From dBm to dBm/Hz - Page 1
Forum section: electronics / rf-microwave

author: miko1977
author_rank: Newbie
timestamp: July 18, 2017, 05:02:34 pm
content: Hello peopleThis is my first post, and i am happy to be in this forum. Greetings from Greece friends.I am assembling a simple radio telescope receiver and i want to measure itsnoise figure. I want to use the Y factor method (https://www.maximintegrated.com/en/app-notes/index.mvp/id/2875)but my analyzer can't measure power spectral density (dBm/Hz), only power (dBm). Is there any reliable way totransform dBm to dBm/Hz?Thank you

author: LaserSteve
author_rank: Super Contributor
timestamp: July 18, 2017, 05:56:02 pm
content: Do you have a spectrum analyzer with an intermediate frequency output port?There are lots of ham (amateur) radio methods of measuring Y factor at a single frequency, but to extend that to " Root Hz" requires a variable bandwidth tunable receiver for the easy method.I've sent you a PM, with a link on where to ask.Steve

author: w2aew
author_rank: Super Contributor
timestamp: July 18, 2017, 06:09:23 pm
content: Quote from: miko1977 on July 18, 2017, 05:02:34 pmFirst order, rough approximation would be to take your dBm reading and subtract the log value of the RBW used for the measurement. For example, if you use 1kHz RBW in your analyzer, and you measure -90dBm, then:dBm/Hz = -90dBm - 10LOG(1kHz)-120dBm/Hz (approximate)Should be done using the Average detector in the analyzer, and with minimum (no) attenuation.

author: miko1977
author_rank: Newbie
timestamp: July 18, 2017, 06:21:00 pm
content: Thank you, i will definetly have this in mind.

author: ConKbot
author_rank: Super Contributor
timestamp: July 18, 2017, 10:22:26 pm
content: Are you sure the SA can't do a noise spectral density marker? I've seen some where you can't do a NSD trace, but you can put markers measuring NSD for a specified bandwidth.

author: niconiconi
author_rank: Frequent Contributor
timestamp: February 06, 2022, 11:51:16 pm
content: Old thread, but I was trying to do the same measurements today and confused. A web search brought me here. I found the solution at Keysight's knowledge base, I'll just quote it for reference if someone finds the same post in the future.Basically, dBm/Hz can be roughly understood as the dBm value measured with a 1 Hz resolution bandwidth. To estimate that manually, just divide power at a data point by your resolution bandwidth. For example, if the RBW is 3 kHz, divide the power at this point by 3000. Just be sure you're dividing the number in milliwatts, not in dBm, otherwise the calculation is nonsense. Alternatively, w2aew's post showed how to do the division logarithmically.https://edadocs.software.keysight.com/kkbopen/signal-to-noise-calculations-using-a-spectrum-analyzer-588267017.htmlQuote

author: G0HZU
author_rank: Super Contributor
timestamp: February 07, 2022, 12:05:31 am
content: Be careful doing this with older analysers that don't have a 1Hz noise marker function. A log amp will respond differently to noise and also the detector type can affect the displayed noise level. This requires additional corrections and these are included automatically when the 1Hz noise marker function is selected.However, if you are only doing an uncorrected Y factor measurement it won't really matter as you are just measuring a noise ratio. Just make sure you keep the displayed noise level at least 14dB inside the true log range of the analyser display if it is an older type analyser.

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: February 09, 2022, 03:15:13 am
content: QuoteAs far as I've been able to tell on all the spectrum analyzers I've used, this is not just first order. What the spectrum analyzer displays (in averaging detector mode) is not really dBm but dBm per resolution bandwidth. That makes sense because a spectrum display has to be a density, not an absolute power. You should have to integrate over the density to get power.The only analyzer I ever used that was upfront about this was a Rohde & Schwarz unit, which defaulted to displaying the vertical scale in dBm/hz. The reason why other analyzers use straight dBm's, as far as I can guess, is that in the frequent case of when an interfering signal is very narrow (less than an RBW in width) then the displayed value will correctly reflect the actual power in the spike.

