[time-nuts] Re: Dumb nooby question ... sorry all

[Bob kb8tq]

Hi

The PPS out of a “normal” GPS module is derived from a free running
TCXO. The edge it puts out is only approximately correct. The magic
term for this a “sawtooth” since the departure from correct phase can
look a bit like a sawtooth when you plot it. 

Many modules provide a “sawtooth correction” output as part of the
serial stream. This lets a GPSDO work with a more accurate representation 
of what’s happening. Data collection wise, if you run a time interval counter
to capture the OCXO vs GPS info, you *could* fold that information into
your data plot. 

A sawtooth of 20 to 40 ns is not at all uncommon. As time has marched
on, the clock rates internal to the GPS modules have gotten higher. That
has dropped the peak to peak a bit. 

So, simple answer: what you see is normal.

Bob

> On Mar 19, 2022, at 6:02 AM, Nick <nick at nick-norman.co.uk> wrote:
> 
> Good morning FOAK :-)
> 
> Ok stupid question time (*):
> 
> Background: I've built up a J R Miller simple gpsdo (with a Jupiter GPS module <http://jrmiller.online/projects/ministd/frqstd0.htm>) basically because I had 90% of the bits and I could :-)
> 
> The GPS gets lock quickly with good data on the serial line.
> The OC bit of the OCXO seems to work the unit draws 600 mA dropping to about 300 mA after a few minutes and the OCXO gets warm.
> If I look at the output on a dual trace 'scope (high bandwidth hantek) trigger taken from the 10 kHz from the gps unit I see:
> 
> At switch on I get the marching waves I'd expect of it being 'un-locked'.
> 
> It then obviously gets into lock and the 'picture settles down' with the 10 Mhz wandering about for several minutes (10 or so) and then getting into close lock with the 10 kHz trigger.
> 
> Now the question:
> 
> How much 'wandering' of the 10 MHz should I expect if I compare the 10 kHz to the 10 MHz?  Here after a few hours the 10 MHz trace jitters (is I think the correct term) about 20-30 nS - normal or not?
> 
> I have no other time source to check it aganst (other than a Leo Bodnar mini gpsdo, but don't - yet - have any kit to do the comparison with)
> 
> Very many thanks for any insigth you may be able to provide
> 
> (*) I know no such thing as a stupid question
> -- 
> 'Somewhere, A Crime Is Happening,' said Dorfl.
> (Feet of Clay)
> Sat 10427 Sep 09:40:01 GMT 1993
> 09:40:01 up 16:13, 10 users, load average: 0.68, 0.59, 0.32<pic_10_1.png>_______________________________________________
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