Zero span spectrum analyzer .FFT, sweep, filters and RBW. - Page 1
Forum section: electronics / testgear

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 11:18:16 am
content: Hello, I am thinking of buying a spectrum analyzer and the main objective is to see very small signals, RBW between 0.2 Hz and 1 Hz. These resolutions are easily achieved with an SDR and this is where the problem appears: I want to see the signals also in the time domain and here things seem to get a bit complicated.The SDR works, afaik, in FFT mode and, afaik too, you have to apply a filter to isolate the required bandwidth (in this case zero span which in reality, afaik, would be 0.2 Hz spam, 1 Hz spam or whatever spam, always logically below the SDR bandwidth). A digital 1 Hz filter is difficult to create in my opinion. On the other hand, a sweep spectrum analyzer, afaik, could easily stop the sweep and focus on a point (we would have a bandwidth equivalent to the RBW).The problem would be buying an analyzer that works in FFT mode and that gives me problems in zero span modeBest regards

author: Andree Henkel
author_rank: Regular Contributor
timestamp: August 06, 2024, 11:30:54 am
content: maybe you are actually looking for a "lock in amplifier"?

author: joeqsmith
author_rank: Super Contributor
timestamp: August 06, 2024, 11:53:02 am
content: While I know what zero span mode is, I have never heard of "zero spam". Obviously not a typo. This is spam but I don't see how it fits into showing time domain signals:https://www.spam.com/

author: ftg
author_rank: Frequent Contributor
timestamp: August 06, 2024, 12:16:31 pm
content: A higher end oscilloscope with good FFT like R&S MXO5.According to it's datasheet sub-1Hz RBW's seem possible.

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 12:30:41 pm
content: Quote from: ftg on August 06, 2024, 12:16:31 pmAn oscilloscope represents, in the time domain, everything that appears at its input.What I am suggesting, afaik, is precisely the opposite, to put it in a perhaps not very scientific way, it would be about making a bandpass filter with a bandwidth of 1 Hz and placing it before the oscilloscope.Best regards

author: gf
author_rank: Super Contributor
timestamp: August 06, 2024, 12:50:30 pm
content: What is the desired center frequency?

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 12:55:13 pm
content: I usually work in the television area, between 450 MHz and 750 MHz.

author: ebastler
author_rank: Super Contributor
timestamp: August 06, 2024, 02:13:43 pm
content: Quote from: xugmu on August 06, 2024, 12:03:36 pmSo, prey tell, whatdidyou mean by "spam"? Or do you want to tell us that you mis-spelled it six times in a row, could not be bothered to correct it after it was pointed out to you, and instead chose to write a snide comment?

author: shabaz
author_rank: Frequent Contributor
timestamp: August 06, 2024, 02:33:12 pm
content: OP should also actually mention the problem they are trying to solve, rather than assume that 'zero spam' (which of course also makes this thread useless for others in future, because it's not as easy to google) is the way to meet their actual need. Because, with the current limited information, it sounds way easier to simply work once at baseband to recover the signal they might wish to examine.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 06, 2024, 03:13:27 pm
content: Sounds like the OP is trying to demodulate a signal within a very narrow bandwidth. Probably more of an XY problem than anything else...

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 06:17:46 pm
content: I don't actually need to demodulate the signal. I'm just going to look at it in the time domain, the question is whether it would be better to do it in FFT analyzer mode or in sweep analyzer mode.

