US2288817A - Oscillation modulator - Google Patents

Description

July 7, 1942. A

J. B. MAGGIO OSCILLATION MODULATOR Filed July 18, 1939 1 MODULATED 75 ,0 LI L I SIGNAL CA HRIER v 0U TPU T OUTPUT GRID 8M5 FIG. 3

INVENTOR J. B. MAGG/O A TTORNEY Patented July 7, 1942 NT OFFICE OSCILLATIQN MODULATOR John B. Maggio, Brooklyn, N. Telephone Laboratories,

York, N. Y.,

a corporation oi New York Y., assignor to Bell Incorporated, New

Application July 18, 1939, Serial No. 285,033 1 Claim. (01. 179-1715) This invention relates to oscillation modulating systems and particularly to means for controlling the transmission characteristics of such systems. r I

The purpose of the invention is to improve the over-all efficiency of thesystem without sacrificing the quality and fidelity of the transmitted signal. A particular object is to obtain efiective modulation of a carrier frequency with a minimum of apparatus and a minimum of expended power. More specifically, the invention consists in stabilizing an oscillation generator at a point on its operating characteristic where a'relatively small impressed signal frequency voltage will cause large changes in oscillation amplitude in such-manner as to yield a well and highly modue lated oscillation wave. The invention will be better understood by reference to the following specification and the accompanying drawing in which:

Fig. 1 shows a typical output characteristic of a simple oscillator circuit;

Fig. 2 is a circuit showing one specific form of the invention; and

, Figs. 3, 4 and 5 are modifications thereof.

Referring more particularly to Fig. 1 there is shown a curve of alternating current output versus control grid bias for a simple oscillator. circuit. Normally the output of an oscillator will be that output at which overloading causes the gain to be so reduced that a stable condition has been reached-where the amplitude of oscillations is fairly constant.

ordinarily in Fig. l by such a point as a. on the curve and under these conditions the oscillator is I operating over a portion of its characteristic involving large curvature. Modulation under these conditions is not only inefllcient but involves a very considerable amount of distortion and change in oscillation frequency. It is possible,'

however, by the introduction of automatic oscillation control to stabilize the output at some point such as b on the steep and nearly linear portion of the curve of Fig. 1, where the amplitude is sensitive to small changes in grid bias. It is evi-- dent, however, that stable operation at such a point as I: could not be secured without some form of automatic control.-

It now a varying voltage, such as a signal varying at too higha rate for such an automatic control circuit to follow, is introduced in the oscillator circuit at some suitable point, either in series with a control grid or with a screen grid or in the plate circuit itself, its .efiect would .be to alter This would be represented,

the point b at the rate and by a magnitude determined by the amplitude and frequency of the external signal voltage' If the signal frequencies are too high for the control circuit to follow, the

oscillator amplitude will vary widely, yielding a modulated "wave. Thus in connection with Fig. 1 it is evident that 100 per cent modulation will be obtained for a change in equivalent grid voltage equal to c. For a suitably chosen tube the voltage e may be only a'fraction of a volt and since no grid current is flowing, no appreciable power is required from the signal source. Even the output-of a microphone or similar device will be sufficient to control a relatively large amount of high frequency output power.

Fig. 2 shows a circuit illustrating one specific form of the invention. In this figure a simple triode tube I0, is connected as an oscillator, the

formal circuit being one which comprises 'the tuned circuitL1C1 in the plate circuit and a-coupling condenser H from the output to the input. In order to stabilize the circuit a bias is maintained by a charge on the condenser C2, the magnitude of this bias being determined by a rectified voltage which in turn is obtained from the output circuit of the oscillator. In thisfigure such rectified output is obtained by connecting across the output of the oscillator the input of a vacuum tube rectifier circuit. This rectifier may take on a large variety of forms'but is here shown as consisting of the vacuum tube l2, the input of which is connected over the large resistance Hi. The output of the tube l2 includes the resistance R2 with theby-pass condenser C2, as a result of.

which there is a direct current voltage across the resistance R2 and the condenser C2, the magnitude of which increases as the amplitude of oscillations impressed on the grid of tube I2 is increased. This direct current voltage is then used as a grid bias for the tube III, the connection being such that if the amplitude of the oscillations increases, the voltage drop across R2 in creases and the negative bias of the oscillator increases, thus-tending to keepthe oscillations independent of slow fluctuations in any portion of the oscillator circuit and permitting stable operation about such a point as b. I

