Bob - What I am saying is, even at the levels you mentioned, what is measured is I believe the combination of phase and AM. In other words, you are just measuring noise, but, are not certain if it is all phase, or phase plus some AM. At least that is my recollection when I was heavily involved in it some 30 years ago. Thanks - Mike
Mike B. Feher, EOZ Inc. 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 office 908-902-3831 cell -----Original Message----- From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Bob Camp Sent: Thursday, January 15, 2015 7:48 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Current state of optical clocks and the definition of the second Hi I guess the question becomes how low is low. If it’s a 50 ohm system If the power level is rational If you are at room temperature There are some limits on how low low can be. You have a -174 dbm / Hz thermal floor. AM or PM noise can only be 3db better than the thermal floor. At a power level of 1 watt, that’s a -204 dbc / Hz limit. You will spend some time correlating to that level. You also may need to play a bit with the input circuits to handle the 1W without damage. At a somewhat more common 100 mw, the limit is -194. People have been measuring phase noise in the > -190 dbc / Hz range for at least 20 years now. Correlation may take a week at some offsets. Time will be longer or shorter at other offsets. As with anything else, the more money (correlation channels) you throw at the problem, the quicker it will go. Numbers in the -180 vicinity with normal gear, offsets, and FFT windows are an overnight run sort of thing. Bob _______________________________________________ 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.