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 06:20:20 pm
content: Quote from: ebastler on August 06, 2024, 02:13:43 pmIt's not that complicated to understand either. I don't speak English perfectly and I have confused spam with span.

author: radiolistener
author_rank: Super Contributor
timestamp: August 06, 2024, 06:58:16 pm
content: You can use digital oscilloscope or digital analyzer with analog input with high enough input bandwidth and capability to record long enough continuous sample. Then you can analyze it in Octave or Matlab.For example my SDS1102X allows to record 14 Mpts record at 1 GHz, 500 MHz, 50 MHz, 10 MHz, 5 MHz, 2 MHz, 1 MHz, 500 kHz, 200 kHz, 100 kHz, 50 kHz, 20 kHz sample rate (sample rate below 50 MHz needs to manually disable Roll mode).RBW = SampelRate / Nwhere N is point countAs you can see at 10 MHz sampel rate it has RBW = 10M / 14M = 0.71 Hz.But this is for rectangular window (or no window) function.If you're applying some window function before FFT, it affects RBW. So it depends on your window function choice, which depends on what is more critical for you - frequency resolution or amplitude resolution.With window function RBW = RBWrectangular* ENBWwindow.For Kaiser window it can be estimated as: ENBWKaiser≈ 1 + β​/2where β​ is β​ parameter for Kaiser window, which depends on your requirements for frequency or amplitude resolution.For example you can estimate Kaiser β parameter with this Octave function:Code:% Oppenheim, Schafer, "Discrete-Time Signal Processing", p.475-476.% A - attenuation in dBfunction beta = kaiserBeta(A)if (A >= 50)beta = 0.1102 * (A - 8.7);elseif (A < 50 && A > 21)beta = 0.5842 * (A - 21)^0.4 + 0.07886 * (A - 21);elsebeta = 0;endendPS: as you can see, there is no universal solution and it all depends on your requirements.

author: ebastler
author_rank: Super Contributor
timestamp: August 06, 2024, 07:20:47 pm
content: Quote from: xugmu on August 06, 2024, 06:20:20 pmYes, I figured that out. My point was that (a) you can and should correct your original post, and in particular its thread title. And (b) there was no need for the snide reply to joeqsmith. "Oops, sorry, I have corrected the typos" would have done nicely.

author: gf
author_rank: Super Contributor
timestamp: August 06, 2024, 07:25:35 pm
content: Quote from: xugmu on August 06, 2024, 06:17:46 pmNeither with FFT nor with a swept SA you'll look at the signal in the time domain. They provide a frequency domain view.

author: G0HZU
author_rank: Super Contributor
timestamp: August 06, 2024, 08:41:30 pm
content: I'm unsure what the intended application is, but see below for a screenshot of an old Agilent SA set to zero span in sweep mode. This effectively halts the sweep.I think this effectively uses a 1Hz (digital) RBW filter at the digital IF. The test signal being fed to the analyser at 451MHz has a regular step change in amplitude and the plot shows the response across a 50 second time frame. Despite the 1Hz RBW filter, it does a reasonable job of displaying the two level states.Note that unless your DUT and your test gear is locked to a common frequency reference, you may find the signal from the DUT will not be at the same frequency as the analyser. With a 1Hz RBW up at 451MHz with a zero span setting, this would require very accurate and stable frequency references in the DUT and the analyser.

author: G0HZU
author_rank: Super Contributor
timestamp: August 06, 2024, 09:12:06 pm
content: See below for an amplitude that varies as a triangle wave. Again, the analyser does a reasonable job of displaying what is happening in the time domain when set to zero span with a 1Hz RBW. The triangle shape isn't perfect, but it's fairly close to a triangle shape.

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 09:34:37 pm
content: Thanks for the pictures, an old Agilent analyzer with a 1Hz resolution must have been a fairly expensive device.The purpose of the thread was to find out the difference in the result between using zero span mode in sweep mode or in FFT mode.The 3000 series from Siglent announces the two modes in its user manual. The 800 series from Owon, afik, does not say anything, and I get the impression that when very low resolutions are used, it uses FFT mode.I have a TinySA (sweep) that also has zero span mode, the problem is that it is a device limited to a RBW of 200Hz.On the other hand, I have SDRs that obtain resolutions of 0.2 Hz but use FFT mode, the reduction of the bandwidth in this case I imagine will be achieved with digital filters.With demodulation software, I have reached approximately 400 Hz of bandwidth.Harmonics and spurious things aside, Afik a swept spectrum analyzer "should not need filters to limit the bandwidth" since its oscillator would be stopped at a frequency and the device would only let that frequency through.

