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/

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 <cdelect@juno.com> > 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 -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

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 <cdelect@juno.com> > > >> 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 -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > _______________________________________________ > 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. > >