author: gf
author_rank: Super Contributor
timestamp: February 09, 2022, 07:29:08 am
content: dBm scale is usually normalized such that a peak reflects the power of a single-frequency CW signal.dBm/Hz scale is normalized to reflect the power density of a flat spread spectrum signal.The relation (ratio1)) between the two scales is theequivalent noise bandwidth (ENBW)of the filter, which depends on the shape of the filter's frequency response, and which is not necessarily the same as its -3dB bandwidth.RBW mostly refers to the -3dB bandwidth, though, so it is just a rough first order approximation for the ENBW.1)In logarithmic space (-> dB), a ratio turns into an offset, of course.

author: radiolistener
author_rank: Super Contributor
timestamp: February 09, 2022, 12:19:31 pm
content: dBm/Hz just means "dBm for bandwidth 1 Hz".If you're talking about white noise and have dBm for bandwidth BW and want to get dBm for bandwidth 1 Hz, you can convert it just by reduce value for 10*log10( BW ). For more complicated signal it will depends on its spectrum.

author: xmo
author_rank: Regular Contributor
timestamp: February 09, 2022, 03:30:23 pm
content: G0HZU is correct when he states: "Be careful doing this with older analysers that don't have a 1Hz noise marker function. A log amp will respond differently to noise and also the detector type can affect the displayed noise level. This requires additional corrections and these are included automatically when the 1Hz noise marker function is selected."Also, gf points out the importance of ENBW. These facts are part of why "just divide power at a data point by your resolution bandwidth." will only give an approximation. Such approximations might be useful for making comparisons - possibly such as when computing noise figure using the 'Y method', but it should be clearly understood that these approximations are far from accurate.The attached application note explains noise measurement and how the dBm/Hz noise marker function works.

author: G0HZU
author_rank: Super Contributor
timestamp: February 09, 2022, 07:37:52 pm
content: If you go back 20-30 years when the classic HP8568B and HP8566B analysers were still in regular use there was still a problem even if the analyser has a 1Hz noise marker function.The (nearly new back then) HP8568B analyser at work would show subtly different readings for noise depending on the span and RBW setting even if the 1Hz noise marker was enabled. This was partly because these old analysers used analogue RBW filters and the bandwidth of the RBW filters had quite a lot of uncertainty. This meant the 1Hz noise marker wasn't always consistent across changes in RBW. As the company needed to measure noise very accurately to measure the performance of receive down-converters a neat solution to this problem was devised.The company asked (I think) NPL to source and calibrate a couple of precision 70MHz bandpass filters. These had low loss and very low VSWR and a 'very' flat passband. The idea was to pass a precision (flat with low source VSWR) noise source through the BPF and then NPL measured the noise power at the filter output in dBm using a thermocouple power meter. They then worked out the dBHz factor for the filter (eg 66.12dBHz) and stamped this on the BPF. The filters were sent for checking at NPL every couple of years or so.Each 70MHz BPF was made by Telonic-Berkeley and they were very expensive and rugged. I still have access to these old filters although they haven't been formally checked for the dBHz factor since the 1990s. I have a precision high level noise source here at home and sometimes check my analysers using this method with the Telonic 70MHz bandpass filters.Here are the numbers from the most recent test. This was tested on an Agilent PSA analyser which has a digital IF with digital RBW filters.The noise power after the 70MHz filter on a thermocouple power meter was -18.40dBm on the thermocouple power meter.-84.46dBm/Hz was measured on the PSA spectrum analyser at 70.0MHzThe dBHz factor for this filter (as measured by NPL) is 66.12dBHz.-18.40 – 66.12 = -84.52dBm/HzThis agrees very closely with the reading on the PSA analyser so there's a good chance the 66.12dBHz figure for the filter is still quite close to the true dBHz figure for the filter.This setup was used with the old HP8568B analyser to calibrate the 1Hz noise marker for a given span and RBW setting with the 1Hz noise marker enabled. These old 70MHz BPFs were one of the gold standard references back in the 1990s and it allowed the company to generate accurate signal to noise ratios when testing BER etc.

author: G0HZU
author_rank: Super Contributor
timestamp: February 09, 2022, 07:54:18 pm
content: Here's an image of the two filters. They are currently at work but I could easily borrow them again over the weekend and do a demo.As you can see they are very old and look a bit tired. They would have been used multiple times on a daily basis for several years so you can see the polished wear from all the handling they have seen over the years.To calibrate the analyser, there also needs to be a precision/flat noise source and a calibrated thermocouple power meter. As long as the dBHz number supplied by NPL stays consistent for the filter then the accuracy of the noise power measurement transfers from the power meter to the spectrum analyser with minimal uncertainty. The port VSWR for the filters was extremely low at 70.00MHz to minimise mismatch uncertainty for example.

