Electrical signal jitter and wander measurement system and method

Abstract

An electrical signal jitter and wander measurement system (30) operates in real time and digitally controls bandwidths over which the measurements are performed. A digital phase-lock loop ('PLL') (34) includes a phase detector (44), low pass filters (48, 56), an analog-to-digital converter ('ADC') (54), a digital signal processor ('DSP') (32), a direct digital synthesizer ('DDS') (38), and a tracking oscillator (39). The phase detector receives an input signal that is compared with a signal derived from the DDS. The phase detector signal contains wander and jitter data that are filtered and digitized by the ADC. The DSP receives the data and performs a proportional integral control function to lock the PLL by digitally controlling the DDS frequency. The DDS generates a clock signal at a precise rate determined by the phase accumulation registers. The tracking oscillator locks to multiples of the DDS frequency to increase the resolution of the phase measurement. A master reference clock (40) controls the PLL with a stability and accuracy sufficient to measure low frequency wander. Wander data are available from the DSP as an integral of the DDS operating frequency. The DSP also performs the required loop filter function and high pass filters the wander data to provide subband jitter data. This invention digitally controls the PLL filter high pass bandwidth down to very low frequencies to accurately measure low frequency jitter and wander.