[time-nuts] What is the best counter for a Time Nuts?

[Tom Van Baak]

> This seems like a pretty straight forward technique. Am  I missing
> something?

Hi Charlie,

Nope, your basic method of measuring frequency *accuracy*
with two phase angle measurements is fine. I think many of us
do the same, one way or another. Relative frequency error is,
after all, just a measure of _phase_ drift over time. So you're
good to go.

One has to be careful, though, not to claim more accuracy than
is fair. Once you take into account
    the instability of your unit under test,
    the resolution and stability of the phase comparator itself,
    and the accuracy and stability of the frequency reference,
you may find that your accuracy method is slightly less precise
than the simple math suggests.

Going one step further, note that measuring frequency *stability* 
is essentially taking multiple frequency accuracy measurements
and statistically looking at _frequency_ drift over time.

>  24 hrs  would get you to 2.77e-10 / (24 * 3600) = 3.2e-15,  which is
>  very acceptable. That puts you in the big leagues.

Hi Mike,

It doesn't quite work this way. If it did, hey, you could wait a
month, and be better than the big leagues!

The missing consideration is the stability of the test setup. The
frequency reference and the phase comparator would have to
have 10^-15 levels of stability before you could claim this sort
of per-day measurement resolution. I can tell you a telecom
rubidium and hp3575A are not even close to this.

>  Can you see any drift in the GPS time?

You're not likely to see "drift in GPS time" when using a free
running rubidium or another GPSDO as a reference. Do you
see why?

/tvb