The design of low jitter hard limiters

Hard limiters, which are nearly identical to level crossing detectors or zero crossing detectors, are an integral part of many communications systems, of devices for characterizing precision oscillators, and of analog-to-digital converters. The purpose of a hard limiter is to divide the continuous range of an analog input signal into two regions and to provide a digital output indicating the region in which the signal is. This digital output must, by definition, have an extremely short transition time. A hard limiter is thus a slope amplifier, since a slowly varying input must produce a very rapidly varying output. Noise in the amplifiers causes the output transition to be jarred from its correct position producing jitter in the output. This paper presents the analysis and details the design of very low jitter hard limiters. Their immediate application is to the measurement of high stability signal sources and to deep space communication and ranging; the results, however, are easily adapted to many other purposes such as waveform digitization. One of the devices described has been built. Its performance agrees closely with theory and is vastly superior to that of conventional instrumentation.