[time-nuts] And, for my next trick, 50Hz

[Tom Van Baak]

> Many thanks for these pointers, very interesting and useful work. This
> does show that for a long sampling time, quite a high degree of
> accuracy could be obtained. The downsides are that the sampling period
> would have to be very long and could easily be affected by power cuts
> causing failure to my proposed testing method. So theory proved but
> impractical. Looks like I'll have to bite the bullet and go with a GPS
> disciplined ocxo to make a good frequency standard.
> 
> Cheers,
> Steve

Ah, if you are a time-nut of course you will bite the bullet and
get a GPSDO. But you should also not give up on the PC idea.
If it works it will be a great gift to many people (OK, maybe not
the pico- and nanosecond crowd, but to regular millisecond
kind of folks).

And, I no longer think long sample times are required.

Consider the following. Assume you can get NTP to give you
ms or sub-millisecond accurate time-stamps. Also assume you
divide down your UUT to something like 1 kHz and feed that
into one channel of your sound card (and as Bruce pointed
out, maybe the slower the rise time the better in this case).

Now, collect 16-bits of waveform data at 44.1 kHz. First, note
that it's not exactly 44.100000 kHz -- but over time, as your
circular input buffers fill up, you can use NTP time-stamps to
calculate what the sampling rate precisely was/is.

Then, looking at your highly oversampled waveform data, you
can calculate the phase of your UUT frequency relative to the
now precisely known sound card sampling rate. Over time you
will see the phase drift, which then directly gives you the UUT
frequency error.

So what you end up doing is using the sound card like a high
resolution vernier between NTP timekeeping on the inside and
your UUT on the outside. I bet you a Thunderbolt that you can
measure to 1 ppm within ten seconds.

/tvb
http://www.LeapSecond.com

p.s. For extra credit, tee your UUT into both channels, do twice
the math, and see if you can measure both differential phase,
and differential phase drift between them.