Corby, Thanks - it makes sense. I was suspecting that such a low-frequency phase shifter would be done using cable lengths. However, I can't say that I've got much experience with this kind of thing.
I have another question: The rule of thumb would be that the reference oscillator must be 3x more stable than the DUT. I haven't got much to measure my standards against, but I have got 3 identical units. According to literature the separation of variances method provide a reasonable estimate. How well does a measurement, based on separation of variances, compare to one that was done using a sufficiently stable reference? Regards, Stephan. 2009/7/24 Corby Dawson <[email protected]> > Stephan, > > I have a DMTD system that was built by NBS (now NIST) in the early 80s. > > The coarse phase shifter is made up of ten lengths of miniature coax > cable bundled up in a shielded box with toggle switches on the front > panel to select the delay used. > > The LSB is about 2us. > > It also has a fine adjust that consists of a couple varactors in a phase > delay circuit. > > I have found that with a good quality offset L.O. I can get the precision > I need (<1X10-13th at a 1 second interval) without using the fine adjust > circuit. > > As far as the small time interval value I usually start my measurements > at around 2 to 4 us delay with the DUT set for a slowly increasing phase > shift, > > The accuracy required depends on what your measuring but with a 10Mhz > input and a 1hz beat note +-1us is equal to +-1X10-13th so most counters > will work just fine. Just be advised that the us and below digits will be > jumping around quite a bit! > > Also make sure that the delay is adjusted so the counter updates every > second, you can be updating every two seconds which nullifies the > cancellation of the noise in the L.O. > > Corby Dawson > ____________________________________________________________ > Always a good call. Click now to establish your local phone service! > > http://thirdpartyoffers.juno.com/TGL2141/fc/BLSrjpTHbDiki6pTvGTKKCArqOqjNQCdDWO6HMJXAtNrfARisO7WahgNOso/ > > _______________________________________________ > 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. > _______________________________________________ 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.
