[time-nuts] help me understand AM noise
life speed
life_speed at yahoo.com
Wed Jan 29 20:28:55 EST 2014
Thanks for the link.
My current lineup is:
13 dBm source -> variable attenuator -> amp -> variable attenuator -> amp -> filter -> pad -> amp -> filter -> 20 dBm output
I think I already paid attention to interleaving gain and loss, as well as not reducing the signal to very low levels so as to gain up the noise floor too much. At the moment I am also not much concerned with ALC behavior or stability as the AM noise measurements do not show much issue with this.
Mostly I am concerned with additive noise from the somewhat complex opamp circuits driving (and linearizing) the series/shunt topology microwave FET voltage-variable attenuators. I would like to understand what changes in noise floor are just a result of attenuating the RF chain over it's amplitude control range, and what noise I am adding by my imperfect (noisy) attenuator driver circuitry. I have already made some improvements in this area by bandwidth-limiting the opamps (I need about 1 MHz 3 dB corner for ALC stability and AM modulation) and switching to some lower noise opamps. So clearly it was not perfect. Just trying to figure out how much is added AM noise by my circuit and how much is laws of physics by increasing the attenuation.
Lifespeed
--------------------------------------------
On Wed, 1/29/14, John Miles <john at miles.io> wrote:
Subject: Re: [time-nuts] help me understand AM noise
To: "'Discussion of precise time and frequency measurement'" <time-nuts at febo.com>
Date: Wednesday, January 29, 2014, 4:46 PM
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
>
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