US2490428A - Modulator - Google Patents

Description

'l'.v M. GLUYAS, JR 2,490,428

MODULATQR Filed June 24, 1947 w BB )w f cmrmmx u: Firm INVENTOR.

ATTORNEY Patented Dec.A 6, 1949 MODULATOR Thomas M. clans. Jr., Collingswood, N. J., ossixnor to Radio Corporation of America, a corporation of Delaware Application June 24, 1947, Serial No. 756,718

' 7 claims. (ci. 332-66) In this application. I disclose an improved modulator which is of use in the radio and allied arts. In the modulator of my invention, the grid of an RF stage is modulated and the arrangement is such that the modulation is linear for an extremely wide range of modulation frequencies. This makes my invention of particular value in the television art, where the video band covers a wide range of frequencies.

Heretofore in general. push-pull modulation stages have been used where a carrier is to be modulated by video signals. In these arrangements, the modulation is applied to a tap at the electrical center of a radio frequency grid coil in a push-pull radio frequency amplifier being modulated. The modulator tube output is as a consequence shunted by the grid to cathode capacity of both the radio frequency tubes and also by the capacity of the tank circuit to ground. This loading of the modulator output narrows the characteristic curve of the modulator and limits the frequency range through which linear modulation of the radio frequency stages can be carried out.

It is obviously of advantage to be able to use single ended radio frequency stages wherein the modulation is carried out. This is not easy, however, because the single ended systems must use an RF by-pass condenser across the modulator output to shunt the radio frequency currents around the modulator. This shunting or by-pass condenser is also shunted to the modulator output as is the grid to cathode capacity of the single RF stage, again placing a limitation on the frequency range of modulation potentials at which the modulator responds linearly.

An object of my invention is to provide a modulation system of this type wherein the modulator works on the grid circuit of a single ended stage.

and wherein the capacity loading on the modulator output is small so that a wide band of lsignals may be used in the modulator to accomplish linear modulation of the radio frequency grid potential to thereby linearly modulate the amplitude of the radio frequency being amplified. The

- band Width which may be used in a modulator of this nature is inversely proportional to the capacity into which the modulator works and as a consequence, reducing the capacity load widens the modulation frequency band which may be used.

In describing my invention in detail, reference will be made to the attached drawings wherein:

Fig. l illustrates by a. simplified wiring diagram ythe essential features of a known modulation system. This ligure is used to illustrate the need of my invention. f

Figs. 2, 2A and 3 illustrate by simple wiring diagrams the essential elements and circuit connections of two embodiments of my improved modulation system.

In Fig. 1. I0 is a radio frequency driver tube having its input electrodes excited by radio frequency currents to be amplitude modulated. The' voltage end of radio frequency grid circuit 20' and the cathode of the power amplier tube 24.

Modulation is applied from a transformer T having avsecondary winding connected to the grid of the tube 24. This known circuit is unsuited for use in television or like wide band services because the reactance of Ci plus the capacity between the grid and cathode of tube 24 plus the capacity of the tank circuit 20 with respect to groundis in shunt to the wide band source. As stated above, the width of the band which can be used in this type circuit is inversely proportional to the shunt capacity into which the modulation source Works and the shunt capacity here is too large to permit satisfactory modulation by a band of frequencies running up in the higher frequency range, such as found in a video source.

The modulated radio frequency energy appears in the tank circuit 30 parallel tuned thereto and is fed from said circuit to a tank circuit 32 correspondingly tuned and from the tank circuit 32 to a load as desired such as, for example, to another amplier stage or to an antenna.

