[time-nuts] Phase Noise vs. AM noise

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 16 01:22:23 EST 2015

As long as the instrument is carefully adjusted so the that the 
measurement phase axis is correctly aligned with respect to the test 
signal an interferometer can be used to ensure that the measurement 
system PN noise floor is well below the thermal limit when measuring the 
residual noise of 2 port components such as amplifiers. In order to achieve 
sufficiently accurate alignment a pure AM modulator may be required.


On Thursday, January 15, 2015 01:13:13 PM Mike Feher wrote:
> Bob -
> 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 
> not necessarily know if it is phase noise or AM noise or how much of 
> is included. Regards - 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-bounces at febo.com] On Behalf Of Bob 
> 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. 
> have been measuring phase noise in the > -190 dbc / Hz range for at 
> 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 
> (correlation channels)  you throw at the problem, the quicker it will go. 
> Numbers in the -180 vicinity with normal gear, offsets, and FFT windows 
> an overnight run sort of thing.
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