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2024, 09:46:41 pm
content: Quote from: xugmu on August 06, 2024, 09:34:37 pmSSA3015X Plus will need your needs.

author: xugmu
author_rank: Contributor
timestamp: August 06, 2024, 09:57:35 pm
content: Quote from: tautech on August 06, 2024, 09:46:41 pmI've read quite a bit about the 3000 series. They would be the perfect devices to solve my question: does zero span mode work better in FFT mode or in sweep mode?That's why I'm asking here before buying, to know what I'm buying.

author: G0HZU
author_rank: Super Contributor
timestamp: August 06, 2024, 10:19:10 pm
content: I'm a bit confused by some of the things you are saying about zero span mode. It might be worth it for you to read Keysight's classic AN-150 application note as this will explain the pros and cons of FFT vs swept modes.I've never used the Siglent analysers, but I would be surprised if they offer a 1Hz digital RBW. This requires a very stable frequency reference and a very good LO synthesiser to make it worthwhile when in zero span mode. Lower cost analysers are going to be limited to maybe a 10Hz digital RBW when the analyser is used in swept or zero span mode.However, I'm not convinced you actually need a 1Hz digital RBW for zero span mode unless you are doing some fairly critical analysis of a slowly changing signal. Usually, 10Hz is adequate.

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2024, 10:23:30 pm
content: Quote from: G0HZU on August 06, 2024, 10:19:10 pmhttps://int.siglent.com/u_file/download/24_07_10/SSA3000X%20Plus_DataSheet_E04G.pdf

author: G0HZU
author_rank: Super Contributor
timestamp: August 06, 2024, 10:35:14 pm
content: Quote from: tautech on August 06, 2024, 10:23:30 pmI know that the Siglent can offer 1Hz RBW in FFT mode, but can it also offer it as a digital RBW in zero span?

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2024, 10:52:37 pm
content: Quote from: G0HZU on August 06, 2024, 10:35:14 pmNo just checked, 1 Hz RBW is only available down to a 100 Hz Span.Zero Span provides 30 Hz RBW.See screenshots.

author: coppercone2
author_rank: Super Contributor
timestamp: August 06, 2024, 11:04:54 pm
content: get a older unit if you don't want spam, the new oscilloscopes and whatnot will always be bringing up some pop up advertisement about features, complaining about the RTC, etc

author: G0HZU
author_rank: Super Contributor
timestamp: August 06, 2024, 11:07:59 pm
content: A lot depends on what you want (or expect) from your spectrum analyser. You might not need a high performance analyser for your task as I'm not convinced you 'need' a 1Hz digital RBW filter. The Siglent might be fine for what you want to do.However, if it turns out that you really do 'need' lots of spurious free dynamic range (SFDR) whilst using a narrow digital RBW then I think that you will find that the performance of the Siglent is going to be fairly poor compared to a proper lab spectrum analyser.The Siglent is going to lose out in both the performance of the downconverter (phase noise, IIP3 and DANL) and I'd also expect it to lose out even further if it can't provide a 1Hz (or maybe even a 10Hz) digital RBW filter when in swept or zero span mode.It looks like it is limited to a minimum of a 30Hz digital RBW. This is still quite reasonable, but it does limit the SFDR in swept mode if you want to look for tiny signals near very large ones for example.FFT mode can offer great speed advantages, but it comes at a price in terms of how it can limit the spurious free dynamic range. Swept mode is slower but it usually offers the best SFDR.Having said all that, most users won't need the extra performance and they may be perfectly happy with the Siglent analyser. They may not be aware of its limitations compared to a proper lab analyser but this probably doesn't matter.

