[time-nuts] Re: Why do have OCXO a Vref output? (was: help reviving Trimble UCCM-LPS GPSDO)

[ed breya]

Andy wrote:
"... I too was surprised to see nothing more than a Zener being used in 
one case, but bearing in mind this was in a temperature controlled 
environment is not as bad as a Zener in open air."

In the OCXOs I've had opened up, they used a temperature compensated 
Zener, typically in the 1N821 class, 6.2 V, or one near 9.1 V (I forget 
the part number series). So, it could actually have a very good overall 
tempco - a TC Zener operating at a fixed oven temperature. These were 
directly regulating the internal circuit voltage, and not buffered, so I 
think the main problem with stability is the external loading effects on 
the  operating point of the TCZD. I think the first noticeable effect 
would be a drop from open-circuit reference voltage, when hooking up an 
external pot.

Ed