Cheap and Small Dynamic Signal Analyzer? - Page 1
Forum section: electronics / testgear

author: gigavolt
author_rank: Contributor
timestamp: August 15, 2020, 01:45:14 pm
content: I was watching Dave's new video on magnetic field shielding and I realized: why haven't I seen any cheap hobbyist grade DSAs on the market? Up at high frequencies there's the nanoVNA which is dirt cheap and fits in my pocket. I'd expect that a DSA which is basically just some nice ADCs with the right software would be a prime candidate for creating something in a similar vein.Right now I'm actually using an audio interface and some custom software for rooting out noise, but it all feels really hacked together and having something that you just plug in and take a measurement with would be amazing.

author: Vgkid
author_rank: Super Contributor
timestamp: August 15, 2020, 03:40:08 pm
content: Maybe a quantasylum qa401.

author: gigavolt
author_rank: Contributor
timestamp: August 15, 2020, 07:00:53 pm
content: Wow, that's a cool piece of kit. I don't think I've seen audio gear that goes all the way from 2 Hz to 70 kHz before.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 16, 2020, 02:03:00 am
content: Surprisingly there doesn't seem to be a new instrument-grade low-frequency spectrum analyzer. You can pay Standard Research Systems $10k for a brand-new CRT display device similar to what Dave was using, or get them used. Audio hardware is nice and inexpensive, but doesn't have the full functionality, for example measurements below 1 Hz.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2020, 08:29:16 am
content: Dynamic signal analysis and low frequency network analysis are pretty specialized and generally have no use for portable instruments.Some 384 kSample/second PCI audio cards support input and output bandwidth to 96 or even 192 KHz but low frequency performance is questionable.

author: gigavolt
author_rank: Contributor
timestamp: August 16, 2020, 12:03:56 pm
content: Yeah, and at least for my use cases it's really the low frequency limitation that ruins it. Being stuck at 20 Hz is no fun.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 16, 2020, 09:28:32 pm
content: Some audio cards can be modded for better LF response, even DC.

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2020, 02:03:04 am
content: Quote from: SilverSolder on August 16, 2020, 09:28:32 pmYou can, but there is a good chance they will suffer from high low frequency noise, although that may be correctable as well.An instrument like the Quantasylum QA401 mentioned by Vgkid would be my first choice. Cleverscope and Picotech make some USB DSOs which might be suitable.Do not forget about ground loops which can make measurements difficult. The Cleverscope mentioned above has isolated inputs.

author: 2N3055
author_rank: Super Contributor
timestamp: August 17, 2020, 06:02:55 am
content: Spectrum analyser part is covered by Hi res Picoscopes very well..There is a third party FRA app for them, but is limited to built in siggens.Picoscope 4262 with 5 MHz bandwidth and 8.5 µV RMS noise is very usefull.Also they have great API, so they are directly usable from LabVIEW,MATLAB, or your code for any custom analysys.Also, Digilent Analog Discovery is very interesting...

author: Berni
author_rank: Super Contributor
timestamp: August 17, 2020, 06:46:53 am
content: You can just buy a high quality USB sound card and use it as one.Today its not hard to find 192KHz 24bit audio ADCs and DACs and they have some really impressive noise floors (Once run trough a FFT you get around -150dB, so you can see nanovolt range signals). This gets you a usable bandwidth of around 1Hz to 90KHz. Harmonic distortion is also excellent since its for audio.Removing coupling capacitors can get you down to actual DC too, but don't try to use it as a voltmeter to measure DC voltage, because the audio chips are not designed with DC performance in mind and so are inaccurate and drifty in that regard.

author: KE5FX
author_rank: Super Contributor
timestamp: August 17, 2020, 07:22:50 am
content: Note that many "192 kHz" sound cards still have a reconstruction filter cutoff in the 20 kHz vicinity. All of mine do, as I just found out the other day when I wanted to use it to capture a 100 kHz signal.

author: Berni
author_rank: Super Contributor
timestamp: August 17, 2020, 08:11:44 am
content: Quote from: KE5FX on August 17, 2020, 07:22:50 amYou can just swap out the capacitors in that filter to bring its response up. The audio ADC chips themselves do have a flat response all the way up to nyquist.Also a good idea its a proper sound card that supports USB Async audio class, this removes a sample rate conversion step that can mess with the audio data.

author: exe
author_rank: Unknown
timestamp: August 17, 2020, 11:43:14 am
content: What are recommended sounds cards for the job?

author: electrolust
author_rank: Unknown
timestamp: August 17, 2020, 09:39:18 pm
content: Quote from: gigavolt on August 15, 2020, 01:45:14 pmMaybe Virtins "Multi-Instrument" fits the bill for you? It works with any sound card with ASIO, which includes the QA401. It works with other interfaces as well.

