Measuring ENR output of a noise source (i.e., HP 346 type) directly using SA? - Page 1
Forum section: electronics / general

author: wb0gaz
author_rank: Frequent Contributor
timestamp: June 29, 2025, 07:09:57 pm
content: I've got a task ahead that entails creating ENR table for an uncalibrated ~15 dB ENR (nominal) noise source. The goal is to end up with same noise source with an ENR table at some frequencies (say every 1 GHz up to the practical limit of the source being tested.)In the past, I created a calibration table for a modified low-$ chinese noise source using HP 8970A noise figure meter initially calibrated with a "reference" noise source (the RF-gadgets product, which already has an ENR calibration table as delivered.) This procedure has three (at least) drawbacks I can identify (1) there is added uncertainty in this sort of procedure, (2) limited to the frequency range of the 8970A (max about 1.6 GHz), and (3) it is PITA. Although the homebrew calibration table (for the low-$ chinese noise source) exists, I don't have any sense of uncertainty degradation in this sort of procedure.Here are my questions (don't know if this belongs in the RF topic group or the Metrology topic group, so starting here (Metrology) - any advice as to where to take this question would, along with any other comments/questions, be gratefully appreciated!):1. Can ENR-by-frequency be measured using a sufficiently capable spectrum analyzer and suitable preamplifier (to get the noise source output above the noise floor of the analyzer)?2. What defines "sufficiently capable" in terms of the analyzer, and what defines "suitable" in terms of the preamplifier?3. If both are on hand, then how would ENR be teased out of the SA readings (at a given frequency), and how would uncertainty be calculated?The page (URL below) goes into some depth about noise measurements, however, I've not pieced together the puzzle to guide me to the answers above.https://rfcafe.com/references/articles/Joe-Cahak/noise-and-noise-measurements.htmThanks for any advice, suggestions, or requests for clarifying information/assumptions!Dave

author: EC8010
author_rank: Frequent Contributor
timestamp: June 29, 2025, 09:36:30 pm
content: Don't see why not. I've done it for noise between 10mHz and 10kHz using an oscilloscope's FFT. The key point is that you need a low noise amplifier (LNA) to get the source noise comfortably above the self-noise of the analyser. I averaged across multiple FFTs to smooth the noise spectrum, exported the averaged FFT spectrum as a csv file, then analysed it in a spreadsheet."Suitable" for the LNA means self-noise of less than one third of the noise to be measured.You also require the LNA to have sufficient gain that its amplified self-noise is more than three times the input self-noise of the analyser. That shouldn't be too much of a problem at RF, but it typically requires x1000 gain when low frequency noise is to be analysed by an oscilloscope (they have quite a bit of 1/f noise).The analyser needs a long enough record length to contain ten cycles at the lowest frequency of interest. Again, that makes life hard at audio frequencies and below, but shouldn't be a problem at RF.The main problem is making sure you've done all your calculations correctly...

author: wb0gaz
author_rank: Frequent Contributor
timestamp: June 29, 2025, 09:48:55 pm
content: Thanks, EC8010 - helpful initial direction!Here's an example that will help me understand how to proceed:Suppose the goal is to get ENR of a noise source at 5 GHz.Set a spectrum analyzer at 5 GHz center frequency, with resolution bandwidth of 10 MHz and total span of 100 MHz. I should then be able supply CW RF test signal at 5 GHz and see a ~10 MHz wide indication on the analyzer at center of the display.Next step is to determine the minimum RF signal level (at 5 GHz, from the RF test signal) that is visible above the analyzer's trace noise.What's unknown at this stage is how that RF signal level corresponds to ENR of a noise source - that would seem to be an initial calculation method needed.For example, if the analyzer required -50 dBm test signal to see a response above the analyzer's noise floor, how does -50 dBm correspond to ENR?If I know that relationship, I can then determine what CW test signal would have equivalent energy to the noise source within the 10 MHz passband, and from there, I could determine what amount of gain would be required to cause the noise source to elevate the displayed noise floor at 5 GHz.Does this make sense so far?Thanks again!Dave

author: EC8010
author_rank: Frequent Contributor
timestamp: June 30, 2025, 08:58:24 am
content: That makes sense to me, but beware that I'm not an RF engineer.It all hinges on sqrt(4kTBR). Assuming white noise, and I think that's a reasonable assumption for the narrow bandwidths you're considering, then measured noise amplitude should be proportional to the square root of measurement bandwidth. I'm afraid I'm not familiar with RF spectrum analysers, but making the assumption that an RF spectrum analyser's spectrum is essentially an FFT having bins of adjustable bandwidth, you should be able to test that for yourself by adjusting resolution and looking at the grass amplitude. Narrower resolution should predictably reduce grass amplitude. A single frequency source should remain constant amplitude with resolution until you narrow resolution so much that you start cutting off the source's phase noise. I would expect the same effect to occur due to master oscillator phase noise in the spectrum analyser.A maths whizz can correct me here, but I think it's reasonable to say that the FFT produces frequency bins having a defined noise bandwidth. That's not perfectly true because windowing affects things, but for measuring noise, 1dB accuracy/error is fine. So if you know your bandwidth and you know your noise, you have everything you need to calculate fundamental noise in terms of nV/root Hz or whatever measurement takes your fancy.If you can export the spectrum to a spreadsheet, you can average across bins to get a more accurate answer. BUT! Beware that your spectrum analyser will produce results in dBm and these must be converted back to an absolute voltage before averaging. The average of a logarithm is not the same as the logarithm of an average. Hence my earlier comment about the need to take care in the calculations.I'm making a (previously unstated) assumption that you can rely on your attenuators and gain settings. That might not be true. I'm aware that RF people like to measure receiver noise by deliberately adding noise to increase noise power by 3dB. Perhaps that's because they're unsure of their measurement bandwidth?

author: Miek
author_rank: Regular Contributor
timestamp: June 30, 2025, 11:04:28 am
content: The problem is that, much like your previous setup with the noise figure meter, you need a way to calibrate the receiver. Using a spectrum analyser instead doesn't remove that requirement because both it and the preamp will contribute their own noise. So, it's possible, but you need a calibrated noise source first...For details on the maths involved & equipment specs required, I like this app note:https://www.rohde-schwarz.com/uk/applications/the-y-factor-technique-for-noise-figure-measurements-application-note_56280-15484.html