing magnitude, the voltage drop across it is found to increase to a maximum and then decrease. Curve A is'the static voltage-current characteristic of a typical thermistor.

Dynamically, the alternating current resistance is negative in the region beyond the voltage maxi- If a mum Em for sufficiently low frequencies. direct current lb, of value higher than rent corresponding to Em), is caused to flow through the thermistor, a superposed alternating current of frequency approaching zero will trace out a curve aob, approximately the static characteristic. As the frequency increases, the thermistor experiences increasing difficulty in changing in temperature, and, consequently, in resistance, with the increasingly rapid change in current flow; eventually, a frequency is reached at which the voltage-current curve is along the ohmic resistance line cod. For low frequencies, therefore, the thermistor offers a substantially linear negative resistance to the current, and at high frequencies a substantially linear positive resistance. At intermediate frequencies, the superposed alternating current will produce voltage-current curves along the broken lines e, f, g in the order of increasing frequency.

The characteristic of thermistors in offering a substantially linear negative resistance to low frequencies and a substantially constant positive resistance to high frequencies enables'the thermistor to be used for linear modulation purposes. In conventional modulators, linear modulation is obtained by making the carrier wave amplitude much larger than the signal wave amplitude. Then the system approximates one in which the resistance to the signal is varying at the frequency of the carrier wave. Under such conditions, distortion components at frequencies near the car-- rier and the desired side-bands are eliminated or minimized. In a thermistor modulator, the relative magnitude of the carrier and signal wave is not of primary concern; the signal wave works into a substantially linear negative resistance, and it varies a positive resistance facing the carrier.

With reference to Fig. 1, let the thermistor be biased sufficiently so that its operating point is 0. As already explained, as the frequency with which the bias is caused to vary plus and minus about the value of bias for the point 0, is increased, the

current-voltage characteristic will vary from that of aob to that of cod. Thus, the resistance of the thermistor for low frequency is negative, given by do, d1 at i=io, 11:00, and the impedance of the thermistor for high frequency approaches the direct current resistance If a high frequency carrier wave and a low frewhere i and v are the total current and voltage and n 4. Neglecting thermal lag, the low frequency resistance is The high frequency and direct current resistance (the cur-- If signal wave currents (is) and carrier wave currents (in) are supplied to the thermistor, the high frequency voltage across the thermistor will be v=ica(i0+is) =icR0(l+is/io)"' Ro[1(1L+1)/ioi5]ie. lisl i0 The equivalent electrical network of a negative resistance thermistor, as shown in Fig. 9, is a negative resistance (R1) in series with an inductance (L1), the series resistance and inductance being in parallel with a positive resistance (R2). The impedance of the thermistor varies with frequency for low or audio frequencies, as illustrated by the curves of Fig. 2, showing reactance versus frequency and resistance versus frequency characteristic curves X1 and RT for a typical thermistor. The constants of the equivalent network were R1=13260 ohmsv R2=25000 ohms L1 =.1'l henries shown in Fig. 10, may comprise a resistance (Re) and an inductance (Le) connected in series, a capacitance (C1) being connected in parallel with the series resistance and inductance.

With the following constants: R3=30000 ohms, 3 11245 Ll-f and henries, the resistance versus frequency characteristic Rs, and the reactance versus frequency characteristic XS of Figs. 3 and 4, respectively, are obtained. The characteristics of the compensating network are the negative to a fair approximation of those of the thermistor, and their combination Xe and Re provides a substantially constant impedance over a substantial portion of the audio frequency range. If more exact compensation, or a constant, pure negative resistance is desired, a somewhat more complex compensating network could be associated with the thermistor.

Fig. 5 shows a modulator circuit embodying the invention. It comprises a thermistor T, a source In of high frequency or carrier wave, a series resonant circuit N1 resonant to the carrier wave, and a load L. Also connected across the thermistor are a source ll of current for biasing the thermistor to a desired point on its voltagecurrent characteristic, preferably on its substantially linear negative resistance portion, a source 20 of audio frequency or signal wave, and a parallel resonant circuit N2 antiresonant to the carrier wave. The resistance R, is an ohmic resistance, or one having a positive temperature coefficient of resistance, and is proportioned so that the total effective impedance around the mesh involving the thermistor will be positive to minimize possibility of singing around the thermistor mesh. The network N is a compensating network of the type discussed with refezence to Fig. 10, and enables the thermistor to present a substantially constant impedance to signal waves over a substantial portion of the audio frequency range.

The modulator circuit of Fig. 6 employs a plurality of thermistors arranged in a bridge. The signal wave is impressed through the transformer it across one pair of terminals of the bridge, and the carrier wave is impressed through a second transformer it across the other pair of bridge terminals. The source ll provides biasing current for the thermistors. Resistance Rm is proportioned so that the total of effective impedance around the mesh involving the thermistors is .positive, and is by-passed to carrier by the series [inductance and capacitance Na. A compensating network N of the type shown in Fig. 10, is also included in the circuit. The condenser C is a blocking condenser, and the network N: comprises a resistance by-passed to the side-band, to ofier high impedance to the signal wave.

Fig. 7 shows a double balanced bridge.modulator embodying the invention. Each arm of the bridge comprises a thermistor T, and network N in series, proportioned to compensate for the impedance-frequency characteristic of the thermistor, and providing a total eflective impedance around the thermistor mesh that is positive, and low impedance to the carrier wave and the modulation product's. Current for biasing the thermistors to the desired point on their voltage-current characteristics is provided by the-current source ii. The carrier wave is impressed across a pair of terminals of the bridge through the transformer ii, the modulated carrier wave is taken oil from the other pair of terminals of the bridge through the transformer i8, and the audio frequency or signal wave is impressed on the modulator through the transformer l9, the ends of the secondary winding of the latter being connected to the mid-points of the bridge windings of transformers ll, it. The condensers C and condensers C20 are blocking and by-pass condensers, respectively.

