[time-nuts] Comparing PPS from 2 GPS units

Hal Murray hmurray at megapathdsl.net
Mon Dec 17 18:19:43 UTC 2012

> 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).

Thanks.  Those are more $$$ than I'm interested in right now, but might be a 
useful tool sometime in the future.

Another approach is to use a scope: trigger on one PPS and adjust the delay 
(which might be negative) and sweep speed so you can see the other PPS 
signal.  Maybe I'll play with this to see what sort of results I can get.

> 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. 

You can make a reasonable delay line by using the lumped circuit 
approximation for the L and C for the appropriate impedance transmission 
line.  I assume that's what's in the delay boxes.  I should try that 
sometime.  Thanks for the reminder.

The delay chips I've looked at before used gate delays.  I think they were 
Motorola rather than Dallas.  I just poked at a few Maxim data sheets.  I 
didn't find out how they implemented the delays.

I think some of the clock recovery chips tune delays by tweaking the 
threshold voltage.

These are my opinions.  I hate spam.

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