Bob wrote:
I've noticed that a few of these OCXOs continue to retrace upwards in DAC movement, even after a number of days. I haven't run any of them for more than a week, as I've built only a limited number of these units. The normal situation is to retrace downward from the start, once initial warmup is over.
There are several factors that could conceivably affect this. First, some OCXOs have a positive EFC coefficient (higher EFC voltage produces higher frequency), while others have a negative EFC coefficient (higher EFC voltage produces lower frequency). It all depends on how the varactor is hooked up internally. For example, the internal Trimble OCXOs in Thunderbolts and HP 10811s have opposite EFC polarities.
Of course, if you design a PLL that generates a more-negative EFC voltage when the OCXO is high (i.e., it expects an OCXO with a positive EFC coefficient), and then install an OCXO with a negative EFC coefficient, the loop will be unstable and will eventually saturate the EFC loop at one or the other of its limits with the oscillator way off frequency and the loop unlocked. If you didn't wait long enough for the loop to reach a steady state, it is possible that you have some OCXOs with positive EFC coefficients and some with negative EFC coefficients. [1]
If you are sure that all of the OCXOs have the same EFC polarity, you need to distinguish two phenomena -- warmup, and retrace. Obviously, when an OCXO is cold it will be pretty far off frequency (typically, parts in E6), and as it warms up it will approach its nominal frequency. Whether it is low or high when it is cold (and by how much) depends primarily on the crystal cut (assuming that the oven temperature is set to the crystal's turnover or minimum-tempco temperature). So, if you are talking about the time between power-on and being fully warmed up, the direction and amount of drift is probably mostly a function of the crystal cut.
Once the crystal is fully warm, it will be in the "retrace" regime -- essentially, accelerated aging to reach its new, stable frequency. This can be in either direction, without regard to crystal cut. Some crystals will settle very, very close to their last stable frequency, others not so much (and the difference can be either + or -).
[1] One final point on OCXO EFC characteristics: Each OCXO has an EFC "gain," expressed in Hz/V, which is one of the factors in the open loop gain of the PLL control loop. If you install an OCXO with higher EFC gain into a PLL that is compensated properly for an OCXO with lower EFC gain, you are very likely to make the control loop unstable (or, at least, conditionally stable), which could make the DAC voltage do strange things.
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
