UBLOX GPS waveform jitter: experiment shows the cause

There are 2 different situations that result in signal jitter from uBlox GPS, as follows:

Situation 1: Jitter happens whenever the chosen output frequency does NOT divide evenly into 24 MHz (where 24 MHz is the frequency of the uBlox onboard TCXO). This jitter happens because the uBlox can only produce these "odd" frequencies by mixing "even" frequencies of different wavelengths. Note that this type of jitter happens whether or not a GPS satellite has been acquired. Avoiding this first type of jitter is simple: choose output frequencies that divide evenly into 24 MHz.

Situation 2: Jitter occurs after GPS satellite acquisition. Note that this type of jitter occurs regardless of the chosen output frequency. There does not appear to be a workaround to avoid this situation. This jitter seems to be due to the way time is maintained by the uBlox device before vs after satellite acquisition. Prior to satellite acquisition, the internal TCXO is relied upon for all timing. Thus, generated output frequencies are produced and counted by the same TCXO clock. Thus, they remain in sync and jitter is avoided. After satellite acquisition, the uBlox relies on precision GPS time and mixes different frequencies in order to produce the desired output frequency (cycles per second). This mixing of frequencies causes jitter. As the video demonstrated, the output frequency is primarily controlled by the physical properties of the onboard oscillator, while frequency measurements/counting is controlled by the method of timing (GPS vs TCXO). The frequency of the resulting jitter depends on the difference between GPS clock ticks and the uBlox oscillator's clock ticks.

Conclusion
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In both situations, the root cause of jitter is fundamentally the same: The desired signal's wavelength does not fit evenly between clock ticks. Since wavelength and frequency are related by the speed of light, there is nothing that can be done other than to mix different frequencies, in order to output the requested cycles between clock ticks. This causes output jitter.

If you want to use your uBlox GPS unit as a limited signal generator and can accept an inaccuracy of a few Hertz, choose an output frequency that divides evenly into 24 MHz and turn off the satellite networks using uCenter software. This should produce a jitter-free output signal.

Correction
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The NEO-M8N has a TCXO -
Temperature-Compensated Crystal Oscillator (not an oven controlled crystal oscillator).