[time-nuts] Z3816 Oscillator Saga

[Peter Schmelcher]

Chris,

The Z3816 has a few design short comings. In the circuit board pic the MTI 
oscillator circuit board grounding is improved. Subsequently I opened and 
modified the oven internally and added an extra oven feedthrough pin. The 
issue is the oven heater current modulates the ground pin voltage and that 
effects the EFC value (poor layout). I finally connected the oven power 
pins to a separate floating PSU with the new internal oven EFC signal 
ground pin the only common connection point, while hunting for the source 
of a short transient EFC event.

I would caution you that the circuit board traces and hole plating are very 
thin, and both are easily damaged by external forces. I used multiple irons 
for un soldering problem pins.

The two tantalum capacitors with the black marker dots are not original. 
These two capacitors had the wrong voltage rating and became noisy. These 
might also be the source of your problem. It can cause unexpected jumps in 
the reported EFC value and are not an oscillator problem, just a dumb 
assembly mistake. FYI at the turn of the century cell phone sales had a 
surge and tantalum capacitors became impossible to get at 10x the price.

I replaced the Z3816 oscillator temporarily with no problems but observed 
at power up the smart clock algorithm used a much higher gain in the EFC 
loop for about an hour then drops to a more normal value.

I also adjusted the oven temperature turning point pot, however, I suspect 
the MTI internal oven circuitry self calibrates the crystal turning point. 
I never figured out how to stop the oven from self calibration and had 
intended to fix the oven circuit before I determined it was a feature. The 
pot adjustment seemed to have more effect at power up. As I recall, during 
the relatively short  calibration event, the frequency changes by 0.008Hz 
with a maximum gps phase error of about 400ns. I had wondered if the two 
algorithms were interacting badly but I never verified this. Even with this 
quirk, the 260 is a great oscillator.

-Peter