/tvb wrote:
I'm suspicious of fast/slow tracking loops. If you want to vary the tracking, perhaps it is best to continuously, transparently, smoothly vary loop parameters according to drift rate rather than use a hardcoded fast/slow algorithm binary switch. I'm sure there's deep theory on this, which I have not read yet.
No reason to be suspicious, they work well if they are designed well. Quite often, the best loop tuning for acquiring lock is very different from the desired loop characteristics during locked operation. This is particularly true in very slow loops such as we see in GPSDO applications. In this case, two time constants, each with its well-defined application, can work spectacularly better than just one. With just one tc, mutually exclusive design constraints pull in two directions and often result in a compromise that sacrifices operating characteristics to achieve bearable performance during lock acquisition. Separate acquisition and maintenance time constants allow you to have both, and there is no practical difficulty in making sure that each one does its job and does not interfere with the other. The fast tc operates only during acquisition, and the slow tc operates 99.99% of the time. Only when there has been a big disturbance (in which case the GPSDO should be in an alarm status) would the fast tc be recalled.
Having several intermediate time constants raises potential worries that the control circuitry will not always choose the best tc for current conditions. There are many instances in which it works well (it has been widely speculated that the HP 38xx GPSDOs use this technique), although care is necessary in design. However, if, as you posit, it would be beneficial for the loop parameters to vary with the drift rate, it would seem clear that several steps of piecewise approximation would surely be better than just one operating alignment as long as proper care is taken and no harmful artifacts are created in doing so.
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.
