> Bruce, > > The Tsc5125A and miles Timepod show a phase noise floor 3dB above the > noise floor of the two oscillators (if two are used with identical noise floors).
Bruce is actually talking about a semi-undocumented trick with the TimePod that allows it to act like an E5052 or TSC 5120A-01 with two internal references. Basically, you remove the SMA jumpers from the TimePod's input jack panel and feed two independent reference sources that are very close to the same frequency to the Ch0 IN and Ch2 IN jacks. Then you connect your DUT to the REF IN jack, and flip the channel equations from "0-1" and "2-3" to "1-0" and "3-2" to reverse the roles of the input and reference sources. That allows the cross-correlation algorithm that normally removes the ADC noise from a PN measurement to remove the uncorrelated noise from your two references as well. It's a nifty technique, but it is important that the two references be close to the same frequency for various reasons, some of which I haven't adequately investigated. So it falls into the "Unsupported technique/Use at your own risk" category. > It would be nice if that worked, then we all could use $10 TCXOs instead of > $1000 Wenzel references. That's basically how the E5052A/B and TSC 5120A-01 work -- if you look at their phase noise floor specs, they are much better than what can be obtained from any one reference source. The E5052A/B instruments actually use the technique to remove the noise from two VHF-microwave downconverters. That was my original intent in adding those SMA jacks -- see http://www.miles.io/timepod/VHF_test_jig.jpg and http://www.miles.io/timepod/VHF_example_400_MHz.png , where a pair of HP 8662A synthesizers is used to downconvert a much quieter 400 MHz source for measurement at 9 MHz. In principle you can indeed use a pair of $10 TCXOs as references to measure arbitrarily low-noise DUTs, but you may need to run the measurement for days or weeks. Measurement time is the price of the free lunch in this case. -- john Miles Design LLC _______________________________________________ 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.