  We describe a low-cost system designed to monitor wander in digital clocks
with a precision of $\le$ 1 ps. With this system we have shown that it is
possible to track phase variations at the sub-picosecond level by adding noise
to a reference clock. As in many cases where a clock is part of a complex
distribution network small changes in temperature and other effects can lead to
small changes in the a clock's phase. As a further demonstration of the system,
we have used it to measure the phase changes induced in optical signals in
fibers.
