Stephan For ultra fine phase shifts one can always use NIST's approach of using a splitter (or directional coupler) to sample the OCXO signal then use a mixer as a dc controlled attenuator, attenuate the output with a fixed attenuator and recombine it in quadrature with the original signal. Very small phase shifts at the reference frequency can be achieved in this way.
Stephan Sandenbergh wrote: > 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? > > It works best when all 3 oscillators are compared (pairwise) simultaneously using 3 DMTD systems and 3 time interval counters. It tends to be less useful for longer tau as the 3 oscillators share the same external environment. > Regards, > > Stephan. > > > Bruce > 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. > > _______________________________________________ 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.
