Source of "hiss" in small-signal RF amplifier? - Page 1
Forum section: electronics / rf-microwave

author: Co6aka
author_rank: Unknown
timestamp: October 28, 2019, 10:17:27 pm
content: "Academic" question because unfortunately it's a potted block amplifier so I have no access to the circuit, but basically it's a 20dB gain small signal RF amp for 100kHz to 60MHz...Receiving in AM mode, strong off-air or generated signals have an essentially silent background, but when those signals are amplified by the amplifier in question there's a significant added "hiss" in the background. (It's NOT atmospheric noise; precisely the same results when testing on the bench.) I see no signs of oscillation or instability on the spectrum analyzer. The power supplies I've tried are all "quiet" and I've tried adding additional filtering but no effect. Taking the baseband audio from my service monitor into my scope and examining the waveform, it looks like random noise or "white noise" or whatever. Receiving in FM mode, the background is silent, so the hiss is being amplitude-modulated onto the carrier (and onto any and all signals amplified by the amplifier.) So, flying blind, what are your top suspects for the reason/source of the noise?

author: TheMG
author_rank: Frequent Contributor
timestamp: October 29, 2019, 01:59:55 am
content: Any amplifier will to some extent introduce its own noise into the amplified signal. Essentially the thermal noise of the various components within the amplifier add to the overall noise.There is a performance figure associated with RF amplifiers known as the "noise figure". Better amplifiers will have a lower noise figure. An ideal amplifier would have a noise figure of 0dB (no additional noise), but as we all know in electronics there is no such thing as an ideal/perfect circuit or components. All amplifiers, no matter how good, introduce a little bit of noise.That is why, when extra amplification is not necessary, it is better to switch off or bypass the amplifier as it would be doing more harm than good.

author: OwO
author_rank: Super Contributor
timestamp: October 29, 2019, 03:17:30 am
content: Is there less hiss with strong signals or is the volume of the hiss constant regardless of received signal strength? If it is constant then this points to multiplicative noise, which is most likely due to power supply noise.

author: Bud
author_rank: Super Contributor
timestamp: October 29, 2019, 04:56:59 am
content: Poor linearity? Try characterizing the amp if the specs are not known. 3rd order intercept point, two tone IMD test.

author: Co6aka
author_rank: Unknown
timestamp: October 29, 2019, 06:34:09 am
content: Zero to do with the amp's noise figure or IM and compression performance. The power source isn't the source of the noise; happens even when powered by batteries. The AM noise is on all signals regardless strength, but it's not easy to detect on weak signals.I've simulated the noise by AM-modulating white noise at 0.3 to 0.4% onto a strong carrier with my Agilent U4438C. This is in a 50-Ohm system. Maybe this will describe the situation better:Test-1: Generate -50dBm unmodulated carrier with U4438C, output directly to a comm receiver in AM mode; background is essentially silent.Test-2: Generate -70dBm unmodulated carrier with U4438C, output directly to amplifier, amplifier output directly to comm receiver in AM mode; background is noisy.Test-3: Generate -50dBm 0.3% AM-modulated carrier (white noise) with U4438C, output directly to a comm receiver in AM mode, background noise equals that of "Test-2."Test-4: As in "Test-2" but with comm receiver in FM mode; background is essentially silent, thus the amplifier's noise generation problem is AM.On a spectrum analyzer the amplifier's output is clean; no oscillations, harmonics, etc. At least none between 10kHz and 3GHz.In AM, the receiver has quieted enough at -70dBm to be able to clearly hear the noise, and it's audible and unchanged up to amplifier compression at about +28dBm output. For the high level tests, I added a 30dB and 20dB attenuator between the amplifier and receiver.Leif Asbrink, SM5BSZ, discovered something similar to what I'm seeing with some MMIC amplifiers, except the noise was at very low levels. He AF-bypassed the input, and also found that AF-feedback further reduced noise:What I'm trying to figure out is, theoretically, what component(s) in small signal RF amplifier circuitry might be causing the (so much) AM noise? Bad transistors? Bad resistors? Problem in the feedback path? Could the amp be oscillating at audio frequencies? (Though I snooped around with my modified "banana" - plastic tip replaced with a rubber duckie - but don't hear anything.)

author: radiolistener
author_rank: Super Contributor
timestamp: October 30, 2019, 01:11:28 am
content: Quote from: Co6aka on October 28, 2019, 10:17:27 pmthis is the noise of amplifier. Any active circuit include amplifier adding it's own noise to the signal. Use amplifier with low Noise Figure in order to reduce such noise.If your receiver input impedance is higher than impedance of feeding line, you can use RF transformer for impedance transformation in order to achieve noise-free gain.You can use RF transformer for impedance transform ratio up to about 1:9. It will be hard to make RF transformer with higher impedance transform ratio. With transformer 1:9 you will get 1:3 voltage gain ratio, because sqrt(9) = 3. And this gain will be noise free.Active amplifier always add noise, so this is why you're needs to take into account amplifier Noise Figure.

