Timing extraction from baseband data waveforms

Abstract

The timing information in synchronous digital regenerators is derived by passing the baseband data signal through a circuit with nonlinear transfer characteristics followed by a band pass filter. The theoretical and experimental analyses of the performance are given for two types of band pass filter, a single tuned circuit and a phase-locked loop(PLL). When the single tuned circuit is operated as an oscillator, which is triggered by pulses derived from the data transitions, the variance of the output phase jitter is almost equal to that of the input phase jitter. The average phase shift from optimum sampling points is found small in this case. For the PLL filter system, it is pointed out that a sawtooth phase comparator implemented by flip-flops produces not only a useful error signal, but also injects ``noise`` into the PLL. This noise is from the randomness of data. A time-invariant linear model is developed and analyzed for the PLL system. The mean-square output phase jitter is shown proportional to the jitter bandwidth, which can be expressed in terms of loop parameters. For a chain of regenerators with PLL timing, the phase jitter accumulates without bound, and the rate of accumulation is dependent on the damping factor of the PLL. Jitter generation and measurement sets for experiments are made by specially designed PLL``s. Spectral density of the random pulse train with jitter is derived for a bandlimited white Gaussian jitter, and applied in the analysis for PLL filter system.