Dr Bruce Griffiths wrote: > 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 > > Tom
Additional info on K34-5991A Manual available at: http://www.to-way.com/k34599a.pdf Since the circuit uses 2 line receivers, a dual flip flop an exclusive or gate and an opamp, designing/building an equivalent instrument should be fairly simple. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
