[time-nuts] Designing and building an OCXO and GPSDO

Philip Pemberton lists at philpem.me.uk
Mon Aug 11 16:28:28 EDT 2008

WB6BNQ wrote:
 > The first is a series of Application notes from Agilent (old hp test
 > div) called AN-200. A total of 5 App notes comprise the AN-200 series.
 > If you go to the following Web page and enter AN-200 at the top of the
 > page in the search box, you will get a return of all the AN-200 booklets
 > in PDF that can be downloaded. The BIG one is AN-200-2, but it would be
 > to your advantage to collect all of them. You need to paste in the
 > entire link below if your browser doesn't see the whole thing when
 > clicking on it.

Hm, nothing found on Agilent's site, but Google found it (search term:
"agilent.com AN-200").

Lots of interesting stuff there, from quartz oscillator basics through
microwave signal generation and measurement...

 > That should keep you busy for a while. A lot of stuff on the WEB, some
 > good and some not so good, just take it with a grain of salt ! NIST
 > (the old NBS) has several things worth reading, however, most of that
 > deals with the measurement process or is a rigorous mathematical
 > analysis of one thing or another.

Well I was looking at the manual for the ELV OCXO400
as a partial reference, and the disassembly photos for one of the HP
dual-oven oscillators (can't find the link at the moment).

I dismissed the ELV's mechanical design based on the fact that it was two
BC337 (read: low power) transistors mounted on each side of the crystal, with
a cheap thermistor on top. No real thermal load at all -- as soon as the
transistors turn on, the temperature is likely to shoot WAY, WAY up, then
drop FAST when the transistors are turned off. The comparator-based
thermostat-type temperature control isn't likely to make that much easier

Apparently the oscillator circuit is quite badly designed too (based on
information at http://glowbug.nl/projects/M51.html). Badly biased output
transistors and mismatched xtal load capacitance mainly.

 > Second it does not take much to get parts in 10^-7 range. Temperature
 > compensated crystal oscillators easily handle that level. With care, a
 > crystal oscillator in a well designed circuit can reach parts in 10^-8
 > with a bit-bang oven control. HP did that in the late 1950's. From
 > that point the difficulty is logarithmic.

It certainly seems that way...
Nice to know it's not quite as hard as it looks. The ELV is specced at
2x10^-8 stability and uses an AT-cut HC49U fundamental-mode parallel-resonant
crystal. I don't have the specs for the crystal cut used for my 10MHz xtals,
but I do know the base specifications are almost identical to the ELV unit's

I should probably mention the main reason I wanted to use the PIC control --
so I can adjust the oven parameters (desired temperature, PID variables, and
so on) in real time, and read off the oven status at the same time (current
temperature, target temperature, last warmup time, hours run since last power
cycle, total crystal age, starting temperature, crystal oscillator state, and
so on). Obviously the turning point for each crystal isn't going to be
exactly 50.0000000... Celsius, so being able to hook up a high-resolution
counter and figure out what the exact turning point is and program the
thermal management kit with that would be useful.

In truth this is more a research and learning exercise. It'd just be nice to
end up with something that compares reasonably well to a commercial OCXO,
that I can use as a basic frequency reference. Then later on I'll decide if I
really need anything better.

lists at philpem.me.uk

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