Problem measuring digital signals with the TinySA,rbw for what? - Page 1
Forum section: electronics / testgear

author: xugmu
author_rank: Contributor
timestamp: December 18, 2021, 06:35:12 pm
content: Hello, I have bought a TinySA in the hope of being able to measure weak signals (below -88dbm) in digital systems (8Mhz) thanks to its RBW.The problem is that, even if the RBW goes down, the internal noise of the device goes down but the high part of the digital signal also goes down,only the dc component stays up. I don't understand what sense such a thing can make.If I use narrow signals the matter works well and the high part of the signal remains on topBest Regards

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 18, 2021, 07:17:36 pm
content: Broadband signals go down as you narrow the RBW because reducing the RBW means less of the signal falls into the RBW.Narrowband signals don't go down as you narrow the RBW because the entire signal still fits into the RBW.You can separate a narrowband signal from broadband noise by reducing RBW, but you cannot separate a broadband signal from broadband noise by reducing RBW.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 18, 2021, 07:50:23 pm
content: Oh, I shouldn't just leave you with "yeah that sucks but it's just how things work," I should give you an actual suggestion: use a preamplifier. Sometimes these are built into SAs, but that is for convenience -- the best preamplifiers are as close to the signal source as possible. Sometimes that means a little USB powered module sitting on your desk, sometimes that means a cryogenically cooled purpose-built LNA at the focus point of a gigantic antenna. In any case, you should expect an ultimate noise floor of "thermal noise floor + noise figure of preamplifier."From the TinySA site: "Lowest discernible signal using a resolution bandwidth of 30kHz of -102dBm." This is -147dBm/Hz vs thermal noise at -174dBm/Hz, which gives a noise figure of 27dB. That is, the TinySA is about 27dB worse than a perfect theoretical spectrum analyzer, and that's the most we can expect to claw back with a non-cryogenic LNA. It looks like cheap LNAs on ebay have 20dB gain, 5dB NF. That lines up very well with what the TinySA needs: you can expect a roughly 20dB improvement in SNR by using one of these cheap LNA modules.

author: xugmu
author_rank: Contributor
timestamp: December 18, 2021, 08:35:10 pm
content: Quote from: jjoonathan on December 18, 2021, 07:50:23 pmThank you very much for your answers but there is one thing that is not clear to me: What is really the data that indicates the minimum level of signal that an SA is capable of reflecting on the screen?Until now I had related it directly to the RBW but it seems that it is not like that. It seems that the sensitivity is indicated by another parameter. I am logically referring to digital signals, for example a DVB-T signal.To give an example, the visible spectrum (when at noise level) of a digital channel in an MSI sdr is practically the same as that obtained with the TinySA. What sense, in terms of seeing weak signals, can it have to spend 1500 euros on an SA if I can see almost the same as with a 100 device?Best regards

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 18, 2021, 11:45:43 pm
content: This specification is typically called "DANL" (displayed average noise level). The TinySA page didn't specify DANL so I used "lowest discernable signal" as a proxy.Finding a DANL specification does not remove the need to do math: if you have a 1MHz wide signal and 1kHz RBW, your signal will be chopped into 1000 buckets. You must adjust your expectation for how much falls into each bucket accordingly (-30dB assuming even distribution) and you have to do this yourself because the person writing the spec sheet doesn't know if your signal is 0, 1, or 10MHz wide and they don't know if it looks flat, sloped, wiggly, or dick-and-balls. Ahem, I mean "side lobes."A cheap SA typically makes sacrifices in:* Bandwidth* Preselection* Image/harmonic rejection* Sweep speed* Phase noise* Analysis packages* System integration & supportIt all comes down to what your problems are and how much you value your time. Bandwidth is the most fundamental: you have to be able to see the frequency you are interested in. Once you have arranged frequency coverage, if you just want to demodulate a known signal, SDRs are attractive. If you just want to see "signal" vs "no signal" and the signal is slow, cheap SAs are attractive. If you are building a RF device, however, you will find that the cheap devices are bad in exactly the places where you need them to be good. This is not a coincidence -- they cut out cost in exactly the places where you want to evaluate components which are better than "dirt cheap." If you need to do this, it becomes a function of valuing your time: it is possible to "bootstrap," to make good measurements with bad equipment, but the process can be extremely laborious and require fabricating filters, running banks of parallel analyzers and doing math on them, etc. If these "workarounds" take engineer-months, suddenly spending thousands on a SA isn't just rational, it's cheap by comparison.--------- Explanations ---------Bandwidth: a 1GHz SA can't see a 2GHz signal.Preselection & Image Rejection: bad preselection causes "fake signals" to show up in your display. These are typically frequency-shifted, reflected, and attenuated copies of real signals. SDRs are notorious for these. Sometimes, especially with SDRs, they are only 20dB below your real signal! On a 100dB screen, a fake signal might be 80dB tall! If your real signal is narrowband, you can figure out which signals are fake and ignore them. Maybe you have to give things a nudge if there is an uncomfortable overlap. As signals get wideband, overlap becomes inevitable, and lack of preselection causes a complete mess.Harmonics & phase noise: these are parameters that you will want to measure in your own system, but if your SA is worse than the system you are measuring, you just see the SA's harmonics and the SA's phase noise. Bad SA looks at Good Product and sees Bad SA. Unhelpful.Sweep speed: a magnificent creature comfort. High sweep speeds at low RBWs let you watch signals jump and play in the spectrum. Otherwise you need signals to "sit still for the picture," and that can be difficult to arrange.Analysis packages: save you time in matlab.Integration and support: reduce risk in a corporate environment. They give you someone else to blame for a wide variety of problems, and this can be a very valuable feature indeed.

