[time-nuts] Re: 20230122: Please help me understand my OCXO

[Bob Camp]

Hi

Looking at the data, past day 10 you get into the “unclear” range. Is the data
limited by the test resolution? maybe. It temperature an issue? who knows. Is
there something else going on …..

After a month or so, you can likely draw a box around data with wobbles in it and
make a rational guess about the aging rate. Take the diagonal through that box
and it’s a pretty reasonable guess at the max rate. 

Back in the 70’s 1 ppb per day was a pretty good spec on an OCXO. As time went
on, things got better and better. Going from AT to SC crystals helped a bit. Mostly,
crystal processing just improved in many small steps over many decades. 

To really know what category the one you have falls into aging wise, you need more
data. A month or three of monitoring will give you a better idea. Best guess is it
will fall away to the “need that box” level. You simply will be guessing with the current
test setup. 

Hermetic (or reasonably hermetic) OCXO’s tend to do pretty well after storage. They
don’t soak up humidity like the open designs do. What you are seeing is not unusual 
on a part like you have.

How good can it get? I’ve seen groups of OCXO’s from back in the 1970’s that stayed
well below 1x10^-8 per year. There are TBolt’s with OCXO’s on them that have even 
less change per year over the 10 to 30 years since they left the factory. 

Does aging when powered off count? How long has this or that example been on 
power vs off power? There’s not a lot of way to get that data on most of these parts. 
One would *guess* that this or that device did spend significant time on power before
it got scrapped out.

Take the yearly numbers and do the division. You can get a daily rates.  1x10^-8 / 365 
gets you 3x10^-11/day. I’d suggest that while it’s a valid use of math, the result isn’t 
really significant for a “per day” estimate. 

The good news: Your OCXO seems to be running ok and it’s not broken.

Bob

> On Jan 24, 2023, at 12:53 AM, Andrew Kalman via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I recently revived an instrument with a non-small OCXO in it, and I'd like
> to better understand what I've measured in terms of the OCXO's accuracy and
> aging.
> 
> Background & Test setup:
> 
>   - The OCXO is an Isotemp Model OCXO36-44, date code 9552.
>   - It originally came from an XL Microwave Model 3060 -- I dunno if it's
>   the Option 112, 115 or 120 OCXO.
>   - It has just three connections -- 12V, GND and RF output.
>   - I moved it into an XL Microwave frequency counter Model 3080 (10Hz to
>   8GHz).
>   - In 2017 I adjusted the OCXO (using its coarse and fine adjustment
>   screws) to an indicated 10,000,000.00MHz using the
>   two-traces-on-an-oscilloscope method and a GSPDO.
>   - Around 2019 this Model 3080 stopped working (it blew a transistor on
>   its power PCB).
>   - A few weeks ago I repaired this Model 3080 (a Schottky power rectifier
>   had failed, and took out a transistor). I replaced three rectifiers and two
>   transistors in the power supply.
>   - Once repaired, the Model 3080 started up immediately and the OCXO
>   measured 9,999,999.66MHz.
>   - My measurement setup is an HP Z3805A GPSDO providing a 10MHz reference
>   that feeds into an XL Microwave Model 3120's external 10MHz reference
>   input; the Model 3120 is in turn measuring the Model 3080's 10MHz reference
>   output. My recorded data is strictly from the Model 3120's display (max
>   digits). All of this equipment is just in my lab, temperature swings are
>   small. The Model 3xxx frequency counters have no fans.
> 
> I've attached a picture of the OCXO and a plot of its behavior over 17 days
> of uptime since the day I repaired it.
> 
> My questions:
> 
>   - *Is my measurement setup one that provides reasonably accurate
>   measurements of the OCXO's absolute accuracy and aging?*
>   - *Is the "warm up behavior"of this OCXO typical for a nearly 30-year
>   old OCXO with unknown total uptime?*
>   - *Does my test setup allow me to conclude that the OCXO is
>   demonstrating roughly 0.5ppb aging from day 11 onwards?*
>   - *From this limited amount of data, is this representative of a great /
>   good / average / poor OCXO, and why?*
> 
> Thanks for your responses.
> 
> --Andrew
> 
> --------------------------------
> Andrew E. Kalman, Ph.D.
> <20170512_IsoTemp_OCXO36-44.jpg><20230109_XL_Microwave_3080_OCXO_Aging.pdf>_______________________________________________
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