US3351808A - Feed current control for pulse-modulated magnetron transmitter - Google Patents

Description

Nov. 7, 1967 A. HURLIMANN 3,351,808

FEED CURRENT CONTROL FOR PULSE-MODULATED MAGNETRON TRANSMITTER Filed Aug. 11, 1964 INVENTOR ARMIN HL RLI IANN United States Patent 3,351,808 FEED CURRENT CONTROL FOR PULSE-MODU- LATED MAGNETRON TRANSMITTER Armin Hiirlirnann, Schlieren, Zurich, Switzerland, assignor to Albiswerk Zurich-A.G., Zurich, Switzerland Filed Aug. 11, 1964, Ser. No. 388,816 Ciaims priority, application Switzerland, Aug. 28, 1963, 10,634/63 1 Claim. (Cl. 315-102) This invention relates to thermionic valves of the type having heated cathodes. More particularly, the present invention is directed to pulse modulated magnetron transmitters.

It is known that pulse currents on insutficiently heated cathodes of magnetrons lead to the destruction of the latter. For this reason, it is usual to preheat the magnetron before operation. It is also known that, in the case of a pulse modulator feeding a magnetron through a pulse transformer, when the cathode of the magnetron is insufficiently heated, the pulse voltages at the secondary side of the pulse transformer may increase to at least double the values attained during normal operation with a heated cathode.

These deleterious eifects on the operation of a magnetron valve while the cathode is insufficiently heated have been avoided by using a temperature dependent relay which controls the operating voltage of the modulator and the anode voltage of the magnetron in such a manner as to avoid application of operating voltage to the magnetron until the cathode thereof has been heated to the proper operating temperature.

In portable magnetron transmitters, such as used in repeaters or responders in meteorological balloon probes, unmanned reconnaissance planes, and the like, such temperature responsive relay circuits are unsuitable due to the fact that vibrations, shocks, and large acceleration forces result in uncontrollable switching errors. Furthermore, relays have a relatively high current consumption so that such magnetron transmitters, when used, for example, in balloon probes, require excessive loading of the balloon in the form of batteries.

As a substitute for a temperature dependent relay control circuit for starting up a magnetron transmitter, it is possible to use a control relying upon the residual current of the magnetron valve and on the basis of its diode characteristics. With magnetron tubes having a pulsed output of the order of a few kilowatts, this residual current is very small, for example, several a. Thus, for

control of the starting, an additional D.C. amplifier is required, with an amplification ratio of at least 1:1000. The cost of such an amplifier is out of proportion to the costs of the apparatus itself and thus unjustified economically.

An object of the present invention is to provide an improved control circuit for reducing the power of magnetrons during the pre-heating period of the cathode, Which is simple, inexpensive and without mechanical contacts.

Yet another object of the invention is to provide a control circuit for reducing the power of a magnetron during the pro-heating, and which control circuit involves solely solid state components.

In accordance with the present invention, the disadvantges of known starting up arrangements for pre-heating the cathodes of thermionic valves, of the type in which a signal operating potential is applied from a suitable source across the cathode and anode, are avoided by reducing the feed voltage to the pulse modulator during the cathode heating up period of a magnetron by the use of a series resistance in the modulator supply voltage circuit. Means are provided for bridging this series re- 3,351,808 Patented Nov. 7, 1967 sistance in accordance with the thermal emission of the cathode.

As applied to the aforementioned magnetron transmitter which is pulse modulated, the series resistance is included in the feed circuit for the pulse modulator being supplied with a DC. voltage. The emission of the cathode, in any case, is derived or measured from the voltage drop efiected by the mean anode current across an adjustable resistance, such as an adjustable voltage divider, connected in the anode circuit of the magnetron.

In a specific example, a semi-conductor or silicon control rectifier is connected for bridging such series resistance. A transistor, connected as an emitter follower, has its control or input voltage derived from the tap of the adjustable resistance or voltage divider, and the output or emitter current of the transistor is supplied to the control element, such as to the gate of a silicon control rectifier. The conductivity of the semiconductor is thus controlled by the output or emitter current of the transistor as a function of the measured anode current of the valve which, in turn, is a measure of the thermal emission of the cathode.

For an understanding of the principles of the invention, reference is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawing.

In the drawing:

The single figure is a schematic wiring diagram illustrating a pulse-modulated magnetron transmitter embodying the invention.

