[time-nuts] Freq cal to 1ppm without GPSDO

Murray Greenman Murray.Greenman at rakon.com
Thu Feb 17 02:52:57 UTC 2011

I've had quite a bit of experience doing what you want to do, and have tried several methods. TV used to be a good way, but now that everything's gone digital that method is out.

Despite advice to the contrary, comparing with WWV or similar on HF is NOT reliable to 1ppm. This is because there is Doppler on the short wave F-layer signal to the tune of several ppm. (It can be done if you are within ground wave or E layer range). An experienced user can achieve results with WWV by understanding when the propagation is E layer, and always using the same time of day.

1. My best advice is to get hold of a cheap GPS module with a 1pps (seconds pulse) output. Connect it up, and when you have a fix, use the 1pps to trigger your digital oscilloscope. Set the timebase to 1us/div to start with, and ultimately 100ns, and observe the 10MHz output of your TCXO. You will see the waveform drifting slowly. Counting how long it takes to slip one cycle will tell you how far off the TCXO is. If you have a counter with Time Interval mode capability, you could use that, using the GPS to start and 10MHz to stop, again observing the drift.

2. The reference oscillator in a GPS receiver isn't any help to you. While they need to be stable, they are NOT usually controlled by the GPS receiver. If you know what you are doing with the GPS NMEA 'clock bias' telemetry, you can work out what the receiver reference is doing, and compare with that, but that's only for experts.

3. If you want to divide the 10MHz down to 2Hz, you can run a 'Quartz' clock mechanism with second hand, and then, although this method is very slow, compare clock time with WWV's minute marks. A slip of 1 sec every two weeks is about 1ppm!

Bear in mind that few TCXOs have a spec better than ±2ppm, and will often have other dynamic and hysteresis effects amounting to the same order. If you can find an OCXO to use as reference, so much the better.

Method 1 is really simple to do and positively foolproof!

Murray ZL1BPU

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