[time-nuts] GPSDO simulation tool

Said Jackson saidjack at aol.com
Sun Mar 23 01:56:06 EDT 2014


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?

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

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.


Sent From iPhone

On Mar 22, 2014, at 22:10, "Tom Van Baak" <tvb at LeapSecond.com> wrote:

> Hi Don,
> Yes, easy! And that's exactly the idea -- to take real inputs (or borrowed copies of real inputs), and a real software algorithm, and measure the virtual output to see how well your algorithm and tunable parameters work. Tweak parameters. Evolve the algorithm. Simmer until well done.
> Theoretically, after one builds the real GPSDO, using the same code or at least the same algorithm, the actual performance should nearly perfectly match the simulation.
> The difference, at least for me, is that I'd rather play with unix commands and C code on a PC, trying things out in a matter of minutes, than spend weeks slowly trying different things with a real GPSDO (which I've also done). In addition, I think gpsim1 makes a useful, almost interactive, teaching tool.
> Now, no simulation is perfect. But oscillators, dividers, 1PPS comparators, and DACs are not really that complicated. You are probably guessing that I'm working on gpsim2 which will allow simulation of phase and frequency jumps, varying GPS reception, power-up, cold-boot, warm-boot, holdover, thermal or mechanical shocks to the instrument, and other events that I see in real life.
> But let's let gpsim1 run its coarse before we worry about 2nd order effects. I'm very interested in alternative or enhanced algorithms that people come up with. The two algorithms now in gpsim1 and default tuning parameters are just something I threw together in a few minutes.
> /tvb
> ----- Original Message ----- 
> From: "Don Latham" <djl at montana.com>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Saturday, March 22, 2014 8:18 PM
> Subject: Re: [time-nuts] GPSDO simulation tool
> Hi Tom et.al.   Isn't the simulator "easily" convertible to the real thing?
> That is, data inputs should be convertible somehow to data streams from
> physical devices?
> Don
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