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.
Bruce
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
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
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.
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