GPS reference generator

1PPS GPS synchronized Si5351 frequency generator

Lately when experimenting with a Si5351 board I was wondering how to calibrate it. Same issue applied when I was building crystal ladder filter – my network analyzer is not calibrated. I have a 15 year old frequency counter but who knows whether it is still accurate.
Searching the internet I found this nice project from SQ1GU which I decided to build by myself.
I used the code from SG1GU’s website. The only changes I made was to add two red/green LEDs. The LEDs are 2 pin bi-color LEDs. Per LED only one Arduino pin is required.
The 1st LED shows whether the device is locked to the GPS signal, the 2nd LED indicated whether the accuracy is within the specified range (I defined it as +/- 0.5 Hz).

The “GPS lock LED” is basically the same as the “*” which is shown in the display behind the frequency once the measurement of the 2nd subsequent 40 seconds was without signal loss.
The “precise” LED is changing to green when the accuracy is between +/- 0.5 Hz.

As written on SQ1GU’s website when you power up the device the first time you need to perform EEprom initialization. Therefore press the encoder button and turn on the power.

Note: as per the source code the calibration value is only written to the EEprom if the device has reached a stable status with very high accuracy (which takes some time). If you only power it up the first time for some minutes the device might not reach this status. In this case you have to repeat the EEprom initialization procedure. My recommendation is to power it up, place the antenna at a good location and leave it there for the next 30 mins.

Thanks SG1GU for sharing the circuit and the source code. Beside the circuit itself I very much like the case you made for your generator.

Rather than a paper label, this time I used a silver metallic label for the front of my case, covered by a adhesive transparent foil. I cannot recomment the silver metallic label. Yes, it’s good to print on but you can see every little scratch.

to be continued…..

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