A 50 /spl Omega/, 50 kV ceramic disc coaxial load

This paper describes the design, construction and testing of a 50 /spl Omega/ coaxial termination for high power ultra-wideband (UWB) pulses. The function of the device is to act as a dummy load for the development of rep-rated pulsers with sub-nanosecond rise-times and output amplitudes in the range 10-50 kV. The design specification includes a broadband 50 /spl Omega/ termination impedance, 50 kV pulse capability, a low reflection coefficient, a high pulse energy rating and a mean power rating of 100-200 W. The major design difficulty for high power sub-ns rise-time coaxial loads is the compromise between the ideally minimal resistor thickness for good high frequency performance, and the resistive volume necessary to dissipate the power. A stack of 20 ceramic disc resistors, each 1 inch in length, forms the resistive element. Possible difficulties include the voltage and temperature dependence of the ceramic disc resistance. A nominal 1:10 tap-off is built into the stack to enable voltage monitoring. On the basis of simple skin depth calculations, the high frequency pulse characteristics of the discs are adequate for absorption of sub-ns rise-time pulses. The performance of the termination is critically affected by the connector and housing design, which must both satisfy impedance matching constraints, and give the required voltage withstand capability, particularly of the connector-load feedthrough section. Oil insulation is used in the highly stressed sections. Testing of the load for reflection characteristics, mean power capability and the rise-time performance of the 10:1 voltage pick-off is discussed.