Modern digital systems typically include multiple digital devices capable of communicating with each other using digital signals made of sequences of digital symbols. A digital symbol takes up a time interval, which is often referred to as a unit interval or a data interval. A digital device may transmit a digital data signal by setting the value of a signal parameter associated with a communication channel to one of a plurality of predetermined values for each data interval according to a transmitter clock. A digital device that receives the digital signal takes successive samples of the signal according to a receiver clock and determines the values of the signal parameter represented by the samples. The receiver clock is preferably synchronized with the digital signal so that the samples are taken at times corresponding to the bit intervals.
To communicate data at high speed, it is desirable to make the data intervals as short as possible, which, in turn, requires greater precision in the synchronization between the receiver clock used by the receiving device and the data intervals of the incoming signal. For example, modern interfacing and communication protocols such as PCI Express, SONET, InfiniBand and XAUI use data intervals on the order of nanoseconds or less, and require that the receiving device use a clock which is synchronized to the data intervals to within a fraction of one data interval. For ease of synchronization, many digital systems are designed such that the transmitter clock and the receiver clock both have a relatively stable frequency. However, such systems may be susceptible to electromagnetic interference (EMI) at or near a particular frequency, and may interfere with other systems that are sensitive to signals at or near the particular frequency.