[time-nuts] GPS noise reduction

Magnus Danielson magnus at rubidium.dyndns.org
Sun Apr 6 07:40:13 EDT 2008

From: "Tom Van Baak" <tvb at LeapSecond.com>
Subject: Re: [time-nuts] GPS noise reduction
Date: Sat, 5 Apr 2008 23:02:34 -0700
Message-ID: <005a01c897ab$cee7d200$0600a8c0 at pc52>

> Richard,
> One more thing, maybe I missed this earlier in the thread, but
> can you really call it "sawtooth correction" when you apply a
> receiver reported correction (with 1 ns granularity) to a 1PPS
> signal (with 9 ns RMS or +/- 15 p-p jitter) as coarsely measured
> by a 100 MHz TIC (with 10 ns resolution)?

Recall that it is the difference between the A/B OCXO which is being measured,
and sawtooth corrections can be directly applied to correct the PPS phase, so
the phase deviation characteristics of the uncorrected PPS becomes virtually
uninteresting except from a system design aspect. The jitter properties of the
corrected PPS is however of great interest and the 1 ns staircase that we can
see as systematic effect should be an indication.

> I haven't done the math, but it seems like it's at least an order
> of magnitude less precise of a correction than the normal case
> where one makes ns or sub-ns measurements and then applies
> a ns-resolution correction in software. This is what both Tac32
> (for Oncore) and Tboltmon (for Thunderbolt) do when you use
> one serial port for the GPS receiver and another serial port for
> a 53131/2 time interval counter.
> It just seems the small ns-level corrections from your receiver
> get rather lost in the large 10 ns resolution of your TIC. More
> like a staircase than a sawtooth, perhaps?

Yes... but if you give it TIC resolutions below 1 ns you enter the same
problem, but at a lower deviation. There is always a limit.

Trying to acheive 1 ns resolution on the TICs should be worth it.


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