How can spectrum analyser measure noise floor? - Page 1
Forum section: electronics / rf-microwave

author: fonograph
author_rank: Frequent Contributor
timestamp: August 27, 2018, 01:11:32 am
content: I have another RF noob question.How can spectrum analyser measure noise floor of DUT? I know that long FFT window lenghts or slow sweeps can pull out signal that is even bellow the noise floor becose coherent signals add +6db while incoherent noise adds only +3db.Thats means long FFT or sweep will average the noise down more than the signal.But what if I want to measure is not a signal,but a noise instead? And not just any noise,a white noise,completly flat is frequency,can spectrum analyser accuratelly measure that?Lets say our DUT have -60dbm noise floor.If the FFT or sweep speed is too short,then the noise floor of DUT cant be measured becose the noise of our SA is higher,on other hand,if we set it to long,then it would ofcourse decrease the noise of the SA bellow the noise floor DUT,but the DUT noise floor will be also lowered by this long sweep averaging!Or would it not? Am I wrong? Becose if DUT have white noise floor and SA sweep technique is causing averaging to happen,then it must artificaly lower the apparent DUT noise too.I know there are real time SA and I am also aware of the the two channel coherent/incoherent noise measurement method,but if I have just normal single channel sweeping SA,how could I ever measure a white noise floors with it,is that even possible?

author: rs20
author_rank: Super Contributor
timestamp: August 27, 2018, 01:23:10 am
content: I suspect that you've correctly observed that any signal processing techniques will not be able to discriminate between noise from the signal analyser vs noise from the DUT. Therefore;If the DUT is significantly noisier than the signal analyser, then you can see the increase in noise seen on the signal analyser when a terminator is swapped out for the DUT, and therefore infer the noise floor of the DUT,If the DUT is significantly quieter than the signal analyser, then all you can infer is that fact alone and you won't be able to perform any further measurements.Just like phase noise, the SA can only measure up to its own level of performance.You can always add a super-low-noise preamp to your set up, that will lift the noise from the DUT up to a range where the SA can see it.Maybe there are some other tricks I'm not aware of though.

author: fonograph
author_rank: Frequent Contributor
timestamp: August 27, 2018, 01:49:03 am
content: You missed my point,ofcourse DUT noise must be higher than the instrument we use to measure it.What I claim is that sweep based SA cant accuratelly measure DUT with white noise floor no matter how much lower the SA noise is.Even if it had infinite dynamic range,by sweeping,you are averaging,and by averaging you are artifically lowering the apparent noise floor of DUT.Lets say we have magic SA with zero noise and DUT with -80dbm/Hz white noise floor.If you set that sweep speed to long,the reading you get will be that the DUT have -100dbm/Hz noise,you make it even longer it will show -110db,as the sweep lenght approaches infinity,so will the apparent noise floor of DUT!If SA is showing zero noise,but the real noise of DUT is whatever non zero value,that is infinitely high error,the reading is 0% accurate and not becose the SA isnt good enough,its becose the entire sweep technique is inherently flawed.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2018, 02:32:43 am
content: A swept spectrum analyser has no problem measuring noise within the limits of its sweep speed, filter bandwidth, and inherent noise and it is routine to use them for this. The noise bandwidth of the measurement results in a spot noise measurement which can be corrected to return noise within a 1 Hz bandwidth. An FFT being used to measure noise can be corrected in the same way.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 27, 2018, 03:08:04 am
content: Quote from: fonograph on August 27, 2018, 01:49:03 amThat's not the way it works.In a traditional swept receiver spectrum analyzer, you have two separate filters: the resolution bandwidth (RBW) filter and the "video" bandwidth. The RBW filter is applied before the power detector to the RF voltage (after mixing to an IF). Using a narrower bandwidth filter reduces the signal voltage in the expected way, but also reduces the bandwidth, The spectral density stays the same.The "video" bandwidth filter is applied after the power detector. This is the filter you are talking about when you sweep slower. The power detector converted the voltage signal which could be positive or negative to a power signal which is only positive by computing v^2. The power signal has a non-zero mean (obviously). So averaging longer by sweeping slower will reduce the fluctuations but will converge towards a non-zero value.

author: RadioNerd
author_rank: Regular Contributor
timestamp: August 27, 2018, 10:01:54 am
content: a noise floor is measured in dBm/Hz and is often frequency dependent. What you are doing by taking longer FFTs (or using a lower RBW filter in the analog world) is lowering the analysis bandwidth.Taking an FFT of an 1 second long time domain measurement yields a resolution of 1 Hz in the frequency domain. In this example, the noise floor in dBm/Hz at certain frequency would be the power measured in one FFT bin divided by 1 Hz.For a 100ms long time domain measurement, the resulting FFT resolution is 10 Hz, therefore the measured value in each bin must be divided by 10 in order to get the noise power density in dBm/Hz.The averaging effect you are talking about does therefore not lower the noise floor. It does only increase the SNR for the measurement of CW signals because they are (ideally) infinetely narrow in the frequency domain.Lowering the resolution bandwidth wont help at all if you are dealing with wideband modulated signals.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 27, 2018, 08:55:12 pm
content: Hi,I have a Keysight N9000A CXA spectrum analyzer, and the noise figure measurement application software for it.It all works by the so-called Y method. There is a very good paper from Keysight explaining how it is done:http://literature.cdn.keysight.com/litweb/pdf/5952-8255E.pdfI also built a noise figure measurement box myself:https://electronicprojectsforfun.wordpress.com/making-noise/a-noise-figure-measurement-solution-for-the-poor/results compare quite well with the commercial solution.