DSO's FFT vs Spectrum Analyzer? - Page 1
Forum section: electronics / testgear

author: ice595
author_rank: Regular Contributor
timestamp: January 29, 2014, 02:49:34 am
content: What is the main difference between the FFT math function in a DSO and a real spectrum analyzer?both could measure voltage vs frequency just fineIs it the frequency response of a real spectrum analyzer that makes it so expensive?I got the Rigol DS1102E DSO and it seems to breakdown a square wave into its fundamental frequency components quite nicely.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 29, 2014, 03:55:06 am
content: One is a toy,& the other a Test Instrument!Grumpy old RF person here!

author: digsys
author_rank: Unknown
timestamp: January 29, 2014, 04:10:30 am
content: Quote from: vk6zgoLOL. I'm interested in knowing opinions on the differences as well. Does it matter if you compare -a cheap SA to an expensive DSO <> cheap SA to a low end "name brand" DSO? Can you elaborate on "Toy"? :-)

author: ice595
author_rank: Regular Contributor
timestamp: January 29, 2014, 05:17:40 am
content: Is input sensitivity another reason why SA is real expensive?a lot of DSO I've seen only have 8bit DACs and maybe 1 or 2 USB DSO have 12bit DACsalso, is there such thing as a all-analog spectrum analyzer?

author: rfbroadband
author_rank: Unknown
timestamp: January 29, 2014, 05:44:20 am
content: To answer this in detail one would have to cover many topics. Here are a few:A scope has a broadband input that feeds into a variable gain amplifier which goes into a high speed ADC. Depending on the analog BW one wants to achieve the ADC needs to oversample by a certain factor (lets say 5). So assume you want 1GHz analog bandwidth, your ADC needs to sample a 5Gsps. At 5Gsps you are limited in terms of the number of bits you can support. Assume you have an 8bit ADC at 5Gsps and you get an effective number of bits (ENOB) of 7. This means the theoretical SNR you can achieve is 6 * 7bits + ~2dB = 58dB. This is the lower limit of course, because if use use a 5Gsps scope to measure a 100MHz signal you are effectively oversampling a lot and you get what is called oversampling gain. You can further improve the SNR or dynamic range by 'averaging' and lower the noise floor even further.A spectrum analyzer has multiple RF conversion stages and several IF filters before the signal is fed to the internal ADC. Since the SA can down convert the RF signal internally with highly linear mixers to a lower IF frequency (< 150MHz) you can measure a high freq signal by sampling it at a lower IF freq using an ADC that has a lot higher ENOB (lets say > 12 bits) so you inherently end up with a lot more dynamic range.The other key difference to a scope is that the SA only has a broadband input (up to the first mixer), but during the down conversion process the signal is filtered several times so one can actually do measurements of very small signals in the presence of large signals. If the large signal is not so large that it compress the first mixer in the SA, the large signal will be filtered by several IF filters and you can now increase the sensitivity for better measurements focusing on the small signal. Imagine you want to measure small signals (few mV) in the presence of a 2Vpp signal with one channel on a scope...you can't scale the y-axis because you will compress the ADC of the scope.In order visualize what I just described I fed a 500MHz sine wave (CW) signal at 0dBm into a high end SA and a high end scope. Both screens are adjusted for a 0dBm reference level. One will see immediately that the SA has a much lower noise floor than the scope.it should be noted that the scope that was used provides an excellent dynamic range, most scopes on the market come nowhere near a noise floor of -90dBm, but I hope that attached screen shots will help to understand some of the differences.So I just scratched the surface on this topic, but I hope you find these basic explanations useful.My last comment: Most FFTs on scopes are an absolute pain to use. I wish scope manufactures would implement better GUIs for FFTs on scopes, because even with the limited dynamic range of a scope there are lots of things that can be learned from doing measurements in the freq. domain if you don't have a spectrum analyzer you can use.

author: ivaylo
author_rank: Frequent Contributor
timestamp: January 29, 2014, 06:12:38 am
content: QuoteYes there was. Instruments like these would let you plot spectrum:- the frequency analyzers of yesteyear -http://www.radiomuseum.org/r/bruelkjae_frequency_analyzer_2120.html- and a bit more mechanical but the same thing really -http://www.museu-tecnologia.com.br/wordpress/?p=1422