author: xmo
author_rank: Regular Contributor
timestamp: February 09, 2022, 08:06:54 pm
content: G0HZU - A very interesting story! Thanks for sharing. I seem to remember that I have a 70 MHZ Telonic filter somewhere. I always wondered what it was used for in its former life.To touch on a couple of spectrum analyzer points others have posted - yes - for the spectrum analyzer amplitude display / marker value to be accurate you must either be measuring a CW signal or ensure that all the components of the signal fit within the RBW filter.Paul said: ".. integrate over the density to get power ..." This is a very interesting subject. For example, the TIA publishes diagrams that do exactly that - they show the SPD of various common modulation formats used in land mobile communications. These can be used for comparison, evaluation of bandwidth requirements, computation of ACPR, frequency coordination, etc.I have attached one such diagram to illustrate. Note that a narrow RBW has been used to show the spectral makeup and the waveform peaks just exceed -20 dBm. The integration of the SPD data yields an actual signal power of 0.33 dBm

author: xmo
author_rank: Regular Contributor
timestamp: February 10, 2022, 12:44:16 am
content: Noise figure and noise measurement are very interesting topics and very important to those of us working in communications.G0HZU mentioned the Agilent PSA. Noise is one reason those were so expensive. Noise as in phase noise.I have attached an analyzer sideband noise comparison. Each analyzer was looking at a 0dBm signal from the same low-noise signal generator. The image on the right is from an Agilent ESA-E, a mid-tier instrument. Note that the sideband noise near the carrier in almost 30 dB higher than the other instrument!When does this matter? I'll add another post later with an example.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 10, 2022, 02:04:02 am
content: Even VNAs (well some) have better phase noise; not too related to noise figure unless you are measuring noise figure of mixers and looking at low IF offsets, then LO phase noise can be important. Here's a plot with the same settings of my N5242B PNA-X with new DDS sources, but also using PNA as a spectrum analyzer (not an advertisement; maybe a little showing off, though)

author: xmo
author_rank: Regular Contributor
timestamp: February 10, 2022, 04:18:15 am
content: Dr. Joel,I registered for your webinar but something came up and I wasn't able to attend. Fortunately, Keysight sent me a link to the recorded content. I am looking forward to viewing it.Thanks for showing us the PNA-X. Something we test equipment addicts can only dream about - but - time marches on. Back in the day I never thought I would own an ESA-E or an 8753ES or an ESG-DP.Now I do (along with many other cool 'toys') Also, thanks for hanging out with us.

author: xmo
author_rank: Regular Contributor
timestamp: February 10, 2022, 03:29:00 pm
content: A couple more comments.G0HZU said: "The company asked (I think) NPL to source and calibrate a couple of precision 70MHz bandpass filters."For those of us not in the UK, NPL = National Physical LaboratoryAlso, Dr. Joel, it did not escape my notice that the screenshot of the PNA-X performance has the same settings as the tests I had run. The direct comparison makes it obvious that the progress in sources is amazing.The direct comparison also suggests that you love working with these instruments as much as some of us addicts - it's clearly not just a job for you. I know you have been at this for a long time. I worked in the communications field for 54 years and would still be had I not been forced to retire recently due to declining health. I loved it and I'd do it all over.Along the way, I always considered the cost of my test equipment acquisitions as tuition payments in a life-long continuing education process. I'd do that all over again too.Thanks again for helping me and others like me in our journey.

