Tom Van Baak wrote: > Don, > > Yes, using an oscilloscope in this way is a time-honored > trick to observe slow changes in relative phase over time. > > It gets even better if you have access to a storage 'scope > or variable persistence because then you can get a rough > sense of short-term jitter as well. > > Either way, the math is the same. Frequency error is just > phase change over time. You have two things working for > you: 1) you don't even have to wait for full cycles, and 2) > there is no limit on how long you can wait. > > If you have a fast sweep of 10 ns/div you can easily eyeball > fractions of a 100 ns cycle. High-end 'scopes allow ns and > sub-ns resolution. Add that to long observation times (many > minutes, even hours) and you've get quite good frequency > resolution. > > Again, as an example, if you observe that it takes about > 3 hours for it to move 1/10 a cycle (10 ns), that's about > 1e-8/1e4, or 1e-12. You see, even using the old CRO > method, you get fine results. The downside is that you > have to keep an eye on it now and then to make sure the > relative phase changes continue to be gradual and that > you aren't cycle slipping. > > Now you ask if there is a better method. If you intend to > automate it, to collect a lot of data, then the CRO method > is maybe not ideal. For example, to see how frequency > changes over time, from week to week, or month to month, > so that you can compute the aging rate, you might want > something more digital and less eyeball. Or if you want to > measure oscillator stability (rather than accuracy) you'll > want to collect a series of periodically spaced readings. > > Your 5245L would partially help here. The trouble is they > don't have a modern digital output; it's a parallel BCD > interface if I recall. > > Does anyone else have ideas for Don? > > /tvb > Tom
One could always use an HP K34-5991A (or equivalent phase comparator) connected to a strip chart recorder. Failing that an equivalent device should be easy enough to build and it could be connected to a DVM or other low resolution ADC which in turn is connected to a computer for data logging purposes. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
