Apparatus and method to hold pll output frequency when input clock is lost

Abstract

A clock conditioning circuit including a phase detector circuit configured to provide an analog tuning signal indicative of a phase relationship between a reference clock to be conditioned and a generated clock. The controlled oscillator is configured to produce the generated clock, with the generated clock having an output frequency adjustable in response to an analog tuning signal applied to a control signal input of the controlled oscillator. Converter circuitry is provided to produce a digital representation of the analog tuning signal when the mode control circuitry is in a tracking mode. In the event the reference clock is lost, the mode control circuitry switches to a holdover mode so as to provide an analog holdover signal to the control signal input based upon the digital representations produced just prior to the loss of the reference clock.