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

[Bob Camp]

Hi

So, right at day 23 the nice upward trend takes a step down. It does it suddenly 
and the rest of the curve pretty much keeps on going. You might wonder how
abrupt the change was. It could have (and likely was) very abrupt. Sample at
whatever speed you wish, it’s a jump between two readings (maybe). 

Various folks have various names for this. Frequency hit, frequency jump, and
phase hit are three. There are many others. They all are talking about an instantaneous 
change. Needless to say these changes can be a bit of an issue in some systems. 

Back on topic: Do they count as aging? Since they are random in direction and 
random in spacing the hits *should* average out to zero. (Yes, there is more than
a little debate about that …). If aging is simply the “smooth trend” (a *very* 
arbitrary definition) then they don’t count. 

Even when it is clear …. it’s not :) :) :)

In some cases an OEM might have a spec on the jumps *and* a spec on the 
aging ….

Bob

> On Feb 16, 2023, at 11:01 AM, Andrew Kalman <aekalman at gmail.com> wrote:
> 
> Just a follow-up if anyone is interested ... I ran my test for over a month, and I conclude that the OCXO's drift has settled to around 0.25-0.33ppb/day. Plot attached.
> 
> --Andrew
> 
> --------------------------------
> Andrew E. Kalman, Ph.D.
> 
> 
> On Tue, Jan 24, 2023 at 8:05 AM Andrew Kalman <aekalman at gmail.com <mailto:aekalman at gmail.com>> wrote:
>> 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 <mailto: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 <mailto: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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> <20230109_XL_Microwave_3080_OCXO_Aging.pdf>