[time-nuts] GPSDO simulation tool

Tom Van Baak tvb at LeapSecond.com
Fri Mar 21 17:55:34 EDT 2014

Designing a GPSDO is a permanent topic of time-nuts, and always invites lots of opinions and methods.

The net performance of a microcontroller-based GPSDO is mostly due to the following ingredients:
- the stability of the OCXO (or TCXO or Rb or whatever the LO is)
- the stability of the GPS 1PPS (including sawtooth correction, or not)
- the disciplining algorithm itself, and user-settable configuration parameters or filtering
- the finite resolution of the TIC or phase comparator
- the finite resolution of the DAC/EFC

Normally what happens is that someone spends weeks or months or even years working on each of these ingredients, measuring, comparing, tweaking, or maybe just hoping for the best. These measurements can take a lot of time, or be difficult to replicate.

I have an alternative.

It's a simple software tool which takes *real* GPS phase data, and *real* LO phase data, and a *real* GPSDO algorithm(s) -- along with optional resolution of the TIC and optional resolution of the DAC -- and then creates GPSDO phase data through *simulation*. You can then plot this virtual GPSDO phase data with Stable32 or Plotter or TimeLab or your favorite phase / frequency / stability tool.

So instead of waiting hours and days to test your new filtering idea, or your new GPSDO algorithm, or to compare the effect of a 10 ns vs. 1 ns vs. 100 ps vs. 10 ps TIC, or to compare the effect a 10-bit vs. 16-bit vs. 24-bit DAC -- you just run the simulation on your PC and get an answer in a few seconds.

Have a look and let me know what you think. The tool is gpsim1.c (Windows: gpsim1.exe) under:

For this to work, one needs actual GPS data and actual LO data. I have a growing collection of sample data files here:

For example, if you run this command:
    gpsim1  gps-mtk3339.txt  ocxo.dat  >gpsdo.txt

and use TimeLab to plot these three files, you will get the attached plot. No solder, no instruments, no antenna, no waiting, no guessing. A complete 4-day simulation takes just 3 seconds (on my 10-year old laptop). Load the simulated phase data with 'L' in TimeLab and view phase, frequency, ADEV, MDEV, TDEV. Answer your GPSDO design questions in minutes instead of weeks.

Try different parameters. Try different GPS boards. Try different oscillators. See if you can make the best ADEV. Try new disciplining algorithms. Make the PID more complex. Change the filtering.

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