author: vk6zgo
author_rank: Super Contributor
timestamp: January 29, 2014, 06:24:32 am
content: Most of the older Spectrum Analysers are analog.though many may have a uProcessor to do various "housekeeping" functions.That said,digital SAs have been around for quite a few years.The only problem I have noticed with the digital ones was with an IFR unit a couple of years ago.We had a PLL which would "let go" & revert to a default frequency about 30MHz higher.This happened quite fast,& the IFR saw it as a bunch of carriers,leading us to chase a non-existent fault.We bought an "El Cheapo" SA,(which was an analog type) to constantly monitor 4 Transmitters (in an ISMapplication)& free up the IFR for other work.The analog SA showed the PLL frequency change clearly.Apart from that,& the fact that to transfer screenshots to a PC,you needed to use a floppy disc,the IFR was a really good unit.The main philosophical difference between the FFT function on a DSO & a Spectrum Analyser,is that the latter works directly in the frequency domain--classic analog SAs are a narrowband radio receiver tuned automatically across the band of interest.Practical differences are:Frequency range---a cheap Chinese analog will probably operate from a few 100s of kHz to around 1GHzas compared to 100MHz for a DS1102E."Cheap" is relative here---this type of SA will probably cost 3-4 times what you paid for your Rigol DSO.Dynamic Range:-the "noise floor" of these SAs is probably between-60 & -70dBm.(much better units -90dBm)Maximum input is often around +20dBm (better units, +30dBm).As is obvious from the above,the display is by default logarithmic,& scaled in dBm.Max input & 'noise floor" can co-exist on the same display.The noise floor is higher in DSOs,& they are normally linearly scaled,so cannot show the relationship in dBbetween two signals.What is easily visible on a SA screen can be invisible on a linear scale DSO screen.As an example: +20dBm is 100,000,000 time as large as -60dBm in power,--to be fair,a lot less in voltage as normally displayed by a DSO,atonly10,000 times!Cheapie SA's up until recently were analog,but I think the current generation are digital.

author: Jane
author_rank: Frequent Contributor
timestamp: January 29, 2014, 06:33:19 am
content: And how about USB logic analyzers? E.g.herehttp://www.acute.com.tw/product/la.phpCan anyone compare them with "normal" analyzers?Are those also "toys"?

author: vk6zgo
author_rank: Super Contributor
timestamp: January 29, 2014, 06:43:39 am
content: Not my Department!I've only ever used a big clunky old HP we hired from "Tech-Rentals"--we had a Tektronix,but it had been faulty for ages,& attempts to revive it were unsuccessful.The HP did the job OK.Addressing your question though,there seems to be a bit of disagreement on this forum about the merits& drawbacks of "Stand alone" LAs,those incorporated in DSOs,& USB types.I'm sure you will get a lot of opinions

author: ivaylo
author_rank: Frequent Contributor
timestamp: January 29, 2014, 06:53:26 am
content: QuoteDifferent animal. It's "the other" analyzer.."...

author: vk6zgo
author_rank: Super Contributor
timestamp: January 29, 2014, 06:56:01 am
content: Quote from: ivaylo on January 29, 2014, 06:12:38 amYeah,the first one is a Selective Millivoltmeter or Microvoltmeter.We used something very similar to calibrate Audiometers when I worked at the hearing aid place.We fitted the Audiometer headphones to a "pretend ear",then fed its output to the Selective Millivoltmeter,so we could check & calibrate levels,distortion,etc.Talking about SAs,though--they were analog for many years--those old classic HP ones you see on Ebay are analog.

author: Mark_O
author_rank: Frequent Contributor
timestamp: January 29, 2014, 05:30:43 pm
content: Quote from: Jane on January 29, 2014, 06:33:19 amAnd how about flying pigs with monkey wrenches? Can we discuss them in this thread too? Or should I start a different thread for that, and not pollute this one?

author: bingo600
author_rank: Super Contributor
timestamp: January 29, 2014, 07:18:57 pm
content: Quote from: digsys on January 29, 2014, 04:10:30 amIMHO DSO's are cheap and SA's are expensive (Hobbyist).The Rigol SA costs 2 x the DS2072.And i could get almost 4 x DS2072 for the same price as my used HP 8594EI have a DS1102E , as the SA used my "toys budget" for a while.But now that one can only get 2072A's , and no "hack" is available.I'm considering to get a HP 3xxx Scope instead , when time comes.I might try a 50Mhz sine into the 1102E (can't do 500Mhz) and the 8594E and later the E4406A (a nice FFT Box)But i might have to get a Windows machine , and use the KE5FX Gpib tool :http://www.ke5fx.com/gpib/readme.htmI'm 95% linux based and haven't gotten into to capture the screen of the 8594E or E4406 there yet./Bingo

author: 1design
author_rank: Regular Contributor
timestamp: January 29, 2014, 09:08:40 pm
content: @rfbroadband: That is some nice SA, 10ms sweep time with those settings is quite noticeable, what are you using there?BR,Miha

author: rfbroadband
author_rank: Unknown
timestamp: January 29, 2014, 09:13:51 pm
content: Miha,R&S FSWOliver

