[time-nuts] Comparing PPS from 2 GPS units
Tom Van Baak
tvb at LeapSecond.com
Thu Dec 13 17:55:19 UTC 2012
This happens to me too. The solution I usually use is to adjust the cable delay parameter. I find it convenient sometimes to advance the 1PPS by 1 microsecond. Kind of like "at the tone, the time will be", rather than "at the tone, the time was". Give up on "at the tone, the time is".
The other solution is to use two (N) counters. Connect a clean (OCXO+divider is ok) 1 Hz signal to all start channel inputs. Each GPS 1PPS then goes to a stop channel; one per counter. From the raw data, you can then calculate all pair-wise combinations of N GPS receivers. The inaccuracy of the OCXO drops out of the calculations.
A third solution is to use a time-stamping counter (e.g., Pendulum CNT-91). In this case the order of events doesn't matter. You can calculate whatever you want from the raw timestamp data of all pulses received.
A long delay cable is fine too. If these are timing receivers you probably don't need more than 100 ns of delay, once you figure out which receiver is ahead of the other. The cable tempco is low enough not to worry about.
A fifth solution is to use a pulse delay generator like a DG535. I use this to create high-resolution early/late 1PPS sync pulses. They show up on eBay, but aren't cheap. For bargains, watch for older model programmable pulse delay generators by BNC (Berkeley Nucleonics Corporation).
Lastly, there are cute little delay boxes (www.ebay.com/itm/150962422699) that might work. Not sure how stable they are at the ns level. But it would be fun to measure. If someone opens one of these please tell us if it's a coil of wire, some kind of LRC filter delay, or if they use those Dallas delay chips. Which is another solution for you -- google or eBay search for: silicon delay line.
----- Original Message -----
From: "Hal Murray" <hmurray at megapathdsl.net>
To: <time-nuts at febo.com>
Sent: Thursday, December 13, 2012 1:57 AM
Subject: [time-nuts] Comparing PPS from 2 GPS units
> Suppose I want to compare the PPS outputs of 2 GPS units. The problem is
> that I don't know which one will happen first.
> If I feed them into the start/stop inputs of a typical timer/freq box, I
> don't know which is which. If I get them wrong, the answer will be
> 0.999999xx seconds rather than the -0.000000xx that I want.
> Is there a simple solution for this?
> My straw man is to use the antenna cable delay setting to offset one of the
> PPS pulses a long way.
> Plan B would be a physical delay unit. But they are probably temperature
> sensitive, and I'm not setup to keep anything at a constant temperature.
> Do any of the counter/timer boxes have a mode for this? If so, is there a
> buzzword I've overlooked?
> These are my opinions. I hate spam.
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