[time-nuts] How do I know my GPS stabilizedoscillator is working?

[Tom Van Baak]

I'd also be interested in any anecdotes out there with
regard to this subject. Here's my 2 cents...

Start/Stop -- Either way works and gives the same data.
Both ways make your brain spin when you have to
figure the sign(s) of the phase, frequency, and drift.

DUT -- Note sometimes it's hard to know what "DUT"
is. If you're comparing a surplus GPSDO you just
picked up with a cesium that you already trust, which
is the DUT? In the short-term the Cs is the reference,
making the GPSDO the DUT. But in the long-term,
if the GPSDO works well, it will be a better reference
and your cesium becomes the DUT. So there are
cases where the DUT/start/stop issue is ambiguous.

Zero -- The one thing that's a pain is sign wrap in
time interval data. One solution is to offset one or
the other by some amount prior to the experiment.
I keep one of my standards ahead (or is it behind)
by about 10 us to virtually guarantee all positive
time-interval readings. Clearly then the earlier
standard, which ever it is, gets the start channel.

Drift -- Another consideration is when you know if
the DUT is drifting forward or backward in time. In
this case arrange the start/stop channels so that,
over time, you always diverge from zero rather than
approach and cross zero. This argues for putting
the device with higher frequency as the start channel
(lower frequency as the stop channel). Or is it the
other way around.

1PPS -- Lastly, there is one convention I found handy
with 1 PPS sources, specifically those GPS boards
that are designed to suppress the 1PPS signal when
they loose lock. In this scenario make the reliable
1PPS ref the start channel and the GPS 1PPS the
stop channel. If lock is ever lost -- your TI readings
will reveal the number of missing pulses. For example:

(REF=start, GPS DUT=stop)
    8.9755  us
    8.9368  us
    8.8986  us
    8.9214  us
    8.8829  us
    34.0000088063 s
    8.8310 us
    8.7332 us

If you wire the TIC the other way around, then those
missing 34 pulses won't even show up in your data.
For example:

(GPS DUT=start, REF=stop)
    8.9755  us
    8.9368  us
    8.8986  us
    8.9214  us
    8.8829  us  [followed by 34 seconds of silence]
    8.8063  us
    8.8310  us
    8.7332  us

In summary, there's isn't a right way and a wrong way.
There may be, in some cases, a convenience of one
over the other.

/tvb

----- Original Message -----
From: "John Ackermann N8UR" <jra at febo.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Sunday, July 30, 2006 18:33
Subject: Re: [time-nuts] How do I know my GPS stabilizedoscillator is
working?


> My understanding is that the convention is to put the DUT on the start
> and the reference on the stop (reversing the two will obviously reverse
> the sign of the offset and drift) but I am not sure how widely accepted
> that convention is.
>
> I'd be really happy to have some confirmation that in fact I'm doing it
> the right way.
>
> John
> ----
>
> Glenn said the following on 07/30/2006 04:28 PM:
> > Just curious, why do you use GPS for the stop input instead of the other
> > way 'round?
> > -glenn
> >
> >
> >>John Ackermann N8UR wrote:
> >>
> >>
> >>
> >>>Hi Faisal --
> >>>
> >>>I use a time interval counter to compare 1pps signals -- the DUT goes
to
> >>>the start input, and the reference (here, GPS) to the stop input.
> >>>