author: electrolust
author_rank: Unknown
timestamp: August 17, 2020, 09:56:50 pm
content: Quote from: maxwell3e10 on August 16, 2020, 02:03:00 ameh?Signal Hound USB-SA44B (1Hz)Bode 100 (1Hz)Tek RSA7100B (1Hz)Keysight E5061B (5 Hz)If you need <1Hz there are various lock-in amplifier based designs, commercially "instrument-grade" available. SRS, Zurich seem the 2 most prominent.Also there are commercial ADC / sound card designs, "Multi Instrument" (Virtins) which interfaces with a large selection of hardware, and PicoScope 4262 are ones I'm aware of. Oh, and of course QuantAsylum, mentioned earlier, But it's really focused on audio.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 17, 2020, 10:32:44 pm
content: Quote from: electrolust on August 17, 2020, 09:56:50 pmI wouldn't get an instrument primarily designed for RF, it likely will not have a good performance down at 1 Hz, not to mention the cost. SRS does make a nice low-frequency spectrum analyzer with 5 nV/Sqrt(Hz) noise level down to a few Hz, but it's a dinosaur.

author: Berni
author_rank: Super Contributor
timestamp: August 18, 2020, 05:39:27 am
content: Quote from: electrolust on August 17, 2020, 09:56:50 pmYou typically wouldn't want to use a spectrum analyzer for this. The noise floor down there might be too unpredictable and you might be lacking the dynamic range of a proper DSA.But there are spectrum analyzers that go down to actual DC:https://www.keysight.com/en/pd-1000002367%3Aepsg%3Apro-pn-89410A/vector-signal-analyzer-with-w-cdma-capability-dc-to-10-mhz?cc=SI&lc=engI got scored once of these cheep and it is a pretty capable DSA like instrument except it goes DC to 10 MHz. Ignore the W-CDMA part (It is capable of analyzing digital I/Q data from a separate giant 3GHz RF downconverter box that optionally goes under it) it does the usual DSA stuff like measuring the noise floor of opamps or distortion tests. Also includes a signal generator to sweep the phase and frequency response of your DUT.Quote from: exe on August 17, 2020, 11:43:14 amI don't know what sound cards to recommend because i ended up building my own that is designed for the job and includes galvanic isolation from USB. It was mainly used to measure noise and THD. Got down into something like 0.0004% THD and down into -150dB of noise.But if you want something off the shelf id say the QuantAsylum QA401 is a pretty good choice. Its not just for audio.

author: KE5FX
author_rank: Super Contributor
timestamp: August 18, 2020, 06:02:38 am
content: +1 to that. The 89410A / 89441A is one of the most underrated boxes around. Unlike so many other HP DSAs, it 'drives' like it was designed by the 8568/8566 team, even though it was created in a completely different era by a completely different engineering group.Beautiful piece of hardware. Shame they never published a CLIP for it.

author: avlijas.sladjan
author_rank: Contributor
timestamp: August 18, 2020, 06:54:10 am
content: https://www.picotech.com/oscilloscope/4262/picoscope-4262-overviewIt is 16bit 5MHz oscilloscope but for low frequency can oversample to 20bit.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 18, 2020, 08:06:25 am
content: Quote from: KE5FX on August 18, 2020, 06:02:38 amLooks promising and maybe a little cheaper than a used SR780, plus a color CRT, that's progress!

author: Berni
author_rank: Super Contributor
timestamp: August 18, 2020, 08:15:14 am
content: Quote from: maxwell3e10 on August 18, 2020, 08:06:25 amNot only that. It even supports LAN connectivity (But you will need some rather ancient networking gear to make that work)

author: electrolust
author_rank: Unknown
timestamp: August 18, 2020, 05:05:05 pm
content: Quote from: Berni on August 18, 2020, 05:39:27 amsure, i mean if you need a local gravity source.the question i was responding to was why aren't therenew,spectrum analzyers, at LF. I agree, the performance of RF devices gets poor at the lower frequencies. But eg the picoscope 4262 is an FFT device, and the Bode 100 I think has good performance? I haven't studied them so not sure if I'm just speaking nonsense.The LF option on the E5061B does seem designed for LF LCR measurement, I guess it can replace a second device you would otherwise need to have, and isn't suitable for signal analysis in general.I did just miss out on an 89410A on ebay. Most of them seem priced around $1000 and this one was an actual auction, not buy it now. it went for $150 I think. Maybe that was you that scored this great deal. There are no recent sales though, so at $1000 they aren't exactly flying off the shelves. A deal could probably be struck.

