[time-nuts] 10 MHz TCXO periodically jumping 20 mHz up and down

[Erik Kaashoek]

During long term testing of some 10 MHz TCXOÂ  the output frequency seems 
to jump within one second 20 mHz ( millihertz) up in frequency  every 
110 seconds up and after a 25 seconds, again within one second, the same 
amount down. The noise in the frequency measurement was well below 5 mHz
In an ASCII drawing of frequency versus time this looked like: 
________|~~~~|___________|~~~~|________
Sometimes the high frequency period was very short (some seconds) or 
absent but the overall period was within 5 seconds constant
This was tested with 4 different power supplies, although all where 
mains connected, not yet tested with battery only, 2 different counters 
and two different reference frequency standards.
The TCXO was thermally shielded and testing with some cold air showed a 
different  behavior for external temperature changes (fast jump away and 
slow return to stable frequency)
Also, with thermal shielding removed, touching the TCXO showed also a 
fast jump away and slow returning to stable
Measuring the supply voltage did not show clear changes but the used 
voltmeter only had 4 digits resolution.
The official spec of the TCXO is much worse so the device is well within 
spec but I'm trying to understand why this could happen.
Does anyone know a possible cause for this behavior?
Could this be a small mains supply variation in a 110 seconds long cycle?
Or what else?