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

[Andrew Kalman]

Hi Bob.

That helps a lot -- gives me a lot of insight, esp re measurement
resolution vs time.

(I'm slowly being dragged into long-time duration measurements, and I'm not
enjoying the associated energy costs :-) ).

--Andrew

--------------------------------
Andrew E. Kalman, Ph.D.


On Tue, Jan 24, 2023 at 6:15 AM Bob Camp <kb8tq at n1k.org> wrote:

> 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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