  The Allan variance of signal and reference frequencies is measured by a
least-squares fit of the output of two analog-to-digital converters (ADC's) to
ideal sine waves. The difference in the fit phase of the two channels generates
the timing data needed for the Allan variance. The fits are performed at the
signal frequency ($\approx 10 $ MHz) without the use of heterodyning.
Experimental data from a modified digital oscilloscope yields a residual Allan
deviation of $3 \times 10^{-13}/\tau $, where $\tau$ is the observation time in
sec. This corresponds to a standard deviation in time of $<$ 300 fs or 20
$\mu$Rad in phase. The experimental results are supported by statistical theory
and Monte Carlo simulations which suggest that optimized devices may have one
or two orders of magnitude better performance.

Instrumentation and Detectors