author: G0HZU
author_rank: Super Contributor
timestamp: August 06, 2024, 11:48:10 pm
content: The other thing to be wary of with the cheaper analysers (like the Siglent) is that they are unlikely to offer much (if anything) in the way of analogue preselection ahead of the ADC when narrow spans are selected.A decent lab analyser will have a suite of adjustable preselectors ahead of the ADC and the bandwidth of the preselector is usually adjusted to be a bit wider than the bandwidth of the digital RBW filter (down to maybe 1kHz).This means the ADC is only exposed to a limited spectrum of signals when set to narrow spans or narrow RBW settings.I'm not sure what the Siglent does here, but I expect it might only have one (or maybe two) IF preselection bandwidths. This limits the spurious free dynamic range and also makes the analyser very prone to ADC overload if there is a very large signal maybe 100kHz or maybe even several MHz away.Many users will never realise this limitation because they probably don't ever explore the limits of the analyser during typical use. However, having a wide bandwidth digital IF can sometimes be a disadvantage if there is no means to reduce the bandwidth that the ADC is exposed to.

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2024, 12:10:00 am
content: If this all sounds a bit confusing, then consider the case of FFT vs swept for a span of 500kHz.In the swept case, the digital RBW could be 1kHz and there could also be a 3kHz wide preselector ahead of the ADC. So the ADC is only ever exposed to a spectrum 3kHz wide even though it is sweeping through a span of 500kHz. This protects the ADC from overload from large, nearby signals.In FFT mode, the analyser may well present the whole digital IF bandwidth to the ADC and this could be several MHz wide. So there is much more chance of ADC overload either from a single large signal or from the voltage summing of all the signals in the IF bandwidth.If the analyser does support an adjustable IF preselector, then in FFT mode, the preselector could be set to about 2MHz bandwidth or the analyser could select a narrower preselector and do several smaller (narrower) FFTs whilst stepping or hopping the LO each time. So the analyser could use a narrower preselector bandwidth and provide more protection of the ADC. This would sacrifice some of the speed advantage of the FFT mode though.I don't know if the Siglent can do this multiple FFT trick, but it would ideally need to have a suite of IF preselectors fitted in order to make it truly worthwhile.

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2024, 12:35:18 am
content: Note that for a 500kHz span in FFT mode, you can't choose a preselector that is also about 500kHz wide as there will be droop in the preselector band edges and this will spoil the flatness of the analyser across a 500kHz span. Usually, the preselector is selected to be several times wider than the span in FFT mode in order to preserve flatness.If multiple FFTs are available, the the preselector bandwidth could be reduced significantly and maybe ten or more smaller FFTs could be done whilst retuning the LO for each FFT. Building up the spectrum in chunks like this obviously takes longer than doing a single FFT but it means the analyser could auto adjust the gain to prevent overload for some of the smaller FFT chunks if they contain huge signals in their passband.

author: ftg
author_rank: Frequent Contributor
timestamp: August 07, 2024, 07:30:47 am
content: Yeah I have hit the dynamic range and phase noise limits of SSA3032X on occasion.And indeed it only offers 1Hz RBW in swept mode and 30Hz in Zero Span mode, which makes sense I guess, as 1Hz wide Zero Span would require rather good frequency stability on higher frequencies.What measurement requires 1Hz RBW on Zero Span?Especially on UHF and up?At least on my SSA3032X the VBW does go down to 1Hz, so for long term monitoring of some signals output amplitude the RBW can be wider and then the VBW setting can be used to filter out all quick variations.

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2024, 10:02:14 am
content: On narrow spans, the dynamic range of the Siglent analyser will be phase noise limited. So I guess one could conjure up scenarios where a large adjacent signal can spoil zero span mode (with a 30Hz digital RBW) through reciprocal mixing (noise).However, this is going to be a rare event.Overall, the difference between the Siglent analyser and a high end lab analyser is that the lab analyser is designed to be able to measure very small signals that are close to 'very' large signals. The Siglent analyser can't compete here. It can easily have 20 to 30dB less dynamic range compared to the lab analyser in some tests.My interpretation of the Siglent datasheet is that it can't offer a digital RBW less than 30Hz in swept mode. Therefore, this doesn't just impact zero span mode.

