Fixed-frequency clocks produce a single constant output frequency. Other clock
circuits, such as phased-locked loops (PLLs) and voltage controlled oscillators
(VCOs) have an adjustable frequency range. Unfortunately, such devices are
usually more jittery than fixed-frequency clocks.
The Audiophillleo1 and Audiophilleo2 USB to S/PDIF transports are two of the few such devices with fixed-frequency clocks, which, along with the isolated differential ECL output stage, is a primary reason for their radically lower jitter, compared to other products. Fixed-frequency clocks may be used because both Audiophilleos employ asynchronous USB, and contain the master clock, instead of being slaved to the computer's USB clock.