In message: <[EMAIL PROTECTED]> Bruce Griffiths <[EMAIL PROTECTED]> writes: : The conventional approach adopted by NIST is to divide each frequency to : be measured down to 1 PPS, then timestamp the PPS transitions for each : channel as well as the PPS transitions from a GPS timing receiver. By : using the same setup at NIST with their frequency standards most of the : noise due to ionospheric delay variations is a common to both systems : and is eliminated on subtraction.
NIST has gone to an approach where the signal under test is heterodyned with a signal that's ~10Hz low. Since one cycle in the test frequency is one cycle in the heterdyned frequency, you can get measurements of signal down to the noise floor of your hardware (since even a simple 32MHz counter gives one several order of magnitude better than the noise in the zcds). At 10MHz, the heterodyne factor is 1e9. This means you can measure the phase difference of the signal to 31.5e-18 (assuming a 32MHz clock). Since the noise in the 32MHz oscillator is at 1e-13, we can measure the 10MHz down to a few parts 1e-13 (since the noise dominates over the resolution of the measurement). With a better oscillator, one can hit the noise floor of the ZCDs that we used in the project with a more stable 32MHz oscillator (down to 1e-14 or 5e-15 or so in some of the tests I ran in the lab). The 32MHz oscillator is a relatively cheap OCXO. The design of the system is such that almost all of the 32MHz noise subtracts out... The noise is such that switching to 100Hz gives a factor of 3 better noise floor. But any faster than that doesn't help much. A 2Hz signal is 5x worse noise floor because the 32MHz noise starts to dominate. With a good quality TIC, one is doing good to get PPS measurements in the picosecond level (1e-12). But that's a lot more expensive than the above device. At least that's what NIST is using to measure the cesiums in its clock ensemble. Warner _______________________________________________ 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.