Evaluation of fast voltage discriminators in the picosecond time range

An automated system for the evaluation of fast-risetime voltage discriminators is described. A high-performance time digitizer is used to determine the elapsed time between the generation of a test pulse and the response of the discriminator under test. The measurement is repeated for a series of discriminator threshold levels generated by a digital-to-analog converter, and the results are compared to a photograph of the leading edge of the pulse obtained with a sampling oscilloscope. To obtain high accuracy, an average of several thousand individual digitizations is used in each measurement to reduce the uncertainty to a maximum of ±3 ps. The measurement system is used with a test fixture to evaluate the performance of a number of tunnel diodes and in preliminary measurements of a prototype discriminator of novel design. At present the prototype discriminator exhibits a time walk of about 15 ps per dB change of input pulse amplitude, and work is underway to evaluate and minimize this effect