Fig; 8 shows a circuit-arrangement enabling amplification of both the carrier wave and the signalwave before modulation. The thermistor T is connected in the plate current of anamplifying electron discharge device V1, impedancecoupled to a succeeding electron discharge device V2. If the device V1 is a high impedance tube such as a pentode, a high order of stability is obtained because of the large series resistance in the signal and the direct current circuit and good negative resistance element whose impedance varies with frequency, and a network compenbeing included in said circuit in serial relation to said element and comprising parallel connected. inductance and capacity elements.

3. A circuit comprising a resistance element whose equivalent electrical circuit is a positive resistance shunted by a negative resistance and a reactance in series, and a network connected in series .with said element and comprising a reactance shunted by a resistance and a reactance in series.

4. A circuit comprising a resistance element whose equivalent electrical circuit is a positive resistance shunted by a negative resistance and an inductance in series, and a network connected in series with said element and comprising a capacitance shunted by a resistance-and an inductance in series.

'5. A circuit comprising a. mesh including a thermally sensitive negative resistance element, and means to prevent self-oscillation around said mesh comprising means to provide a positive total effective impedance around the mesh involving said element. v

6. A modulator circuit comprising a mesh. including a thermally sensitive negative resistance element, and positive resistance means to prevent 2. A circuit comprising a thermally sensitive self-oscillation around said mesh comprising means to provide a positive total effective impedance around the mesh involving said element.

7. A modulator comprising a resistance element whose resistance changes with its temperature, means to biac said element to a stable temperature, a source of low frequency signal wave to vary'said temperature and consequently the resistance of said element, and a source of high frequency wave whose amplitude varies in accordance with th variation of said resistance when said signal wave is applied to said resistance element.

8.ln combination, a source of high frequency electric wave, a source of low frequency electric wave, and a resistance element whose resistance is variable with its variation in temperature and that offers a linear resistance to said high fresignaling efiiciency without sacrifice of any gain in the amplifying device. If the. device V1 is not a high impedance device, it may be desirable to introduce the compensating network described with reference to Fig. 10 in series with the thermistor between it and the anode of the tube.

with a modulator of the type described there is no necessity for relatively large carrier wave amplitude for linear modulation, there is only small distortion at and around the carrier wave harmonics, and there is the possibility of signal to side-band gain without the use of relatively expensive devices such as vacuum tubes. Although the invention has been disclosed with reference to several specific embodiments, it will be understood that it is not limited thereto but is of a scope evidenced by the appended claims.

What is claimed is: l. A circuit comprising a thermally sensitive negative resistance element whose impedance varies with'frequency, and a network for compensating for such impedance variation, said network being included in said circuit in serial relation to said element and comprising parallel connected positive and negative reactances.

quency wave and a difierent linear resistance to said low frequency wave.

9. In combination, a source of high frequency electric wave, a source of low frequency electric wave, and a resistance element whose resistance is variable with its variation in temperature and that offers a linear resistance tosaid high frequency wave and a different linear resistance to said low frequency wave, said resistance element varying in impedance with frequency, and means to compensate for said impedance over a sub stantial portion of the audio frequency range.

10. In combination, a source of high frequency electric wave, a source of lowfrequency electric wave, and a resistance element whose resistance is variable with its variation in temperature and that offers a linear resistance to said high frequency wave .and a different linear resistance to said low frequency wave, said resistance element tending to cause singing around the'circuit in which it is involved, and means comprising positive resistance to provide a total effective impedance that is positive around the circuit.

11. An electric wave system comprising a reparallel connected inductance and capacity ele- 5 ments, and a positive resistance in series withsaid resistance element; a source of high frequency electric wave connected across said resistance element; and a source of low frequency electric wave connected with said resistance ele- 10 ment.

12. A circuit comprising a variable resistance unit, means for applying an audio frequency signal current to said unit, said unit having a substantial negative temperature coefiic'ient of re-v l5 sistance, and being adapted to change in tem-.

perature and resistance substantially instantaneously with change in the amplitude of the signal current over substantially the entire audio frequency range, and said unit varying in imped- 20 ing the temperature of the unit such that the change in temperature of the lmit when the sig-.

ance to the signal current with variation in the frequency of the signal wave, current means for adjusting the temperature of the unit such that the change in temperature of the unit when the signal current is applied thereto, varies the re- 25 ing included in said circuit in serial relation with said unit and comprising parallel-connected positive and'negative reactances.

13. A circuit, as claimed in the preceding claim, in which the electrical analogue 01' the resistance unit is a positive resistance shunted by a negative resistance in series with an inductance, and

in which said compensating network comprises inductance and capacitance elements.

14. A circuit comprising a variable resistance unit, means for applying an audio frequency signal current to said unit, said unit having a-sub- I stantial negative temperature coeflicient of resistance and being adapted to change in temperature and resistance substantially instantaneously with change in the amplitude of the signal wave over substantially the entire audiofrequency range, and said unit varying in impedance to the signal current with variation in the frequency of the signal current, current means for adjustnal current is applied thereto, varies the resistance substantially linearly above and below that for the adjusted temperature, and a damped reactive network for compensating for theimpedance variation of -said unit with variation in the frequency of the signal current.

. LUTHER W. HUSSEY.

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