Thanks for all the excellent info
The TCXO is actually a VC-TCXO at 10MHz intended for use in a cheap
GPSDO where the ambition is to have 1e-9 frequency accuracy so the 2e-9
jump was just too high.
The VC-TCXO had its own low noise voltage regulator but the Vtune was
connected to a variable voltage divider between GND and the Vcc of the
VC-TCXO through a low pass filter so even with its own voltage
regulator any change in current in the TCXO can change the supply
voltage and feedback through the variable voltage divider.
It was expected that only slow changes in current could happen because
of temperature changes as the temperature control is very fast, loop
bandwidth well within one second, and these where filtered out by a low
pass filter after the variable voltage divider.
To test if the jump up and down was caused by a current to Vtune
feedback the Vtune was set to Vcc, 1/2 Vcc, 1/4 Vcc and GND.
The result was interesting:
Vtune | Max jump
Vcc | 6e-9
1/2 Vcc | 3e-9
1/4 Vcc | 1.5e-9
GND | no jump
This suggests the jump is indeed caused by feedback from the current
changes through the variable voltage divider into Vtune and there is a
digital circuit inside the VC-TCXO with changing current consumption
causing the 107.34 seconds periodicity.
To confirm the feedback was indeed the cause the variable voltage
divider was connected to a 3.7 V battery instead of Vcc and indeed, no
more frequency jumps!
The ADEV of this cheap VC-TCXO with Vtune at 1/2 Vcc is not bad:
0.1 s | 2e-10
1 s | 1.5e-10
10 s | 0.9e-10
100 s | 2e-10
With the Vtune at GND the ADEV is even much better so there is still
some more investigating to do.
Some DAC's with internal voltage references have been ordered to test if
it is possible to connect the DAC to the same Vcc (to save cost) and
still have a stable but variable Vtune without feedback .
If this does not work the DAC will need its own voltage regulator.
Again thanks to all the people that replied, I'm learning a lot.
Erik.
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