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.