Referring to the drawing, a line-type modulator M has a direct feed voltage applied thereto from a rectifier G connected across a suitable source of AC. potential. One of the two feed lines between rectifier G and modulator M is grounded, and this grounded feed line has a resistance R1 connected therein and bridged -by a semi-conductor thyratron Th. Thyratron Th may advantageously be a silicon control rectifier, which is a solid state device whose conductivity is determined by the current applied to a control element or electrode usually termed gate.

The output of line-type modulator M is applied to the primary winding of a pulse transformer Tr]. This transformer has a first secondary winding W1 and a second secondary winding W2. Corresponding terminals of both of these secondary windings are connected to respective terminals of a source of AC. potential indicated at Q, and the corresponding opposite terminals of these secondary windings are connected to respective opposite ends of the primary winding of a heater circuit transformer TrH. The secondary winding of the heater circuit transformer supplies the heating voltage for the cathode of a magnetron valve S. In the particular arrangement illustrated, secondary winding W2 is also connected with the adjacent terminal of the secondary winding of the heating transformer TrH.

The anode of magnetron valve S is grounded. A voltage divider R2, R3, which is bridged by a condenser C, is connected between ground and the A.C. feed of secondary winding W2 of pulse transformer TrJ. Resistance R2 of this voltage divider is provided with an adjustable tap which is connected to the base of an NPN transistor T. The collector of this transistor is connected through a collector resistance R4 to a source of positive potential. The emitter of transistor T is connected with a control electrode of the semi-conductor thyrtatron Th, which control electrode may be the gate of a silicon control rectifier.

The line-type modulator M and the rectifier G are shown only in the form of blocks, as the: types used are well known. For example, for a typical 1ine-type modulator M, reference is made to the journal Electronics of Feb. 23, 1962, page 44.

In the described arrangement, during heating up of magnetron valve S, line modulator M is fed from rectifier G through resistance R1, as semi-conductor thyratron Th is, at such time, non-conductive. Modulator M thereby operates with a reduced supply voltage, and thus supplies pulses of only a very small amplitude to pulse transformer Tr]. The amplitude of these pulses must be such that, in spite of the insufficiently heated cathode of magnetron valve S, no dangerously excessive voltages can appear aoross second secondary winding W2 of pulse transformer Tr]. This can be efiected by a proper selection of the value of series resistance R1.

As soon as the emission capability of the cathode begins to increase, a mean anode current flows through magnetron valve S. This mean anode current produces a voltage drop across the voltage divider R2, R3, so that a voltage is applied to the base of transistor T from the tap of resistance R2. Transistor T thus begins to be conductive with the increasing voltage drop across resistor R2, and accordingly a current begins to flow from the emitter of transistor T to the control electrode or gate of semi-conductor thyratron, or silicon control rectifier, Th. As soon as this control current reaches a threshold value, the semi-conductor thyratron becomes conductive and feeds the full DC. voltage from rectifier G to linetype modulator M, thus bypassing or short circuiting series resistor R1.

The sensitivity of the described arrangement, which can be adjusted by means of the adjustable resistance R2, is a function of the temperature dependence of the current amplification of transistor T and that of semiconductor thyratron Th.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

A pulse-modulated magnetron transmitter comprising, in combination, a magnetron having an anode and a heated cathode; a heating circuit for said cathode; a pulsemodulator connected to said cathode to supply operating pulses to said magnetron; a DC. voltage source; a supply circuit connecting said source to said modulator; a series resistance in said supply circuit to reduce the current supplied to said modulator during heating of said cathode; a semi-conductor thyratron connected in shunt with said series resistance; an adjustable voltage divider connected in the anode circuit of said magnetron to provide a voltage drop corresponding to the measured mean anode current of said magnetron; and a transistor having its input circuit connected to said adjustable voltage divider and its output circuit connected to said thyratron to supply a control current to said thyratron corresponding to the measured value of the mean anode current of said magnetron.

References Cited UNITED STATES PATENTS 2,507,282 5/1950 Stivin 315102 2,697,171 12/1954 Little et a1. 33187 2,940,010 6/1960 Kenny 315106 X 3,171,040 2/1965 Goebel 30788.5

FOREIGN PATENTS 106,432 2/ 1943 Sweden.

JAMES W. LAWRENCE, Primary Examiner.

P, C. DEMEO, Assistant Examiner.

Images