[time-nuts] help me understand AM noise

[life speed]

Hi Guys.  It has been a while since I posted, hope you can help with a slightly time-related topic.  Can't have frequency without amplitude . . .

I recently designed an Automatic Level Control circuit consisting of dual-slope detector logger, open and closed loop references with AM modulation, and a linearizer (volts/dB) driver for series/shunt microwave attenuators.  This is part of a DC - 20 GHz microwave synthesizer.  I measured the AM noise at 3 GHz, both open and closed loop, and find the noise level is higher at the output of the attenuator/amplifier chain at similar power levels to the input (13 dBm).  The input RF chain saturates at about 17 dBm, while the output amp following the attenuators saturates at about 20 dBm.

I understand that an amplifier in compression will suppress AM noise.  What I wonder is are my measurements of increased AM noise (red trace) at the output of the attenuator/amp lineup to be expected based on the higher available saturated power?  Is it possible to attenuate the signal using the power control (open loop in this example, ALC is not used) without degrading AM noise performance?  Does anybody have any suggested reading on this subject?  I am trying to understand how well my circuit performs, in general.  I do observe that control the power to a lower  level increases the AM noise.  But it is a relative measurement to begin with, so what is "good"?  I have been reading the Agilent E5500 user guide on AM noise measurements, but don't find a great deal of information there regarding AM noise performance of a Device Under Test.

Thanks,

Lifespeed

http://home.comcast.net/~claybu/pics/electronics/am_noise_1.png