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 01:24:55 am
content: my guess would be thermal noise,from carbon resistors etc,can you reduce the noise by cooling the amp? just a thought.

author: radiolistener
author_rank: Super Contributor
timestamp: October 30, 2019, 01:30:15 am
content: thermal noise is too small at short wave it is smaller than ether ambient noise

author: Co6aka
author_rank: Unknown
timestamp: October 30, 2019, 03:57:13 am
content: Quote from: radiolistener on October 30, 2019, 01:11:28 amNote the signal levels I'm testing with, and that noise is obviously being amplitude-modulated onto signals being amplified; has zero to do with noise figure, transformers, etcetera, and if it would be device noise then it wouldn't be an amplifier but rather a noise generator.I'm guessing the most likely suspect is bias circuitry; maybe someone used a Zener diode? After that, maybe instability at audio frequencies? I'm wondering if anyone has encountered this in their or other's amplifier designs, and what was the culprit.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 30, 2019, 12:18:32 pm
content: Quote from: Co6aka on October 30, 2019, 03:57:13 amCalm down on the facepalms, it just makes you look like a tool, esp when you seem to not-quite-as-knowledgeable about a subject as you might think you are.Unless you actually provide us with measurement results, nobody can do anything but make educated guesses. You keep saying that it is not 'amplifier noise', but up to this point I don't see you providing any proof of this.While it is possible that noise is being AM modulated onto your carrier (1/f might do this) I have serious doubts about this being the case here. I think what is much more likely is that you are simply adding noise at the frequency you are using (If your amplifier has gain, it will also have noise).BTW, batteries also have noise, and depending on the type, it can be quite bad (esp at low current draw, I believe they can have a kind of 'shot-noise' thing going on where a low current draw results in you 'seeing' the impact of individual electrons being released in the chemical reactions).It makes sense that the amplifier introduces less noise in FM - in FM, you have a very non-linear noise transfer function that I honestly don't understand myself fully, but in the end it comes down to the fact that your information is purely in the frequency domain, and noise in amplitude on top of your FM carrier (which, btw, is CW which also helps) will have a much smaller.What can also happen (but I've never seen this in RF amps) is something like crossover noise in a audio amplifier, which really just manifests itself at low amplitudes (as large amplitude means it spends less time in this cross-over region).

author: Co6aka
author_rank: Unknown
timestamp: October 30, 2019, 06:04:15 pm
content: Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmPeople see the universe as they are, so if that's how a viewer sees it then that's the viewer's problem to resolve.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pm"seem" = "in the viewer's eyes/mind" rather than as actual, and since "as you might think" you know so much about my thoughts then I shouldn't have to communicate anything about anything to anyone because you and they already know everything about everything I (or anyone else) might ever be thinking. Makes a lot of sense. Are you as good with lottery numbers? You pick 'em, I'll buy 'em, and we'll split 50/50. Deal?Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmIn my OP I wrote,"Academic" question because unfortunately it's a potted block amplifier...which obviously you read and understood.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmStated test condition was -70dBm input signal, and I added that "AM noise is on all signals regardless strength." You of course read that and understand the technical implications.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pm1/f is "pink" noise, very low in frequency, and I wrote "basically it's a 20dB gain small signal RF amp for 100kHz to 60MHz" You of course read that and understand the technical implications.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmThough the Agilent U4438C can generate noise, and modulate signals it generates with noise, in the unmodulated test cases it wasn't generating or modulating such noise. Also, as I wrote, and you certainly read and understood, the noise is clearly audible on strong over the air signals. You of course understand the technical implications.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmAs Thunderfoot says, "Really?"Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmA power source with enough noise to affect the amplifier in such a manner will be easily detectable, and as I wrote, power supplies I've tried are all "quiet" and I've tried adding additional filtering but no effect. You of course read that and understand the technical implications.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmDeserves at least a triple though, because I'm completely at a loss for words...I'm now in the process of converting my entire music collection to FM.Quote from: TheUnnamedNewbie on October 30, 2019, 12:18:32 pmObviously, and among many other things too. Yes indeed, a frequency-modulated signal when passed through a linear amplifier becomes nonlinear and is transformed purely into the frequency domain. OKAY! I'm gonna have to ask Thunderfoot to help out here.What I don't fully understand myself is,obviously WITHOUTreading and comprehending and considering, and possibly requesting clarifications, how do people know so much about everything and everyone without actually knowing much or anything about anything, especially about what people are thinking? With such an ability, I could pick all of the lottery jackpot numbers!Maybe Thunderfoot can help?As for the subject-amplifier's noise issue, a theory is that it may be an internal electrostatic issue. As for the amplifiers tested by Leif Asbrink, what design errors may("may" as in "theoretically" since these days every word must be explicitly defined and detailed)have led to the noise he was able to reduce, as he detailed in his videos? Maybe that's a way to ask my question in such a way that it can be understood by another person.

author: coppercone2
author_rank: Super Contributor
timestamp: October 31, 2019, 03:34:11 am
content: if you need to sweep a rail connect a low frequency generator through an op amp into your power rail.The regulator has PSRR, you can blast through it if you don't want to effect its output impedance. It may offer clues. You can put tones into the thing maybe to see what comes out. Maybe you can test for mixer behavior by putting two tones into it at the same time, or in different places.I have been meaning to do these tests for a very long time on stuff that I built. Only problem is that the amplifier needs to be kind of designed for the circuit in mind. Generally available devices do not seem to offer impressive results based on literature and expected effects. Or, why you want to build roided versions of williams circuits?

author: Frillo
author_rank: Newbie
timestamp: November 01, 2019, 03:35:55 pm
content: It could be intermodulation, as your amp will produce 2nd and 3rd order harmonics. If the match between the amp and the receiving circuit is bad, you will have reflections which mix with the original signal.This can cause interference in the receive band which is audible. A dirty but simple "solution" that will reduce reflections is to add an attenuator after the output of the amplifier. A pi shaped resistor attenuator at 5db attenuation should do the trick I think.But really we won't know unless you draw a schematic, and take measurements at all nodes with a spectrum analyzer.

author: CJay
author_rank: Super Contributor
timestamp: November 01, 2019, 04:51:50 pm
content: I assume there's no way to alter the gain of the amplifier as it's potted but on the off chance that there is (perhaps by lowering supply voltage?), does the SNR change with a change in gain?