The signal with which the oscillations areto be modulated may be impressed ata number of points in the oscillator circuit but I find that one I of the most advantageous points is to introduce the amplitude up and down on the curve about 55 it in the grid circuit of the oscillator, as shown at the transformer T2. I

In general, any voltage changes impressed on the grid from the signal circuit will vary the impressed on G3.

yielding a modulated high frequency wave in the output circuit. Furthermore, since, as shown by Fig. 2, the steep portion of the curve is substantially linear, the circuit itself yields substantially linear modulation. 1

While the invention has been. described in connection with a simple triode oscillator circuit,

it is to be understood that it may be applied with other types of tubes and other types of circuit connections. This is illustrated in Fig. 3 in which the so-called 6L7 tube is used. This tube is characterized by the presence of a special grid G3 and has the propertythat the gain of the tube can be modified over a widerange by control of the potential of grid G3. circuit comprises the tuned circuit L1Cl with an inductive coupling to the grid G'i. Such a circuit will oscillate in essentially the same manner as the circuit of Fig. 2; but the amplitude of the oscillations is controlled by the potential on grid G3. shunted around the output circuit is a rectifier circuit comprising the resistance R: in parallel'with the series combination of the diode I8 and resistance R3. A by-pass condenser C3 is In Fig, 3 the output that due to battery 24, will stabilize the oscillator in the manner described above. Signal coming in at the transformer T4 in the control grid circuit will then serve to modulate the oscillations as indicated above. It is again important to note that the time constant of the R4C4 circuit shall be large compared with the signal frequencies.

In some cases it is desirable to have balanced modulation, that is, modulation with elimination of carrier frequency as illustrated in the patent to Carson, 1,463,796, August '1, 1923. The principle of my invention may be applied in such a circuit also as shown in Fig. 5. In this circuit two tubes 21 and 28 are connected for self-oscillation again the oscillations may be stabilized by the shunted around the resistance R3 and there is then available across R: a biasing voltage which is impressed on the grid G3, the direction of connection being such thatas the amplitude of the oscillations increases, a larger negative bias is Supplementing this variable bias may be a fixed bias 20 of such value as to bring the circuit to the desired operating point. i

If now signal voltage through a transformer-Ta is also impressed on grid G3, the amplitude of the oscillations will vary in accordance with the' signal voltage. However, it is important to point out that the time constant of the circuit RsCa shall be large so that its response is sluggish as compared with the frequency of the modulating signal.

Fig. 4 illustrates the application of the invention to a double diode pentode tube in which the double diode serves as a singlerectifier. Here again the input circuit includes the tuned circuit L101, which is coupled to the output circuit to set up oscillations. In response to the oscillations the diode portion of the tube, serving as a rectifier, will give a direct current voltage across the resistance R4, which resistance is by-passed by the condenser C4. The magnitude of this resistance is such that the rectified voltage genercillations.

circuit is again made large, the amplitude of principles given above. Specifically this is shown by the generation of a biasing voltage across the resistance R5 in the circuit comprising the diode 29 supplied from the amplifier 30. introduced in the portion of the grid circuitcommon to both tubes will serve to stabilize the os- Since the time constant of the RsCs oscillations will be subject to the amplitude of incoming signals.

While a number of modifications have been shown to illustrate my invention, it is to be understood that still other modifications may be produced without departing from the spirit thereof.

What is claimed is: In combination, an. oscillator comprising a space discharge device having at least one control electrode and circuit means fixing the frequency of oscillation, means for voltage biasing said control electrode in fixed proportion to the output intensity of, said oscillations, said biasing means being so proportioned that said oscillations tend to be stabilized at a fixed operating point on a steep portion of the oscillation output intensity versus biasing voltage characterisic of said oscillator, and means for concurrently applying modulating signal potential to said oscillator tending to effect corresponding changes in the amplitude of said oscillations, said biasing means being so sluggish in its response to said changes in the said output intensity that the said operating point fluctuates along said char- -acteristic substantially in conformity with the instantaneous amplitude of said modulating sigated, taken with supplemental biasing such as nal potential, whereby said output oscillations are signal modulated.

JOHN B. MAGGIO.

Thus, bias

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