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 <[email protected]> wrote: Subject: Re: [time-nuts] help me understand AM noise To: "'Discussion of precise time and frequency measurement'" <[email protected]> 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: [email protected] [mailto:time-nuts- > [email protected]] On Behalf Of life speed > Sent: Wednesday, January 29, 2014 10:32 AM > To: [email protected] > 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 > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
