US Patent 7746972 Numerically-controlled phase-lock loop with input timing reference-dependent ratio adjustment

A numerically-controlled phase-lock loop with input clock dependent ratio adjustment provides for narrower-bandwidth loops that lock to a wide range of frequencies and/or operation with an absent or degraded input timing reference. A timing reference characteristic detector determines an input frequency range of the input timing reference signal, the data type of the timing reference, and/or whether a timing reference signal of sufficient quality is present. A numerically controlled oscillator is controlled by a numeric ratio that is adjusted to provide the desired clock frequency output in conformity with the detected frequency range and/or data type. If the timing reference signal is absent or degraded, then the numeric ratio can be set to a fixed value or a local timing reference can be applied in order to generate the desired clock output frequency.