Abstract: The invention is a termination circuit which may be used to terminate a tester end of a transmission line between the tester and a digital device under test (DUT). The termination circuit is a clamp circuit which acts as a non-linear resistor of infinite resistance for termination voltages between zero volts (or low logic level voltage) and the power supply voltage V.sub.cc (or high logic level voltage) of the DUT, and which provides a substantially constant slope resistance set to equal the characteristic impedance (Z.sub.O) of the transmission line for voltages outside this range. Given that the output impedance of the DUT is less than the characteristic impedance Z.sub.O of a transmission line, and given the characteristics of the clamp circuit, all reflections on the transmission line as a result of unmatched impedances are totally eliminated by the clamp in twice the delay time of the transmission line. After the reflections are eliminated, the clamp acts as an open circuit.

Abstract: An electron-beam test probe system (400) in which a pulsed laser-beam source (404) and a photocathode assembly (430) are used with an electron-beam column (426) to produce a pulsed electron beam at a stabilized repetition frequency. A pulse picker (414) allows the pulse repetition frequency of the pulsed electron beam to be reduced to a submultiple of the pulsed laser repetition frequency. A test pattern generator (416) is programmable to apply a desired pattern of test vector patterns to an electronic circuit to be probed, the test vector patterns being synchronized with the stabilized laser-beam pulse repetition frequency. A timebase circuit (412) allows the test vector patterns to be time-shifted relative to the pulsed electron beam. The electronic circuit under test can thus be probed at any desired point in the applied test vector pattern by control of the pulse picker and by time-shifting the test vector pattern.