[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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