u-blox GPSDO: Update!

With the hardware already set up to provide a 10 MHz signal and electronic frequency correction, some optimization of the algorithm used for phase locking. It needs to be a really low frequency low pass filter (say, 0.001 Hz), and we need to deal with the discrete nature of the measurements and the quantization.
This is accomplished by three mathematical approaches
(1) The data is sent through a 32-parameter FIR low pass.
(2) The frequency drift is calculated for the last 32 seconds, and used as derivative signal, as long as the oscillator drift is less than 10e-9 (1 ns every second!).

The u-blox settings – these are no timing receivers, but I set the device for 2D stationary navigation, it gave the best results here. Also, better disable all messages you don’t need, it will be beneficial to avoid overloading of the slow serial interface (still running at 9600 baud).

Here some examples:

With the PLL open, the signal is drifting away, albeit, at a very small rate.

To check the stability and general behavior, I’m monitoring the 10 MHz signal on a scope, triggered by the time pulse (set to 100 kHz) of a 2nd u-blox receiver, sitting closeby. Horizontal deflection is 10 ns per div. Sure, the signal is broadened by the interpolation of the 2nd receiver, which has a free-running TCXO. Because of the synthesis of the 100 kHz signal from the internal 48 MHz, the trigger has about 20 ns jitter-no problem here, because the drift is much stronger and the phase of the 10 MHz signal relative to the 2nd receiver can easily be measured down to 1-2 ns.

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