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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
