The present invention relates to detecting error in the frequency of a clock.  More particularly, this invention relates to a digital circuit for detecting when the frequencies of two clocks are within a certain range of each other. 
Assume a receiver circuit with differential inputs biased close to the threshold of the receiver.  In cases where no cable is connected, the inputs can cause high-frequency oscillation of any outputs and clocks of the receiver.  These outputs and clocks can, in turn, drive other circuits, causing extra power dissipation in those circuits or in the receiver itself.  The chattering outputs can also cause multiple bit errors in the received data, giving a higher probability that incorrect symbols may escape error detection. 
Accordingly, there is a need for a circuit which enables the outputs of a circuit only when the received clock is within a reasonable frequency range. 
This and other objects and goals of the invention will be readily apparent to one of ordinary skill in the art on the reading of the background above and the description below.