A simple tunnel diode circuit capable of accurately timing the zero crossing point of bipolar pulses is described. The combination of a fast tunnel diode and a nonlinear load line results in a circuit that can detect the zero crossing of a wide range of input waveshapes. This technique has been utilized in the design of a versatile, fast zero crossing discriminator. This instrument, which is compatible with existing logic systems, exhibits time slewing of less than 200 psec for an input amplitude range of at least 1 to 70 times threshold. Direct coupling and extremely short recovery time enable the instrument to function well at rates as high as 50 MHz.