[time-nuts] OCXO and fluctuations after EFC adjustment

[ed breya]

Rick said:
"In the case of the 10811, I have already posted about the reference 
diode of special characteristics. I don't remember all the exact details 
of how it was chosen, but it was based on proprietary knowledge."

This reminds me of something unusual I found in a Vectron 10 MHz OCXO 
many years ago. It was busted, and it was a soldered-shut type. I had 
nothing to lose in taking it apart, hoping to find the problem and 
restore it if possible. It came apart OK, and I found in the densely 
packed circuitry, a small RF choke that was burned up. I traced the 
circuitry enough to figure out that it was in series with the internal 
reference regulator diode - a 1N82X 6.2V TC type. This added some small 
DC voltage drop to the Zener voltage, and that's what went to the 
internal circuitry, and also out to an external pin. This pin was 
intended to supply an external pot for tweaking the EFC line in 
stand-alone applications, and apparently sometime in its past, it had 
been faulted to an excessive voltage, causing the choke to burn out, and 
maybe damage to other parts.

The only things I could figure were that the choke maybe served as a 
small copper resistance (+3000 ppm/K or so) to provide some extra degree 
of temperature compensation for some reason, or that it was to suppress 
possible oscillations from the Zener's negative R in avalanche.

The choke was one of those little molded types, about the size of a 1/4 
W resistor, and unfortunately burned up and apart too much to read any 
color bands. I guessed it to be around maybe up to a few uH, and a few 
ohms, but that's as far as I got. I set it aside, thinking someday I'll 
dig into it some more, but it still sits in pieces in a box somewhere, 
remaining a mystery to me. All I know is, there must have been a reason 
for it.

Ed