author: xugmu
author_rank: Contributor
timestamp: August 07, 2024, 02:02:30 pm
content: I deduce from your fantastic comments and images that it is not possible, not even with high-end analyzers, to get a stable "analog" RBW (sweep) in the 1Hz range. Therefore, everything that is done in that resolution range is done with digital technology (FFT) so it does not matter if, in that range, a manufacturer announces sweep or FFT.The target to analyze are pilot OFDM carriers with a bandwidth of approximately 1kHz where there is quite a difference (especially in the waterfall) between 1Hz and 10Hz.Getting a high SFDR or many preselector input filters is totally beyond my economic possibilities and, as you have said, buying a measurement device must depend on the needs.Best regards

author: hpw
author_rank: Frequent Contributor
timestamp: August 07, 2024, 03:50:52 pm
content: As I see, you will get any better results using a PN noise measurement system. Additional using an accurate down mixer as here posted. Not to forget the AM/AN spurs as looking PM/FM O;)The link here to the product, will show only single band.https://qsl.net/bg6khc/pn2060c_phase_noise_analyzer.htmMy measurements as you like to do, as simple using a 10MHz reference 1Hz@-120dBc and comparing the SSA30xx showed clearly the side PN/AM of the internal synthesizer.just my 2 centshp

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2024, 11:37:25 pm
content: Quote from: xugmu on August 07, 2024, 02:02:30 pmI'm still a bit unsure what you are trying to say here.If you go back over 30 years, almost all spectrum analysers had analogue RBW filters built using LC resonators or with crystals. These have poor shape factor and the sweep speed has to be really slow for an analogue 10Hz RBW filter. A 1Hz RBW filter would be extremely slow. I've used analysers with a 3Hz analogue RBW filter and the sweep time is painfully slow.In the 1990s, analysers started including digital RBW filter technology for the narrowest RBW filters. These offered much improved shape factor and faster sweep times for a given RBW.The digital RBW filter is generated in the digital domain rather than the analogue domain. It's possible to have a digital RBW down to about 1Hz BW on a decent lab analyser. A digital RBW filter is usually fixed at the IF frequency and it works in conjunction with a swept LO, just like the analogue RBW filters except the digital RBW filter is synthesised in the digital domain.This process doesn't involve FFTs. Digital RBW technology is different to the FFT method.Later analysers had more advanced ADC and DSP technology and this allowed fairly large FFTs to be run over a fairly wide bandwidth.In this case, the RBW is defined by the sample rate and the FFT size and the window function. This is very different to the way a digital RBW filter is generated in the digital domain.I have very little experience with OFDM stuff. You may be better off with a decent SDR or RTSA although I'm still not sure what you are trying to do.A digital RBW filter with 1Hz RBW will have a very slow response time to any changes in signal level and this can cause some confusion. For example, if the signal was instantly gated off, then the analyser might take several seconds to actually show that the signal has completely gone. Similar things (even more confusing?) will happen if you try and use FFT mode.So I'm not sure what you want to see if you select zero span with a 1Hz RBW. The system will be very sluggish to report any changes in signal level to the point where I'm not sure how much use it would be in your case of an OFDM signal.

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2024, 11:48:02 pm
content: I think I can generate OFDM signals here using an old Agilent signal generator and some Agilent SW. This generates 802.11x test signals for wireless LAN. Is this similar to what you are looking at?I've got various spectrum analysers here but rarely use them for looking at stuff like WLAN. I don't know much about WLAN. However, I could try and look for the same thing you want to see?

