US2406839A - Impedance matching device - Google Patents

Description

p E. LABIN Em 2,406,839

' IMPEDANCEMATCHING DEVICE Filed Jan. 12, 1942 AMMETER F/LAMENT PLATE SUPPLY su /1v INVENTOR. 5/14/45 4145/ BY I MES m #76 ATTORNEY.

Patented Sept. 3, 1946 IMPEDANCE MATCHING DEVICE Emile Labin, New York, and James H. Nye

Jackson Heights, N. Y., assignors to Federal Telephone and Radio Corporation, a corporation of Delaware Application January 12, 1942, Serial No. 426,455

4 Claims.

This invention relates to an improved form of impedance matching arrangement and in particular to means for adjustably matching the apparent input impedance of a radio frequency oscillator to the output impedance of a modulator feeding the oscillator.

The. invention contemplates particular application in the case of impulse modulators and transmitters of the nature disclosed in the copendingapplica-tion of Emile Labin, Serial No. 406,499, filed August 12, 1941, and entitled Pulse transmitters. These devices are particularly adaptable to radio locator equipment.

vIt is an object of the invention to provide an improved form of impedance matching device.

Another object is to provide relatively simple means for adjusting aradio frequency oscillator to give maximum output when supplied with high voltage impulse energy.

Other objects and various further features of novelty and invention will hereinafter be pointed out or will become apparent from a reading of the following specification in conjunction with the drawing included herewith.

The said drawing depicts schematically an impulse modulator and radio frequency oscillator circuit embodying features of the invention.

As disclosed in the said copending application, high voltage pulses of short duration may be generated from a relatively low D. C. voltage in somewhat the following manner. The basic circuit involved includes a relaxation oscillator having inductive feedback and an R. C. time con stant circuit in the filament return. Operation depends upon establishinga high current in a coil in the plate circuit of the relaxation oscillator and rapidly interrupting this current. The resultant discharge of the coil into the load presented by a radio frequency oscillator may then develop a peak voltage which is on the order of five times the magnitude of voltage applied to the relaxation oscillator. In forms shown in the said application it is this large voltage which serves as plate supply for the radio frequency oscillator. I

Referring more in particular to the drawing, the impulse generator or modulator includes a pair of triodes I0, I I having all circuits connected in parallel, an inductance L2 in the plate circuit inductively coupled through a coil L1 to the input or control grid circuit, and resistance and capacitance time constant elements [2, l3, and I4 across the input circuit. The plate supply voltage, as impressed between terminals 15 and [6, may be on the order of one fifth the peak voltthe maximum voltage obtainable across the plate coil L2 and substantially to eliminate oscillations in the grid circuit-thus assuring regularity of the pulsing cycle.

In the form shown the radio frequency oscile lator includes two triodes I9, 20 connected in push-pull. The filament, grid, and plate circuits of tubes l9 and 20 are tuned by balanced line structures 2|, 22, and 23, respectively, each of which is adjustably tuned by a short- circuiting member 24, 25, or 26, as the case may be. As shown input is supplied from across inductance L2 in the modulator to short circuiting member 26 of the radio frequency oscillator plate circuit, and output may be derived from an inductive coupling 21 with the plate tuned circuit 23.

Now in order to obtain maximum output from the modulator-oscillator circuit shown it is clear that the output impedance of the modulator (looking back across inductance L2) should equal or substantially approximate the apparent input impedance of the radio frequency oscillator (looking into the oscillator from inductance L2). Inasmuch as the band of frequencies necessary faithfully to transmit impulse energy is rather large, it is preferable that this impedance be essentially resistive so as to be substantially independent of frequency. The magnitude of this resistance R should correspond to the critical damping resistance of coil L2, as defined by the relation e where C represents the total stray capacitance in parallel with inductance L2, and L is the inand to provide some means for adjusting the os cillator in actual operation.

