[time-nuts] help me understand AM noise

[John Miles]

If you're just using a single attenuator at the input of your PA, it makes
sense that the additive noise in dBc terms is worse at lower output levels.
Disregarding saturation for the moment, the PA is adding just as much noise
to a low-level input signal as it does to a high-level signal. 

ALC-controlled amps need to be designed with multiple alternating stages of
attenuation and gain.  Whatever drives the PIN diodes or other attenuators
also needs to be quiet and well-filtered, of course -- which also means
keeping an eye on feedback stability.

My (least) favorite counterexample is the IF amp in the HP 11729C noise test
set.  It's not even an ALC amplifier, but a true limiting amplifier.  It was
implemented by cascading several high-gain MMICs with clamp diodes between
each stage.  Makes a nice comb generator.  They were driving a mixer so they
would have reasoned that odd harmonics were acceptable, but because there
was no interstage attenuation for low-level noise, the amplifier's additive
AM and PM levels were awful.  You'll see similar performance if you measure
the additive noise of a comparator or other limiting amp.  Several limiting
amps were used in the HP 8662A/8663A synthesizer as well, which is why they
have good close-in noise but a relatively poor white noise floor.  As long
as you avoid these mistakes you'll be ahead of 90% of the crowd.

As far as further reading goes, check out http://rubiola.org/index.html .

-- john, KE5FX
Miles Design LLC


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-
> bounces at febo.com] On Behalf Of life speed
> Sent: Wednesday, January 29, 2014 10:32 AM
> To: time-nuts at febo.com
> Subject: [time-nuts] help me understand AM noise
> 
> 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
>