An embodiment of my improved modulator wherein the arrangement is such as to eliminate the defects listed above appears in Fig. 2. Insofar as possible, I have used in Fig. 2, numerals and symbols corresponding to those used in Fig.

l. In describing Fig. 2 therefore, only those features which I consider novel and not found in Fig. 1 will be discussed. The anode of the stage I0 is again coupled to a tuned tank circuit I4'. The high radio frequency voltage end of this tank circuit I4' is coupled by coupling and direct current blocking condenser C2 to the grid of the power ampliiierstage tube 24. The power amplifier tube output circuit is again as shown in Fig. l. The transformer T, Fig. l, has now `been eliminated and the modulator tube 40 has its input electrodes coupled to a modulation source such as a source of video signals and has its anode coupled by a video peaking inductance 46 shunted .tube 24 in case, its screening electrode is maintained at a positive potential and is bypassed to its cathode by a capacitor. l

The radio frequency driver stage in my improved modulator is of high gain and is operated to give more than the required radio frequency voltage for excitation of the grid of the power ampliner tube 24. The-capacitor C2 with the input capacity of tube 2l, Cin, shown dotted in Fig. 2, provides a capacitor voltage divider across the output of the RF stage tube il. Since the gain of the stage il is more than normally required, the capacitor C2 may be made small. C2 is made smaller than the input capacity Cin so that C2 does not impose a substantial load on the output of the video amplifier tube Il. In fact, the capacitor C2 may be on the order of two micromicrofarads and is ordinarily much smaller than the capacity to ground of a tuned grid circuit conventionally used in a push-pull grid bias modulated radio frequency amplifier stage. y

The modulator circuit may include peaking inductor It and resistor 48, inductor 50 and resistor il. These elements form a low pass filter which allows the video frequency energy to be coupled to the grid modulated radio frequency ampliner but prevents radio frequency currents from reaching the modulator. A radio frequency choke RFC may be included to present a high impedance to radio frequency energy at the grid of order that the output circuit of the modulator will not load the radio frequency driver I and its associated circuit including Il', C2 and Cin.

The .modulator output circuit is drawn separately in Fig. 2A and it will be clear from a study of this figure that the output circuit is indeed a low pass filter wherein Cm represents the output capacity of the modulator and is one shunt arm of the network. lThe input capacitance Cin of tube 24 together with capacitor C2 forms the other'shunt arm of the network. Video peaking inductors 4t and Si are the series elements and resistor 54 is the terminating resistor. These elements constitute a one and one half section; constant K; mid-series terminated; low pass illter. Damping resistor Il is added to maintain a constant input impedance to the network for video frequencies close to the cut-off frequency of the network. The inductance of theradio frequency choke RFC is so small that it may be neglected at video frequencies and within the pass band of the filter network. The network is so designed that the radio frequencies present in the circuits of tubes il and 2t are not within the pass band of the filter and are consequently not conducted to the video modulator. The general type of video coupling network `iust described is well known in the television art.

Since the only capacity in shunt to the modulator tube is C2 and the input capacity of tube p24 and since this total capacity is relatively small,

the modulator ll will operate over a wide band of modulation frequencies to apply to the control grid of tube 24, potential variations which are substantially linear with respect to the corresponding potential or current `variations at the input of 40.

Where it is inconvenient to use a high voltage RF amplifier tube at il, the required voltage step-up necessary to drive the grid of the power amplifier tube 24 by way of the relatively small capacitor C2 may be obtained by use of an RF voltage step-up transformer of the autotransformer type. Then the arrangement may be as illustrated in Fig.' 3 of the drawings. In this embodiment. the anode of the radio frequency driver stage tube l0 is coupled down on the inductance of the parallel tuned circuit Il' so that an autotransformer effect is obtained to apply a high excitation voltage to the small capacitor C2. The

arrangement of Fig. 3 is otherwise substantially as illustrated in Fig. 2 and described in detail above. The numerals used in Fig. 3 correspond to those used in Fig. 2 and the operation of the embodiment of Fig. 3 will be apparent to those skilled in the art by inspection thereof after reading the description given in connection with Fig. 2 above.

What is claimed is:

1. In a modulation system, an electron discharge device having a control grid, an anode and a cathode, a radio frequency circuit wherein oscillatory energy to be modulated appears, a connection including an impedance which is low with respect to alternating voltages of the frequency of said oscillatory energy coupling a point on said circuit to the cathode of said device, an input circuit for said device comprising in series between a different point on said radio frequency circuit and the cathode of said device, a capacitor and the input capacity of the device, a modulator tube having output electrodes and having input electrodes excited by modulation energy, and connections coupling the output electrodes of the modulator tube in shunt to said input capacity of the device.

