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

[Bruce Griffiths]

Philip
> 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
> either.
>
>   
Thermal gradients in the vicinity of the crystal will be quite high.
> 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.
>   
Not to mention the low isolation buffer amplifier with relatively high 
phase noise.
Extracting the signal through the crystal is a good method for ensuring 
a low phase noise floor.
Ignore the misguided comments in the literature and on the web that say 
otherwise.
Their calculations indicate a phase noise floor 20dB or more higher than 
that achieved in practice.
This is the result of applying an standard equation to a situation where 
the assumptions implicit in its derivation don't hold.
>  > 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
> crystal.
>
> 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.
>
>   
You'll need a really stable reference for the counter to get close to 
the turning point.
Don't ramp the temperature too quickly as AT crystals are also sensitive 
to thermal transients.
> 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.
>
> Thanks,
>   

Bruce