Digital Rate Synthesis for Frequency Measurement and Control

Digital rate synthesis is a transformation of harmonic synthesis (beat methods) from the frequency to the time domain; the synthesis of harmonics is replaced by pulse techniques. The signal whose period is to be measured or controlled is in the form of a pulse train that has the repetition rate X. A single reference source drives a rate generator which consists of one or more dividing stages in fixed cascade; the output of the generator is a sequence of nonuniformly spaced pulses whose average repetition rate AS is known from the independently controllable connection between the individual stages and their output bus. A rate difference detector compares the rates X and AS. The smoothed detector output has the proper sign and serves to indicate the magnitude of the difference or to reduce the difference in a closed loop, for example, by controlling a tuning motor. The range of direct operation is limited by pulse techniques, but digital and harmonic synthesis can be combined with division and with phase-lock methods to stabilize an oscillator over a practically unlimited band of frequencies, the precision and purity approaching that of the reference source.