[time-nuts] lightening protection of a GPSDO system / optical isolated distribution amp

Jim Harman j99harman at gmail.com
Sun Dec 7 21:51:52 EST 2014

The topic has shifted from lightning protection, but I thought I would
share my experiences on diurnal timing shifts.

On my home-built GPSDO, similar to the design posted by Lars Walenius some
time ago, I can see variations in the TIC output that correlate well to to
the variations 24 hours ago. The variations are about 100 ns p-p and are
noticeably larger when it is raining outside. I think the variations were
also greater over the summer, when the sky view was more obstructed by

My system uses an Adafruit (Globaltop) GPS module, so not the greatest for
timing applications. I have an outdoor puck antenna located a few feet
above ground level and a few feet from a wooden house, with a mostly
unobstructed view to the west.

The time constant for steering the OCXO is set to 2048 sec. The timing
variations appear as noisy "lumps" that last about 90 minutes, so the
disciplining is reducing but not eliminating the variations.

On Sun, Dec 7, 2014 at 2:49 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
> The 12 hour periodicity of GPS is (in general) less obvious in an ADEV
> plot than 24 and 48 hour effects. Part of this is due to the lower “floor”
> at longer tau. Another part of it is due to things like the  ionosphere
> being at different places at the 12 hour points.
> Bob
> > On Dec 7, 2014, at 2:27 PM, Chris Albertson <albertson.chris at gmail.com>
> wrote:
> >
> >>
> >> Bob,
> >> can you explain more about the effect of antenna performance on a GPSDO
> >> system?
> >>
> >> Now you have told me it is important,  I would like to know more! My lab
> >> has an East facing window.
> >
> >
> > Physically what is happening here is that the error bars on the fix your
> > GPS gets will cycle over a 12  hour period because of the sats all have
> 12
> > hour orbital periods.   If you can only see part of the sky the amplitude
> > of this is greater.
> >
> > As an example thing of the worst possible case where you antenna can only
> > see a few degrees of the sky.  Every 12 hours one GPS satellite comes
> into
> > view and your GPS gets a decent "fix" but then for 8 hours the GPS sees
> > nothing and drifts off.  Now think about moving the antenna to a
> marginally
> > better place where it always sees at last ONE GPS stiletto but for 8
> hours
> > there are two and for 2hours there are three satellites.   The quality if
> > the "fix" would still vary but would be better.   In the best case your
> 12
> > channel GPS receiver ALWAYS is able to select the 12 BEST places GPS
> > satellites that are in view.
> >
> > This is not a great effect as long as there is enough sky that there are
> > always some in view.
> >


--Jim Harman

More information about the time-nuts mailing list