author: mawyatt
author_rank: Super Contributor
timestamp: December 19, 2021, 01:09:18 am
content: Had a Keysight Engineering Fellow that designed many of the custom chips in their higher end SA and VNAs, tell me that the measuring instrument needs to be 10~20dB better than the DUT measurement, and the chips in the instrument need to be 10~20dB better that the instrument!This is why there are so many custom chips in their high end SA and VNAs, OTS chips just aren't good enough, and one reason why these high end instruments are so expensive!! Chip development ain't cheap, and when pushing SOTA it goes up from there!!Best,

author: SilverSolder
author_rank: Super Contributor
timestamp: December 19, 2021, 02:06:27 am
content: Quote from: mawyatt on December 19, 2021, 01:09:18 amQuis custodiet ipsos custodes?

author: xugmu
author_rank: Contributor
timestamp: December 19, 2021, 11:23:28 am
content: Quote from: jjoonathan on December 18, 2021, 11:45:43 pmA dvb-t signal like the one in my attached image is quite a dense signal, more than 8000 1k carriers almost overlapping with the adjacent ones. I suppose some SA sampling point should fall on top of one of them.I had read about the DANL and its relationship with the RBW, everything that I had read indicated that thanks to that relationship you could lower the noise floor and, therefore, see signals below normal.What I have not read anywhere is that this beneficial relationship was limited by the bandwidth of the signal to be analyzed.Best Regards

author: gf
author_rank: Super Contributor
timestamp: December 19, 2021, 01:34:21 pm
content: Quote from: xugmu on December 19, 2021, 11:23:28 amBut this implies, if thetotalsignal power is -88dBm (sperad over 8MHz bandwidth), then the power at one carrier is 39dB lower, i.e. only -127dBm, on average.And the power falling into a 30kHz RBW window is only -112 dBm on average, which is already 10dB below the "Lowest discernible signal using a resolution bandwidth of 30kHz of -102dBm" then.[ Btw, more precisely, what matters is rather the ENBW of the SA filter, not its -3dB RBW. TinySA has a relatively rectangular (flat top) filter shape, so I guess it won't be too different. ]

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: December 19, 2021, 05:14:57 pm
content: This is the the issue with measuring "noise-like" signals. Over-the-air digital signals have an noise-like nature that cause them to move in frequency vs. time so that an instrument like an SA measuring with some RBW (say 1kHz) will measure the power of that signal only during the time it is within 1kHz of that particular frequency, and it has to be there for the time constant of the RBW filter (about 1 msec). So something that dodges in and out will not be measured at full power. For testing purposes, where you create a waveform and play it back, you can get around this limitation by measuring very repeatedly the signal and using coherent time averaging to drag the signal out of the noise. But this is a special case where you know a-priori the signal waveform's period. Otherwise, as you lower the RBW, the noise is lowered (because it spends less proportional time inside the RBW filter) and the signal is lowered (as it also spends less time) but if you use a band power measurement you will see both the noise power and the total signal power remains the same even though it appears lower on the screen, as you narrow the RBW, because you have more RBW buckets to sum up. If you have 100 MHz signal, with 1 MHz RBW, at -50 dBm screen level, the band power sums up 100 RBW buckets to show -30 dBm. But if you narrow the RBW to 1kHz, the screen level will drop to -80 dBm, but now you have 100,000 RBW buckets to add up and the total is still -30 dbm. It is a little complicated. (oh, I did cover all this in the new chapter 8 of the second edition of my book).

