[time-nuts] Phase Noise vs. AM noise
mfeher at eozinc.com
Thu Jan 15 13:13:13 EST 2015
I am maybe using the wrong word "discernible". By that I mean that you cannot discern phase noise from AM noise at the real low levels. You can certainly measure, or see the contribution of the total noise power, but do not necessarily know if it is phase noise or AM noise or how much of each is included. Regards - Mike
Mike B. Feher, EOZ Inc.
89 Arnold Blvd.
Howell, NJ, 07731
From: time-nuts [mailto:time-nuts-bounces at 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
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.
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