[time-nuts] GPSDO simulation tool
azelio.boriani at screen.it
Fri Mar 21 19:53:14 EDT 2014
On Sat, Mar 22, 2014 at 12:25 AM, djl <djl at montana.com> wrote:
> Well done!
> On 03/21/2014 03:55 PM, Tom Van Baak wrote:
>> 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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