[time-nuts] GPSDO simulation tool

Tom Van Baak tvb at LeapSecond.com
Sun Mar 23 01:16:54 EDT 2014


Hi Lars,

The dacbits and avg1 parameter are now enabled. Pick up a new copy of gpsim1.c / gpsim1.exe (http://leapsecond.com/tools/)

Ok, I added a 400,000 second dataset of a plain M12 (no sawtooth correction): gps-m12m.txt.gz (http://leapsecond.com/pages/gpsdo-sim/)

Note the 1PPS variation is about 40 ns vs 10 ns peak-to-peak (raw vs. corrected) or about 9 ns vs. 2 ns RMS. So, in general, M12 1PPS granularity improves by a factor of 4 or 5 with sawtooth correction.

In Timelab, load both gps-m12m.txt (uncorrected sawtooth) and gps-m12cns2.txt (sawtooth corrected). Notice the ADEV plots are have picture perfect 15e-9/tau and 3e-9/tau slopes. Now see MDEV/TDEV. Surprised? Look what happens after about tau 300.

Go ahead and make two virtual GPSDO runs, without and with sawtooth correction, using filter/PID defaults:
    gpsim1 gps-m12m.txt ocxo.dat > gpsdo-m12.txt
    gpsim1 gps-m12cns2.txt ocxo.dat > gpsdo-cns.txt

In Timelab, load both gpsdo-m12.txt and gpsdo-cns.txt. Check phase, frequency, ADEV, MDEV, TDEV. Does it makes sense, with this OCXO, that sawtooth correction is completely *irrelevant*? Is this an error in simulation or an error in intuition?

Two sample plots attached. But you'll have fun making your own, zooming in, changing parameters, and so on.

/tvb

----- Original Message ----- 
From: "Lars Walenius" <lars.walenius at hotmail.com>
To: <time-nuts at febo.com>
Sent: Saturday, March 22, 2014 3:25 PM
Subject: Re: [time-nuts] GPSDO simulation tool


> 
> 
> 
> 
> Many thanks Tom for an excellent tool and also the data you have provided. You don´t happen to have data for a non sawtooth corrected M12?
> 
> Another question: how do you insert the options for ticres and dacbits? With “gpsim1 avg1=10 gps-mtk3339.txt  ocxo.dat  >gpsdo.txt” I managed to get different prefiltering in the same way as you had in your graphs. But inserting dacbits=10 instead of avg1=10 gave an error for me.
> 
> Lars
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