author: Mr Simpleton
author_rank: Unknown
timestamp: January 29, 2014, 09:34:59 pm
content: Quote from: rfbroadband on January 29, 2014, 09:13:51 pmInteresting comparason.. as the FSW is a Real Time Spectrum Analyzer... showing only 5 MHz means you do not sweep the front end but only sample over a frequency band almost similar to a DSO, but with better analogue front end and of course better signal processing, and the DSO doesn't have the frequency translation to a lower IF for better A/D perofrmance! The trend seems to be more to use a SA to show the signal in both time and frequency domain rather than have a good frequency domain in a DSO

author: PA4TIM
author_rank: Super Contributor
timestamp: January 29, 2014, 10:34:00 pm
content: https://www.eevblog.com/forum/reviews/fft-high-precision-mode-with-hameg-hmo-vs-rigol-ds2000-scopes/msg139177/#msg139177See here some pictures from a 350z Hameg DSO in FFT and an old analog HP-8558 both fed with the same signals.(I hope the guy who asked if there have been analog SA's is just kidding)I have hybrids too, a Tek 2710 and a Tek 2712I have a selective voltmeter too, a sort of manual controlled spectrum analyserThis is the picture of a very early valve equiped SA, a real boat ancore from 1961. 30 MHz and 150 kilo

author: MarkL
author_rank: Unknown
timestamp: January 29, 2014, 11:52:57 pm
content: Quote from: bingo600 on January 29, 2014, 07:18:57 pmTake a look at Ed Nisley's 859x screen dumping methods for linux:http://softsolder.com/2011/12/21/hp8591-spectrum-analyzer-screen-dump-sizes/He talks about doing it off the serial port so I guess he has option 043. I don't have that, but the HPGL-formatted screen dump can be retrieved via the HPIB port just as easily (if not more so). The trick I needed was to increase the linux GPIB timeout to at least 10 seconds to allow the data transfer to complete.

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2014, 12:25:22 am
content: We can all see that the modern trend is to move away from old school spectrum analysers towards the 'signal analyser' with digital IFs boasting huge IF bandwidths.Once you have a digital IF you get the advantage of much better RBW filter shape factors and also the log display will be a lot more accurate and the overall measurement uncertainty of the instrument improves a lot.I have several old school (but hi end) spectrum analysers from the late 1980s and they are still pretty good by today's standards. But, for certain tests my creaky old Tek TDS2012 scope can prove to be more powerful when put into FFT mode. This is because it can update the display faster on narrow spans when looking at audio stuff. Also I can exploit the aliasing issue with it to get it to 'downconvert' narrowband RF signals so I can look at them with a faster refresh rate than I can see on my big old HP and Advantest analysers.Obviously, for the vast majority of RF testing the TDS2012 (in FFT mode) is pretty hopeless compared to a basic spectrum analyser let alone something decent but it can still prove very useful

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: January 30, 2014, 12:28:20 am
content: Quote from: vk6zgo on January 29, 2014, 03:55:06 amOh, that's just cool! lol

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2014, 12:44:38 am
content: QuoteOne of the main advantages of a decent spectrum analyser is that it will not suffer from much in the way of internal and external spurious signals as long as it is used sensibly.But a DSO can suffer from significant alias issues that can litter the display with false information.Also, a decent analyser will have a very good input match (50 ohm) which will minimise measurement uncertainty and, as has already been mentioned, it can measure much smaller signals whilst maintaining low internal spurious (false and misleading) signals.One thing that isn't so clear on a spectrum analyser is that it isn't always obvious that you are overloading it and degrading the accuracy of any distortion measurements. But with a DSO it is usually obvious that you have overloaded it on that range setting. So it requires a fair bit of user experience and knowledge about the analysers limitations to make sure you get the best from it.

author: Smokey
author_rank: Super Contributor
timestamp: January 30, 2014, 01:21:21 am
content: Didn't Dave pretty much explain this in the Tek MDO review?

author: bingo600
author_rank: Super Contributor
timestamp: January 30, 2014, 06:14:07 am
content: Quote from: MarkL on January 29, 2014, 11:52:57 pmThanx MarkLooks absolutely interesting/Bingo

author: 1design
author_rank: Regular Contributor
timestamp: February 03, 2014, 02:41:45 pm
content: Quote from: rfbroadband on January 29, 2014, 09:13:51 pmI have just sent a quotation request as some nice features like 500MHz demod. BW and BB Analog input have been added lately! Very nice instrument, R&S pushed Agilent back a bit with this one.BR,Miha

author: rfbroadband
author_rank: Unknown
timestamp: February 03, 2014, 03:32:49 pm
content: last year I spent one entire week comparing the FSW against the PXA. I had access to both Agilent and R&S apps engineers which confirmed my tests and measurement results. I then sent the comparison presentation to both companies...it was a tragic wake up call for Agilent, realizing how much they are behind the FSW in performance. If you want a copy of the presentation send me a PM.