[time-nuts] GPSDO simulation tool

Magnus Danielson magnus at rubidium.dyndns.org
Sun Mar 23 09:55:17 EDT 2014

On 23/03/14 06:56, Said Jackson wrote:
> Tom,
> In my experience one of the biggest GPSDO error sources in low-cost and even some higher end OCXOs is oscillator hysteresis. Which can change with operating temperature, operating time (crystal age) and even over crystal tilt.
> Does your OCXO model allow for hysteresis simulation?

No, not really. He uses a quite crude model at this point. I think it is 
fair, as it aims to get the basic principles in such a way that one can 
fool around.

For a more serious simulator it would be nice to have such behaviours 
modelled, but then it needs to be modelled with real oscillators 
behaviours and under the various stresses. That's a nice little topic 
for a series of articles in UFFC.

> It is very hard to discipline oscillators with hystereses and a good modeling tool would be helpful.

I think good modeling isn't very easy either. Maybe it is possible to 
get a crude model to capture the basics good enough for simulation use.

> Most ocxo vendors don't even know about hysteresis or how to quantify it (and react with a deer in the headlight look when asked about it), but even levels of ppt to tens of ppt can wreak havoc due to the fact that the loop is always chasing the ocxo if sufficient hysteresis exists.

Most sales-rep's don't have a clue, and even their market-oriented 
app-note-writes does not really have a clue.

If you have data to illustrate things, I'm happy to look at it and see 
what I can learn.

Temperature-induced hysteresis, which as far as I have read exists and 
matches my experience with drift of oscillators after start-up, sure 
puts a limit to how well TCXOs can perform relative to OCXOs.


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