This subsequent adjustment of the oscillator may be effected by controlling the bias of oscillator tubes l9 and 20, as by varying the grid bias resistor 28. However, in the particular application contemplated tubes l9 and 2'9, have abnormally large filaments for relatively high emission during the periods of transmitting impulses, i. e. when the radio frequency oscillator is in OS- cillation. In this type tube it is well known that the grid characteristic is of an extremely critical nature with respect to bias variation for optimum oscillation conditions. As a practical matter therefor the magnitude of resistor 28 is relatively small so that the bias may be relatively stablethus permitting use of the desired gridcharacteristic.

Now, in accordance with the invention it is possible to change the apparent impedance of the 0scillator without making any appreciable change ingrid bias. It is-proposed that such adjustment be made by variation of an additional resistance 29in the cathode circuits of oscillator tubes 9' and 20. Since resistor 29 is in eifect in both the plate and cathode circuits of tube l9 and 29, a slight variation thereof may have the effect of altering plate circuit impedance to a relatively large degree while negligibly changing grid bias.

It has been found that the increase in output gained from varying resistance 29 to matchthe oscillator impedance to that of the modulator is far greater than any losses or other undesired eifects obtained from the slight change in grid bias occasioned by adjustment of resistor 29.

It is to be' noted that it is the A. C. impedance in thecathode circuits that determines the decay shape ofthe pulses transmitted. This impedance may comprise either capacitance in parallel with resistor 29 or inductance in series therewith; Forsteep-walled pulses it is accordingly considered preferable that precaution be taken that capacitance in parallel with resistor 29 is as small as possible. Furthermore, it is to be noted thatsince resistor 29 is in the output circuit its mag-..

nitude should. be kepta'slow as possiblet'o: ensure maximum power output and minimum power loss in this resistor.

Although in the above description element 29" has repeatedly been referred to as a resistance,

it is to be considered as an impedance having substantially resistive characteristics It will be seen that we haveset forth a. relatively simple method for matching theimpedance of a pulse modulator to aradiofrequency oscillator after these devices. have been built and set up. The proposed. method enables ready ad justment. for optimum oscillation should tubes be replaced or operating frequency be changed. Whilewe have described our invention in considerable detail in connection with the preferred form shown, it is to be understood, of course, that many modifications, additions, and omissions may be made without departing from the spirit and scope of this invention.

What we claim is:

1. In an impulse transmitter comprising an impulse modulator feeding a radiof'requency oscillator having vacuum tube means including cathode, grid, and anode circuits and a substantially resistive impedance in the anode-cathode circuit, the method of matching the impedance of the oscillator to that of the modulator which comprises varyingsaid impedance in the anode-cathode circuit untilmaximum output is obtained from the oscillator.

2. man impulse transmitter comprising an impulse modulator feeding a radio frequency oscillator having vacuum tube means including a cathod circuit, a grid circuit, and an anode circuit, the output of said modulator being in said anode circuit, and said cathode circuit having resistance therein, the method of obtaining. maximum output from the oscillator which comprises adjustably varying said resistance, whereby the apparent impedance of said anode circuit may be varied Without appreciably changing. conditions in said grid circuit.

3. An impulse transmitter comprising an impulse generator and a radio frequency oscillator, said generator including a Vacuum tube having an electron emissiv electrode, a control electrode, and an electron receiving electrode, an inductance, a source of energy, means connecting said source and saidinductance in series between said receiving electrode and another electrode of said tube, a coupling coil inductively coupled. to said inductance, a timing circuit, and means com necting saidcoil and said circuit in series between said control electrode and said emissive electrode, said oscillator including vacuum tube means including a cathode circuit, a grid circuit, and an anode circuit, said anode circuit being. coupled across said inductance, said grid circuit including biassing means, and said cathode circuit including relatively high emission cathode means and adjustable resistance means.

4. An impulse transmitter comprising an impulse mcdulator and aradio frequenc oscillator, said oscillator including vacuum tube means having an anode circuit to ground, a cathode circuit to' ground, a, grid circuit to ground, and means coupling said anode circuit to said modulator, said grid circuit having a biasing resistance means, said cathode circuit having an adjustable resistance means adjustment of which operates to vary the" impedance of the anode circuit. for impedance matching with respect to the modulator impedance while negligibly affecting the grid biasing adjustment.

EIVHLE LABIN. JAMES H. NYE.

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