Measuring voltages with a spectrum analyzer - Page 1
Forum section: electronics / general

author: raff5184
author_rank: Regular Contributor
timestamp: June 13, 2022, 08:45:46 pm
content: Hi all,I have a R&S FPC1500https://www.batronix.com/files/Rohde-&-Schwarz/Spectrum-Analyser/FPC1500/FPC1500_UserManual_en.pdfBeing a spectrum analyzer, the instrument normally measures in dBm. However, I can select what unit to use (dB, dBm, W, V...).My question is, how does the voltage measurement work? If I try to measure for example a 1 Vpp sinusoide coming from a wave generator, the spectrum analyzer measures a peak at 150mVConversely, if I generate a sinusoid from the SA at 0 dBm (440 mVpp) and measure it with an oscilloscope, I read 520 mVpp

author: TimFox
author_rank: Super Contributor
timestamp: June 13, 2022, 08:51:38 pm
content: The spectrum analyzer will be calibrated in rms voltage, not pk-pk.The easiest thing to measure on an oscilloscope is pk-pk, but many DSOs will compute rms for you from the actual waveform.Your 1 V pk-pk sinusoid from a wave generator was probably into a high impedance (like the 1 megohm input to the scope).Into the 50 ohm input impedance of the analyzer, that will be 0.5 V pk-pk, which is 177 mV rms.0 dBm into 50 ohms is 1 mW mean power, or 224 mV rms = 633 mV pk-pk.Note the references to 50 ohms: the SA input impedance is almost certainly 50 ohms (unless you have one of the uncommon ones with 75 ohms).Your scope probably is 1 megohm, unless you terminated it with an external (or internal) 50 ohm resistor.

author: raff5184
author_rank: Regular Contributor
timestamp: June 13, 2022, 09:56:24 pm
content: Thank you TimFox. I agree with what you wrote and that's what I was expecting.I made sure that all the impedances are set to 50but the numbers are off.At 1Vpp from the wave generator, which btw is the wave generator embedded in the DSO I use, I measure 150mV (rms) on the SA.With 2 Vpp I measure 208 mV (rms), while I was expecting 707 mV (rms)

author: TimFox
author_rank: Super Contributor
timestamp: June 13, 2022, 10:34:31 pm
content: Quote from: raff5184 on June 13, 2022, 09:56:24 pmOf course, the SA could be out of calibration. Does it have a calibration routine or a calibrator output itself?What RBW and frequency sweep parameters did you use, at what generator frequency?(Please be quantitative on the relevant parameters.)In cases like this, I would also check the DSO if it is also the generator, since there could be a problem common to both functions.It would be a good idea to use a third independent method to measure the voltage from the generator.It is very suspicious that when you double the voltage, you did not get twice the result.Also look for distortion: if you are overdriving the SA input or overloading the generator output with a 50 ohm load, you may have less voltage at the fundamental frequency than you expect.In my own use of an SA, I sometimes find that a squirelly measurement is due to my accidentally selecting dBV when I meant dBm, or similar.

author: Stray Electron
author_rank: Super Contributor
timestamp: June 17, 2022, 02:25:32 pm
content: Raymond,Can you suggest a good book or other source on the topic of FFT windowing? I have a couple of Textronix Fourier Analyzers and they have all kinds of options for windowing but I know very little on the subject.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: June 20, 2022, 09:43:48 am
content: Quote from: Stray Electron on June 17, 2022, 02:25:32 pmhttps://habr.com/ru/post/430536/

author: raff5184
author_rank: Regular Contributor
timestamp: June 20, 2022, 03:58:44 pm
content: Quote from: Stray Electron on June 17, 2022, 02:25:32 pmfor theory also the book "Discrete Time Signal Processing - Oppenheim"