author: G0HZU
author_rank: Super Contributor
timestamp: February 10, 2022, 06:52:20 pm
content: It's amazing how quickly the years go by... the PSA analyser series is 20 years old now.I've been using the PSA E444xA series since they were first manufactured so I know them quite well. The phase noise plot in post #14 does look a bit noisy to me. Normally the PSA would default to the dual loop 'wide' PLL on a 100kHz span so I'd expect to see a fairly consistent -118dBc/Hz across about 2kHz to 50kHz offset. In other words, the plot looks noisier than I'd expect. Could the sig gen be limiting the result? It looks to be running in Basic mode rather than Spectrum mode and the phase noise plot looks to be what I'd expect to see from the E4406A VSA (14bit version) at these settings. The E4406A looks very similar on the outside to the PSA and has the same digital IF as the E444xA PSA.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 10, 2022, 07:55:55 pm
content: Quote from: G0HZU on February 10, 2022, 06:52:20 pmSomeone clued me into the E4406A years ago (and how to identify the 14 bit version). I bought a few on eBay for about $1000 each. Talk about something amazing for that price. I was working for a company that was renting lots of MXAs many of which were only used as front ends for VSA (company had a custom OFDM waveform). I talked them into saving money by swapping out a few MXAs for E4406As (rented to them by me for a "low" cost). Then VSA came out with a new rev that dropped support for E4406A; fortunately there was a registry hack or something that let it still work. I still have two units at home. I occasionally think about taking the attenuator (6 dB I think) out of the input line to lower the noise floor.

author: xmo
author_rank: Regular Contributor
timestamp: February 10, 2022, 08:04:23 pm
content: G0HZU. Dude! You are one sharp cookie! Nothing gets past youYou had mentioned PSA so I pointed out that low phase noise is one advantage of a high-tier instrument - which I believe to be a true statement.Then I posted a comparison. My intent was to show how much difference there can be between two instruments. I said: " Note that the sideband noise near the carrier in almost 30 dB higher than the other instrument!" but I didn't identify the other instrument.You hit the nail on the head. It was indeed an E4406A with the 14 bit IF. The E4406A is a way for hobbyists to get amazing performance for a bargain price.

author: G0HZU
author_rank: Super Contributor
timestamp: February 10, 2022, 11:50:10 pm
content: Ah, that explains it, thanks! Using those settings at 150MHz and 100kHz span the close in phase noise (i.e. <10kHz offset) will be lower for the PSA. However at 25kHz to 50kHz offset the E4406A phase noise will be lower than the PSA. I think the first LO for the E4406A uses a similar synthesiser module to the one used in some of the Agilent ESGD signal generator models.

author: xmo
author_rank: Regular Contributor
timestamp: February 11, 2022, 07:36:12 pm
content: Many interesting points to discuss - maybe a little off topic but the O.P. seems to have been a hit-n-run anyway.So... Here's my SPD / phase noise story.A few years ago I had the opportunity to assist with the creation of an RFP for a state-wide public safety system. Our group obtained the TIA documents to learn about delivered audio quality (DAQ) and coverage acceptance testing. These documents also contain the SPD comparisons of various modulation formats which I found to be very interesting.It got me thinking about D-Star which Icom was promoting as a 6.25 kHz format. I wondered what D-Star would look like if evaluated using the TIA method. The TIA documents contain instructions on how to create the diagrams so that manufacturers can submit new formats.. I thought - I can do this! How hard can it be? Just follow the instruction, connect a radio to a spectrum analyzer, acquire the data, put the data into the TIA software - ta-da.Then I realized that you can't test an 'unknown' and trust your results unless you first test a 'known' and achieve results identical to the published results. That is the only way can you have confidence that your instrumentation and your procedure are going to produce accurate results.OK. Just start with one of the published modulation formats. Still easy. Not!First, the procedure is rather involved. Once I got past that complexity to a finished diagram, I realized that my Agilent ESA-E wasn't up to the task. I (correctly) concluded that phase noise was causing the problem. OK. I need a better analyzer. So I used my R&S FSP. Better but still not good enough. So then I borrowed my employer's new CXA. 20,000 USD box. What? Still not good enough!What to do? At that point I decided that the TIA tests must have been performed with something really good like a PSA. Wow - those are expensive. Out of my league (still are)Then I remembered having seen some discussion on-line about the E4406A. Sure looks like a PSA. The datasheet says: "The Agilent Technologies E4406A vector signal analyzer (VSA) is a full-featured transmitter tester. Isn't that what I want to do - test transmitters? The phase noise graph in the datasheet looks pretty good. For those selling prices - what the heck - I'll buy one. Good decision! Now my results for testing published formats compared well to those published by the TIA.Look at the attached comparison image. The signal source was the attenuated transmit output of a Motorola APX portable transceiver operating in P25 Phase one C4FM CAI mode. The exact same transceiver was used for both tests. The only difference is the analyzers.The CXA result on the left shows much higher sideband noise than the VSA on the right. This causes a serious error in the ACPR calculation.