[time-nuts] Re: 10811A settling time

[Bob Camp]

Hi

Your OCXO (pretty much any OCXO) goes from room to “pretty hot” ( that might be 20C up to 90C) 
in a short period of time. The temperature coefficients of expansion of the various elements of the 
crystal never match up perfectly. The same is true for the “stuff” in most of the other components. 
This stress translates to a change in value for the components and a change in frequency for the crystal. 

As time goes on, this stress relaxes. If you cooled the part(s) down and still could observe the values,
you would see a change from that as well. It also would eventually relax out. 

Regardless of how well the crystal is scrubbed, there always will be “stuff” inside the crystal enclosure. 
It will settle into some sort of equilibrium over time. Change the temperature and the equilibrium point
changes. That can take a long while to get to some sort of near stable state. 

The typical 10811 is not a sealed unit. Humidity (and dirt) can get into the package. If the unit has been 
in storage for a long time, it can take a while for the boards to bake out / crud to burn off. Until it does, the
unit really isn’t “in good condition”. Spending years in a damp and dirty garage might mean months of power 
on to get things to “fly right”. 

Are there a lot of other contributors? Sure there are. You could fill a pretty good sized bookshelf with 
all those details.

The 10811 should change about 1x10^-9 from 1 hour on to 24 hours if it is in good condition. If you are
multiplying up to 1 GHz, that would be a 1Hz change. If you are looking at 10 MHz, a 1 Hz change is 0.1 
ppm. A properly functioning 10811 should easily hold 0.1 ppm per hour after 1 hour on power. 

If you are looking at *phase* rather than frequency, that’s a different thing. A 360 degree change in one 
second gets you 1 Hz. A 360 degree change in 20 minutes is 1/20 th of a hertz. That gets you to a 
frequency offset of 5x10^-9. If you are looking at the the first hour on power, that’s not an unreasonable 
number.

Bob

> On Mar 31, 2024, at 10:59 AM, alan bain via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I have several of these HP ovenised 10MHz oscillator modules which are
> not able to be run 24/24  although I leave the ovens operating (they
> are generally in power-hungry HP test equipment which has a standby
> mode which leaves power to the oven).
> 
> I've been noticing that they take a decent number of hours (minimum
> 8h) of running as oscillators from standby before the short term
> frequency is adequately stable (in terms of not drifting) for some
> purposes.  It's very obvious when trying to phase lock multiples of two of them
> together using EFC to tune one and a narrow bandwidth PLL. In trying
> to find zero beat the beat frequency can be seen drifting on a scope /
> counter fluctuations usually around 0.5-1Hz over 20-30min period.
> 
> During this time they compare less well to a boring modern non-ovenised
> oscillator for short term stability. No actual quantitative
> information because there isn't a better standard in this location to
> use as a timebase, although I do have an HP53132A counter (but with
> the rubbish timebase option) which is what I use to measure the
> beatnote frequency.
> 
> I'm wondering what the physical process might be for this settling? Is
> heat dissipation in the oscillating crystal needing to find thermal
> equilibrium?  Or do they need repair in some way? I've never taken one
> apart but I do have the service manual.
> 
> The manual for the 10811A suggests running for 10days before checking
> drift, but to do so requires a Caesium frequency reference which -
> alas - I lack!
> 
> I'm aware that there's a lot of knowledge of these HP oscillators
> around here and would be grateful for any pointers.
> 
> Alan
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