Ublox Neo M8T unstable frequency output

[Miti]

I wanted for quite awhile to make a 10MHz  GPSDO for calibrating my frequency counter and spectrum analyzer. Today I started playing with a Ublox Neo M8T that I recovered from a board. This module has two configurable timepulse outputs from 1Hz to 10MHz. So trying to decide if I go with a 10KHz - 1MHz PLL or a 1PPS FLL, I set a 1MHz pulse on Timepulse 1 (pin 3) and I connect the scope and to my surprise the pulse is very unstable. Timepulse2 seems to be very stable. Any idea why?

[MosherIV]

Hi, the Ublox range is based on an Arm core, I think it is an STM32 of some kind, they are clocked by a XO (hopefully a TCXO for a M8T device).
The Freq outputs that you get are based on the timer/counters in the STM32. The Ublox will syncronise the counter with both the TCXO and the GPS signal, this means that the counter will be reset when the STM32 detects the GPS clock is out of sync with the TCXO. This causes the jitter that you see.
Many people report that the Freq out from GPS devices have this jitter.

Most GPS devices now no longer output the clocking freq from the GPS signal.

Most poeple now use the 1pps to gate a count of your local OX and average over long time periods.

By FLL do you mean 'Freq Lock Loop' ?

[Miti]

This is embarrassing, I did not remove the module from the circuit and the timepulse output goes to a PIC input. I powered only the GPS module but I guess the PIC was trying to run from the PPS signal through the clamp diode and was loading the PPS output. Once I cut the trace to the PIC, the output is clean.
Ublox specifies the PPS accuracy to 500ns indoor and 20ns clear sky. That correspond to 500ppb indoor and 80ppb clear sky by my calculations. That is not bad at all, it beats most free running oven oscillators and definitely enough to verify almost any instrument.
Yes, FLL is frequency lock loop and is exactly what you describe, for example accumulating the counts over 16 seconds and correcting the oscillator to get an expected number.