[time-nuts] Re: 10 MHz TCXO periodically jumping 20 mHz up and down, cause identified

[Erik Kaashoek]

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-TCXOÂ  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.