AN ARRANGEMENT IN A POWER UNIT FOR A GRID PULSED O-TYPE MICROWAVE TUBE

Abstract: A power unit for a grid pulsed O-type microwave tube (1) having a cathode (2) and a collector (3), comprises a cathode unit (5) and a collector unit (6), each having two output terminals (7, 8; 9, 10). A first and a second capacitor (C1, C2) are connected between the output terminals of the respective unit (5, 6). One output terminal (7) of the cathode unit (5) is connected to the cathode (2) of the tube (1), while the other (8) is connected to a ground terminal. One output terminal (9) of the collector unit (6) is connected to the collector (3) of the tube (1), while the other (10) is interconnected with that one (7) of the output terminals of the cathode unit (5) that is connected to the cathode (2) of the tube (1). To minimize crosstalk caused by the collector unit (6) across the first capacitor (C1), the output terminals (9, 10) of the collector unit (6) are connected to said second capacitor (C2) via non-capacitive impedances (Z1, Z2), and to said ground terminal via decoupling capacitors (C31, C32).

Description: AN ARRANGEMENT IN A POWER UNIT FOR A GRID PULSED 0-TYPE MICROWAVE TUBE TECHNICAL FIELD The invention relates generally to power units and more specifically to an arrangement in a power unit for a grid pulsed O-type microwave tube. BACKGROUND OF THE INVENTION In high-power radar transmitters, high-voltage power units are used to power grid pulsed O-type microwave tubes, e.g. travelling-wave tubes. Such tubes have a cathode and at least one collector, and a power unit for such tubes comprises a cathode power unit and a collector power unit for each collector. The cathode power unit has two output terminals. One of the output terminals is to be connected to the cathode of the microwave tube, while the other output terminal is connected to a ground terminal. Between the output terminals of the cathode power unit, a capacitor is connected. The collector power unit also has two output terminals. One of these output terminals is to be connected to the collector of the microwave tube, while the other output terminal is interconnected with that one of the output terminals of the cathode power unit that is to be connected to the cathode of the microwave tube. Between the output terminals of the collector power unit, a capacitor is also connected. Both the cathode power unit and the collector power unit normally comprise converters, high-voltage transformers, rectifiers and filters. For good performance of the microwave tube, a high quality cathode voltage is required. However, in such power units, there are problems with crosstalk from the collector power unit to the cathode voltage. When the capacitor connected between the output terminals of the collector power unit is recharged by the collector power unit, an undesired current appears through the capacitor connected between the output terminal of the cathode power unit due to undesired stray capacitances between the collector power unit and ground, mainly between the high-voltage transformer in the collector power unit and ground. This undesired current results in undesired cathode voltage variations. SUMMARY OF THE INVENTION The object of the invention is to eliminate crosstalk from the collector power unit to the cathode voltage. This is attained in accordance with the invention by connecting the output terminals of the collector power unit to the associated capacitor via non- capacitive impedances as well as to said ground terminal via decoupling capacitors. Hereby, crosstalk caused by the collector power unit across the output terminals of the cathode power unit will be eliminated. BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing on which Fig. 1 illustrates an embodiment of an arrangement according to the invention in a power unit for a travelling-wave tube, and Fig. 2 illustrates an equivalent circuit diagram of the arrangement in Fig. 1. DESCRIPTION OF THE INVENTION Fig. 1 shows an embodiment of an arrangement according to the invention in a power unit for a travelling-wave tube generally denoted 1. It is however to be understood that the invention is applicable to power units for any so called grid pulsed O-type microwave tube, e.g. a klystron. The tube 1 comprises a cathode 2, a collector 3 and an anode 4 that is connected to ground. In a manner known per se, the power unit for the tube 1 comprises a cathode power unit 5 and a collector power unit 6. Also in a manner known per se, the cathode power unit 5 and the collector power unit 6 both comprise a converter, a transformer, a rectifier and capacitors (not shown). The cathode power unit has two output terminals 7 and 8. The output terminal 7 is connected to the cathode 2 as well as to one terminal of a capacitor C1 , while the output terminal 8 is connected to the other terminal of the capacitor C1 as well as to a ground terminal. In a similar manner, the collector power unit has two output terminals 9 and 10. The output terminal 9 is connected to the collector 3 of the tube 1 as well as to one terminal of a capacitor C2, while the output terminal 10 is connected to the other terminal of the capacitor C2 as well as to the interconnection point between the output terminal 7 of the cathode power unit 5, the cathode 2, and said one terminal of the capacitor C1. In accordance with the invention, to reduce the effect on the voltage across the capacitor C1 by undesired currents through the capacitor C1 , the output terminals 9 and 10 of the collector power unit 6 are connected to the respective terminal of the capacitor C2 via respective impedances Z1 , Z2. These impedances Z1 , Z2 comprise only resistors and/or inductors, i.e. the impedances Z1 , Z2 are non-capacitive impedances. The values of the impedances Z1 , Z2 are adapted to the impedance of the capacitor C2, and can be of equal impedance values. Also, the output terminals 9 and 10 of the collector power unit 6 are connected to the ground terminal via respective decoupling capacitors C31 , C32. The capacitance values of the capacitors C31 , C32 are adapted to the capacitance to ground of the output terminals 9, 10 of the collector power unit 6, and can be of equal capacitance values. To illustrate the effect of the collector power unit 6 on the cathode voltage across the capacitor C1 , reference is made to Fig. 2 which illustrates an equivalent circuit diagram of the arrangement according to the invention. In Fig. 2, the effect of the cathode power unit on the cathode voltage is disregarded. Also, the impedance of the capacitor C2 is disregarded since it is much smaller than the impedances Z1, Z2. As apparent from Fig. 2 a recharging current I from the output terminal 9 of the collector power unit 6 will not give rise to any current through the capacitor C1 , and consequently no crosstalk will appear.

Claims: CLAIMS 1. An arrangement in a power unit for a grid pulsed O-type microwave tube (1 ) having a cathode (2) and a collector (3), the power unit comprising, on the one hand, a cathode power unit (5) having two output terminals (7, 8), one (7) of which being intended to be connected to the cathode (2) of the microwave tube (1 ), while the other (8) is connected to a ground terminal, a first capacitor (C1) being connected between said output terminals (7, 8) of the cathode power unit (5), and, on the other hand, a collector power unit (6) having two output terminals (9, 10), one (9) of which being intended to be connected to the collector (3) of the microwave tube (1 ), while the other one (10) is interconnected with that one (7) of the output terminals of the cathode power unit (5) that is intended to be connected to the cathode (2) of the microwave tube (1 ), a second capacitor (C2) being connected between the output terminals (9, 10) of the collector power unit (6), characterized in that, in order to minimize crosstalk caused by the collector power unit (6) across the first capacitor (C1 ), the output terminals (9, 10) of the collector power unit (6) are connected, on the one hand, to said second capacitor (C2) via respective non-capacitive impedances (Z1 , Z2) adapted to the impedance of that second capacitor (C2), and, on the other hand, to said ground terminal via respective decoupling capacitors (C31 , C32) adapted to the capacitance to ground of the collector power unit (6). 2. The arrangement as claimed in claim 1 , characterized in that said non- capacitive impedances (Z1 , Z2) are of the same impedance value. 3. The arrangement as claimed in claim 1 , characterized in that said decoupling capacitors (C31 , C32) are of the same capacitance value.

Inventors: ALLANSSON JAN, ALLANSSON, JAN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 2000
Main classification: H01J23
IPC codes: H02M3