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 19, 2021, 05:35:31 pm
content: The RBW is a "bucket," centered around each and every trace point, that "scoops up" signals and adds them together to calculate the display level. What happens when the bucket shrinks from 3MHz wide to 1MHz wide? If we have a 1mW signal that is narrow and right in the middle of the bucket, nothing happens -- it was in the bucket before and it is still in the bucket after the bucket shrinks. The displayed amplitude was 0dBm before and 0dBm after. What happens if the signal is broad, or composed of many narrowly spaced carriers? There is a 1MHz chunk of signal on the left and 1MHz chunk of signal on the right that no longer fall into the bucket after the bucket shrinks. The displayed power in our bucket drops to 1/3 of what it was. However, the remaining 2/3 didn't vanish, it just went into adjacent buckets. Before we had 1 bucket with 1mW, after RBW shrinks we have 3 buckets with 1/3mW each. In the language of dBm, before we had 0dBm, 1RBW wide and after we have -4.7dBm and 3RBW wide.Signal Narrower than RBW = doesn't drop with RBW as RBW decreasesSignal Wider than RBW = does drop with RBW as RBW decreasesNote that this applies to background noise just as it applies to your signal: the background noise is just always wider than RBW, so it always drops with shrinking RBW.[ and yes, gf is correct, we should be talking about ENBWs and filter shapes and convolutions to be precise, but it seems to me that we are still in the intuition-building phase here, so I am pretending that filters are perfectly rectangular in the frequency domain ]

author: erikka
author_rank: Regular Contributor
timestamp: December 20, 2021, 06:30:40 am
content: If you would have used the channel power measurement in the tinySA you would have seen that the power within a channel is independent of the chosen RBW.Only as long as the RBW is wider than the digital signal the signal level does not change when changing RBW. As soon as RBW is smaller as signal width the signal level goes down with RBW.Adding a 20dB LNA to the tinySA works very well . I've even used two LNA with a final 0.5dB NF and the DANL get almost to a level of -174dB/Hz.See this videoFor chanel power measurement see this videohttps://youtu.be/fV7YUMbaGEo?t=457

author: xugmu
author_rank: Contributor
timestamp: December 24, 2021, 12:21:15 am
content: Thank you very much for your responsesThanks to them I have reached two conclusions:One, when measuring both noise and signal, use power measurements and forget about the ordinate axis.And two, it is not possible to use the RBW to gain an advantage when measuring weak signals as long as the signal is not within that RBW.I understand that a preamplifier helps to measure low levels but reading its specifications I had believed that with a professional SA I would see signals (8 mhz for example) over a noise level of -140dbm, something totally ruled out seems to beAs for the TinyAPP software, simply wonderful (and that I had not yet tried tinysa app screen capture), it has made me appreciate the possibilities of a real analyzer versus the spectrum analyzers of the tv analyzers that I normally use,although, curiously, the noise level seemed practically the same.Best Regards

author: gf
author_rank: Super Contributor
timestamp: December 25, 2021, 01:09:34 pm
content: Quote from: xugmu on December 24, 2021, 12:21:15 amWhat matters here is thepower density, i.e. power per Hz bandwidth.A signal power of -88dBmtotal, evenly spread over a bandwidth of 8MHz, has a power density of only -157dBm per Hertz.So the DANL of the SA (or the effective DANL of preamp+SA together) needs to be better than -157 dBm/Hz. A DANL of -140 is not sufficient for this signal.

author: xugmu
author_rank: Contributor
timestamp: January 04, 2022, 09:33:03 pm
content: Quote from: gf on December 25, 2021, 01:09:34 pmThe -140 figure was set a bit subjectively, I could have put -130 or -150. Thanks for the clarification anywayThe problem is that noise figures of -150 or 160 dbm are given in the SA specifications but in reality these figures seem to be useless for signals with bandwidth above a few kilocycles.What I am finding (in my case with the Tiny SA but it seems that it would happen the same or similar with a 10,000 dollar analyzer) is that the lowest signals that I can measure (with 8mhz of bandwidth for example) are similar to those What would I measure with a more or less professional television analyzer. We would be talking about noise floors of -89 dbm approximatelyBest Regards

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 12, 2022, 07:48:29 pm
content: I'm a bit late responding, but if you are measuring a real signal, and it is repetitive, there are methods to do coherent power detection which will tell you the power in your digital signal even if it is below the kTB noise floor level. Note also the best SAs can have noise floors approaching kTB (certainly less than 5 dB noise figure) but that won't help you with a signal distributed across the bandwidth, as you found out. Note, this would only work for repeated signals (that is, for example, a test modulation waveform which is repeatedly played back by an Arbitrary Waveform Generator.). There's 22 pages of discussion (section 8.1.3 and 8.1.4 in my book) to describe this. But it comes down to coherent time averaging to eliminate noise and recover the true signal level.