author: Berni
author_rank: Super Contributor
timestamp: August 18, 2020, 05:36:30 pm
content: Yeah i got my 89410A from a cheap biding where nobody seamed interested in it, forgot the figure but i think it was in the range of possibly being more expensive to ship than it was to actually buy. The overly optimistic sellers that want 1000$ for it are probably just seing others listing it for so much so they do it too. I don't think it is worth 1 grand, it is a neat instrument with impressive performance but its far from world leading performance these days and it is indeed very big, very heavy and consumes a surprisingly large amount of power.I'm guessing that these sort of instruments have faded in to obscurity due to modern digital scopes with FFT. Sure the scopes are still behind in terms of performance, but if you are clever with external low noise amplifiers you can still make them do most jobs. So i guess similar reason why high speed equivalent sampling scopes have gone forgotten, the regular real time digital scopes simply got good enough to replace them.Still someone could make a modern miniaturized version of a DSA using modern ADC/DAC chips and a powerful MCU, much like the NanoVNA is a miniaturized but still perfectly usable network analyzer (really it is surprisingly good for the size and price):https://nanorfe.com/nanovna-v2.html

author: electrolust
author_rank: Unknown
timestamp: August 18, 2020, 07:05:40 pm
content: Quote from: Berni on August 18, 2020, 05:36:30 pmIsn't "Multi-Instrument" that? and PicoScope 4262? Or do you (or OP) specifically mean self-contained like NanoVNA.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 18, 2020, 07:26:59 pm
content: I'd want a self-contained instrument, not USB, with a DC-1 MHz frequency range, a SAR ADC and 100dB SNR.

author: electrolust
author_rank: Unknown
timestamp: August 19, 2020, 01:25:17 am
content: NanoDSA!

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 19, 2020, 03:41:34 am
content: AD4003 ADC and OPA828 input amplifier. Anyone has better suggestions?

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2020, 05:15:48 am
content: Sounds like a pretty good fit alright. Perhaps also add a DAC to also let it do frequency response or distortion measurement.Pair it up with a ST MCU with hardware floating point to do the FFT. The more gruntry ones are fast enough to do live real time FFT with no gaps at a few MSPS. Slap a LCD on the front and you got a NanoDSA.

author: electrolust
author_rank: Unknown
timestamp: August 19, 2020, 04:01:40 pm
content: always best to start from requirements. THEN decide the components. The hard part is going to be the software.

author: Berni
author_rank: Super Contributor
timestamp: August 19, 2020, 05:41:11 pm
content: Well if you are making something similar to the NanoVNA then the number one requirement is cost.Hence you just pick the best components that the BOM budget will allow and squeeze the best specs possible out of those. But yeah most of the work here is indeed the software. Its not difficult to do but a lot of work to give it all the useful features of a DSA.

author: gigavolt
author_rank: Contributor
timestamp: August 19, 2020, 09:54:05 pm
content: Quote from: electrolust on August 19, 2020, 01:25:17 ammy thoughts exactly. I'm surprised nobody has tried to do it yet, but I suppose that the combination of low frequency and low noise makes it difficult.

author: Berni
author_rank: Super Contributor
timestamp: August 20, 2020, 05:11:17 am
content: Quote from: gigavolt on August 19, 2020, 09:54:05 pmGuess more of a problem is that very few people actually want one. They are perfectly happy with there existing Rigol scope.The low frequency and noise is not that much of a problem, you just pick a really nice opamp for the AFE. More of a difficulty is getting a spurious free response over the whole range. The noise floor is so low that even the tinyest of signals show up as a spike. This means a great deal of care must be taken in the shielding and PCB design to avoid any power supply noise or digital signals from getting in there and showing up. For example in my diy sound card design i had a LDO become just slightly unstable, not enough to cause obvious problems or a huge sine wave on the output when when poked with a scope, but it was singing enough to produce a small spike in the FFT, was quite a hunt to find where it was coming from.

author: KE5FX
author_rank: Super Contributor
timestamp: August 20, 2020, 06:39:51 am
content: Quote from: Berni on August 20, 2020, 05:11:17 amLet me guess, the part number contained the digits "1117"?

author: Berni
author_rank: Super Contributor
timestamp: August 20, 2020, 07:23:01 am
content: Quote from: KE5FX on August 20, 2020, 06:39:51 amForgot what it was exactly but it was some fancy pants high PSRR LDO whos job was to prevent crap from getting into the sensitive front end. So yeah... you had ONE job.As for 1117.. yeah i don't even need a scope anymore to tell when one of those damn things becomes a oscillator. Last time i started wondering why my board is making a whistling sound even when there are no switching regulators on it. Turns out it was oscillating so wildly that the ceramic capacitors around it started signing loud enough to be noticed even in a not so quiet room.

