Answer 6—The jitter, and therefore the phase modulation, can be randomized using a pseudorandom number generator (PRNG). For example, a linear feedback shift register (LFSR) could be used. One way to accomplish that is to put an additional flip-flop in the path following the requantization flip-flop. This creates a version of the 1-MHz clock that is delayed by 40 ns, the period of the 25-MHz clock.

A multiplexer is used to randomly select either the “early” version of the clock or the “late” version, based on the output of an LFSR that’s driven by the 25-MHz clock. The output of the multiplexer drives the sine wave generator.

Contributor: David Tweed

Published September 2003