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

[Andrew Kalman]

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.
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