[time-nuts] GPSDO simulation tool

Bob Camp lists at rtty.us
Sun Mar 23 12:15:29 EDT 2014


Unless you can include all the second and third order effects in your model, there will still be a lot to “learn” as you build your gizmo. Computing power is still impacted by garbage in / garbage out.


On Mar 23, 2014, at 12:07 PM, Don Latham <djl at montana.com> wrote:

> I agree..  Had a maybe weird thought.   Computing power is now a trivial part
> of the "problem", in that a Ras.Pi can be put to the task for $35 or so. In
> that case, PIDdling with time constants and such in a feedback loop with phase
> difference as the error signal is easily done, either in simulation, or for
> real, as you point out. So, since ADEV or one of its brothers is the measure
> of success of PIDdling, shouldn't the ADEV or brethren be in some way the
> error signal for the whole?  For example, how about a TCGPSDO? put the whole
> thing in a first-order thermal enclosure, with power-over-ethernet being the
> only  external wiring?
> Hope I'm at least making at least logical sense…
> Don
> Tom Van Baak
>> 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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> -- 
> "The power of accurate observation is commonly called cynicism by those who
> have not got it."
> -George Bernard Shaw
> Dr. Don Latham AJ7LL
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