Zero-Crossing Detector with Sub-Microsecond Jitter and Crosstalk

A zero-crossing detector ZCD has been built and tested with a new circuit design which gives reduced time jitter compared to previous designs. With the new design, time jitter is reduced for the first time to a value 4.2 x 10-8 seconds for a 1 Hz input signal and a 1 second measuring time which approaches that due to noise in the input amplifying stage. Additionally, with fiber-optic transmission of the output signal, crosstalk between units has been eliminated. Incorporation of commercially available double-balanced mixers allowed two approximately equal 100 MHz signals differing by 1 Hz to be compared, giving an Allan Deviation of 1.17 x 10-15 at a 1 second measuring time. The measured values are in good agreement with circuit noise calculations and approximately ten times lower than that for ZCDs presently installed in the JPL test facility. Crosstalk between adjacent units was reduced even more than the jitter. Where the old units showed crosstalk of 10-4 seconds between units, no crosstalk could be detected between the new ZCDs, even when operating from the same power supply.