author: xugmu
author_rank: Contributor
timestamp: August 08, 2024, 11:11:36 am
content: It is logical that you do not understand my answer since my answer was wrong. I confused digital filter with fft.The only way I have right now to get close to 1 Hz resolution is with an SDR. In fact, I opened the thread with that intention, to know what I see on the screen using 1 Hz RBW using fft technology or scanning technology. The same could be applied to the topic of zero span, the doubt was between owon and siglent, owon only offers fft technology at those resolutions and siglent thus offers two options.Bringing the zero span to 1 Hz resolution has also been the result of my lack of awareness. In fact, seeing the images that have been posted in this thread, I have doubts about the use of a variable RBW when using the zero span and how that influences what is seen on the screen.The signals that interest me the most are DVB-T signals (COFDM), specifically the pilot carriers, which are simpler. They interest me in two ways: interference can be appreciated relatively well since they have quite a bit of "free space" and on the other hand, observing the pilots in the time domain (a simple sinusoidal signal) may perhaps provide information (an almost abandoned objective would be to obtain the impulse response (my television analyzers only reach 400us))Although I have demodulated signals complete with gnuradio I have been trying to do the same for some time with an individual data carrier with absolutely no success . The field of quadrature signals seems very interesting to me.

author: G0HZU
author_rank: Super Contributor
timestamp: August 08, 2024, 11:38:14 am
content: I don't know if this helps, but if we confine things to just the ancient analogue RBW filters and digital RBW filters (and we ignore FFT for now) then it's possible to predict the sweep time of the analyser for a given span setting and RBW.The sweep time goes down as the RBW is increased and there should typically be a RBW^2 relationship. So if you increased the RBW by a factor of ten from 100Hz to 1kHz then the sweep time would be 10*10 = 100 times faster.For an analogue RBW filter the sweep time in seconds is typically = Span/(0.33*RBW*RBW) where Span and RBW are in Hz.For a digital RBW filter the sweep time in seconds is typically = Span/(0.8*RBW*RBW) where Span and RBW are in Hz.This shows about a 2.5 times improvement in sweep time for a digital filter.However, if you want to explore very narrow RBW filters then the sweep time goes up very rapidly because of the RBW^2 term in the equation.A 1kHz Span and 10Hz RBW would have a sweep time of 12.5 seconds for a digital RBW filter. If you reduced the RBW to 1Hz the sweep time would go up to 1250 seconds. This is obviously impractical. If the span was reduced to just 100Hz then the sweep time would be 125 seconds and this is still really slow, but it is tolerable. But you only get to see a 100Hz span and not a 1kHz span.The modern solution for this slow sweep time is to select FFT mode.i.e. to improve on the 1250 second sweep time, a modern analyser can digitise the whole IF spectrum using a suitable sample rate and do a large FFT on the sampled data.The display update rate would hugely improve and if you wanted a 1kHz span and a 1Hz RBW and you selected a typical window function (eg Blackmann-Harris window), the screen would update about once every two seconds with the FFT mode selected.This is a massive improvement. However, the FFT mode comes with it's own compromises in terms of spurious free dynamic range.Most analysers will be set up to automatically use digital RBW filters above a certain span setting and then swap across to FFT on narrower spans. This give the best spurious free dynamic range in wider spans (using digital RBW) and it gives the best sweep time on narrower spans (using FFT mode) where the sweep time of a digital RBW filter would be too slow.

author: G0HZU
author_rank: Super Contributor
timestamp: August 08, 2024, 12:02:08 pm
content: Note that the really ancient HP 8568B and HP 8566B spectrum analysers could also support FFT analysis but only on the post detection data. I think they supported several window functions including flat top and hanning. It allowed modulation analysis at frequencies 'very' close to the carrier.However, hardly anybody will have ever used this FFT feature in these ancient analysers.

author: G0HZU
author_rank: Super Contributor
timestamp: August 08, 2024, 12:15:38 pm
content: Note that I don't recommend that you buy an HP 8568B or HP 8566B for your task. These analysers are too big and heavy and they produce lots of fan noise. They have old school analogue RBW filters and they don't support digital RBW.The FFT mode they support is impressive for the age of the instrument, but the FFT performance will be fairly grim by modern standards.You are probably best off using an SDR although the Siglent analyser might be OK for some of the things you want to do.