2. In a modulation system. an electron discharge device having a control grid, an anode and a cathode, a capacity divider comprising in series a capacitor and the input capacity of the device, said capacitor being small relative to the input capacity of the device, connections for applying radio frequency voltage to be modulated to the terminals of said series circuit, a modula tor tube having input electrodes excited by modulation energy, said modulator tube having output electrodes, and a network coupling said modulator tube output electrodes to the said input capacity, said network comprising a low pass filter with the cut-on point just above the highest modulation energy frequency.

3. In a modulation system, an electron discharge device having a control grid, an anode and a cathode, a capacity divider comprising in a series circuit a capacitor and the input capacity of the device, said capacitor being relatively small with respect to said device input capacity, an inductor in shunt to said series circuit, means for applying high frequency alternating voltage to ybe modulated across spaced points on said inductor, saidl inductor and connections forming auto transformer means for applying high frequency voltage to said series circuit, a modulator tube having input electrodes excited by modulating currents, said modulator tube having output electrodes and circuits coupling the output electrodes of the modulator tube in shunt to said input capacity. "f

4. Ina modulation system, an electron dhcharge devicev having a control grid, an anode and a cathode, an output circuit coupled with the anode and cathode, a radio frequency driver stage having its anode coupled by a capacitor to said control grid and its cathode coupled to the cathode of said device so that said capacitor with the input capacity of the device forms a radio frequency voltage divider. said capacitor being small relative to the input capacity of the device, said driver stage being of gain sufficient to supply adequate voltage to said capacitor to drive said device, a modulator tube having input electrodes excited by modulation energy, said modulator tube having a cathode and an anode, a coupling between the anode of said modulator tube and the junction point between said capacitor and input capacity and a coupling between the cathode of said modulator tube and the cathode of said device.

5. In a modulation system, an electron discharge device having a control grid, an anode and a cathode, an output circuit connected with said anode and cathode, a radiol frequency driver stage having output electrodes, an inductor coupled to said output electrodes, a capacitor coupling a point on said inductor to the control grid of said device, said couplings and inductor forming an auto-transformer to step up the voltage fed from the driver stage to the device, said capacitor with the input capacity of the device form-` ing a voltage divider wherein the capacitor is relatively small with respect to the capacity, a modulator tube having input electrodes excited by modulating currents, said modulator tube having output electrodes, and circuits coupling the output electrodes of said modulator tube in shunt to the said input capacity of said device.

6. In a modulation system. an electron discharge device having a control grid, an anode and a cathode, a radio frequency input circuit therefor comprising in series, a capacitor and the input capacity of the device, a modulator tube having output electrodes and having input electrodes excited by modulation energy, connections coupling the output electrodeslof the modulator tube in shunt to said input capacity and a low pass lter with a cut-off point just above the highest modulation frequency in said connections.

7. In a modulation system, an electron discharge device having a control grid, an anode and a cathode, an output circuit connected with said anode and cathode, a radio frequency drivel` stage having output electrodes, an auto transformer coupled to said output electrodes, a capacitor coupling the high radio frequency voltage end of said transformer to the control grid of said device, said capacitor with the input capacity of the device forming a voltage divider wherein the capacitor is relatively small with respect to the capacity, a modulator tube having input electrodes excited by modulating currents, said modulator tube having output electrodes and circuits including a peaking coil and a damping resistance in parallel and a radio frequency choking coil connecting the anode electrode of said modulator tube to the control grid of said device.

THOMAS M. GLUYAS, JR.

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

UNITED STATES PATENTS Number Name Date 1,932,155 Culver Oct. 24, 1933 2,227,027 Schlesinger Dec. 31, 1940 2,243,504 Gluyas May 27, 1941 2,309,764 Gottier Feb. 2, 1943

Images