Characterization of a Very Fast Step Generator

For the bandwidth measurement of voltage and current sensors either AC sources or pulse generators are required which exceed the bandwidth of the sensors. At low voltages this can either be achieved by high frequency oscillators or amplifiers. These can easily create signals up to 300 GHz or 1 ps rise time [1]. However, these signals do not exceed an amplitude of 10 V, which inhibits these for the use in high voltage metrology. To establish calibration capabilities for Very Fast Transient (VFT) measurement equipment a step generator with an amplitude of at least several hundreds of volts, a rise time shorter than 1 ns and a pulse width of at least 100 ns is required. The transmission line pulse generator presented in this paper can produce voltages up to 1000 V with a rise time of less than 100 ps. This exceeds the typical performance of mercury wetted relays which are usually used in high voltage step measurements [2]. These have a minimal rise time of 1 ns, which limits their highest frequency component to less than 350 MHz. The Avalanche effect within standard NPN transistors was found to be usable for rise times less than 100 ps and therefore frequencies far beyond 1 GHz. The output voltage can be varied by different configurations from 1 V up to more than 100 V per single transistor. This paper will cover the basics of the non-destructive avalanche breakdown of NPN transistors, the schematics of avalanche pulse generators with single transistors and the cascading of several transistors for high voltage use. Finally, measurements with the finished pulse generators are performed and compared to standard step generators based on mercury wetted relays.