author: 2N3055
author_rank: Super Contributor
timestamp: August 20, 2020, 08:24:07 am
content: Quote from: Berni on August 20, 2020, 07:23:01 am"The singing regulator", new Broadway show..... LOL...Yep, I avoid them too...

author: gigavolt
author_rank: Contributor
timestamp: August 20, 2020, 03:26:56 pm
content: Quote from: Berni on August 20, 2020, 05:11:17 amThat too, because ultimately the reason I would want it is for better noise performance/higher dynamic range, but in a much smaller bandwidth (<100 kHz) than my scope. I'm guessing most hobbyists can "get by" with 8 bits.

author: Frex
author_rank: Regular Contributor
timestamp: August 20, 2020, 04:42:16 pm
content: Hello,Just for information, I currently design such type a device.It's a full open source 2 channels analyzer that use the LT2380-24,a 24 bits 2MSPS high performance ultra low THD ADC.It's not really a very low cost device and the full project is not yet finished.(I'm writing the FPGA firmware for now).Anyway, maybe that can interest some people.I publish the project advancement on a DIYaudio threadHEREThe design has been firstly checked with tweaked EVM boards few years ago,all details and history can be read on this other threadHERE.Regards.FRex

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 20, 2020, 07:30:00 pm
content: This is a cool project! I guess for a general purpose device one might make a more versatile input stage, with several different gain settings and a good single-ended noise level.

author: KE5FX
author_rank: Super Contributor
timestamp: August 20, 2020, 09:25:01 pm
content: Very nice. I was about to say that any effort along these lines needs to be multichannel. I'm short on free time at the moment but will bookmark the diyaudio thread(s) for later perusal.

author: trampas
author_rank: Regular Contributor
timestamp: October 26, 2022, 12:21:39 am
content: I was looking for something like the DSA that can measure amplifier noise floor and was wondering what options are out there?

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 26, 2022, 01:18:49 am
content: Quote from: trampas on October 26, 2022, 12:21:39 amI was thinking the same. Couldn't find anything I like in the market, so I decided to roll my own. I ordered a AD4630-24 eval board (back ordered till late Nov unfortunately) to pair it with a beefy Zynq ultrascale dev board I already have, so the hardware should be plug n play.It will take the AD4630-24 32.768ms to collect 65536 samples at 2 MSPS. Vivado estimates the zynq chip can do 2 channel, 30 bit input, 65536 bin FFT in roughly 1.5ms, so it should be no problem at all to have 32768 lines FFT resolution and 1MHz real time bandwidth. The AD4630-24 has 105.7 dB typical dynamic range as stated in the datesheet.For context, the SRS785 cost 14k USD, can only do 800 line FFT, 102.4 kHz real time bandwidth, 90 dB typical FFT dynamic range. The zynq dev board I have cost around 1k USD (massively overkill for the job), and the AD4630-24 eval board is 200 USD. All in all, the cobbled together solution should beat the pants off the SRS785.Of course, talk is cheap, and I haven't written a single line of code for the project yet, so don't have your hopes set high for now

author: Berni
author_rank: Super Contributor
timestamp: October 26, 2022, 05:38:16 am
content: Quote from: trampas on October 26, 2022, 12:21:39 amFor just noise measurement you can get away with a preamplifier going into an oscilloscope. Mainly because you don't need a lot of dynamic range for this, just high enugh sensitivity.There are 1000x gain preamplifiers out there from Stanford, but you can just as easily build your own using low noise opamps.As for measuring the noise of an opamp itself you just build a high gain amplifier circuit out of the opamp you want to test and it will amplify up its own noise to a point where it is easily measurable.Quote from: TopQuark on October 26, 2022, 01:18:49 amThat ADC does look like it could make a nice "NanoDSA" when paired up with a fast MCU with hardware floating point. Don't think you quite need the power of an FPGA to do FFTs that fast.Tho the chip is out of stock everywhere as per chip shortage apocalypse.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 26, 2022, 06:29:27 am
content: QuoteYea you can probably get away with a fast MCU like a STM32H7 or something similar. Though once you add in windowing, digital anti aliasing filtering, network analysis, averaging, digital mixing etc. I think it will be a pain to maintain real time analysis capability with a MCU. Perhaps a happy medium would be a cheap Chinese FPGA (e.g. the ones in sipeed tang boards) paired with a decent MCU.To be honest, the main motivation for wanting to use a overkill zynq ultrascale dev board is because the ADC eval board has a pain in the butt FMC connector, and the FPGA dev board just so happens to support that connector.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: October 26, 2022, 06:40:34 am
content: ESI Juli@—www.esi-audio.comSound Blaster X-Fi—www.creative.comEMU 1212, 0202, 0404—www.emu.comLynx L2, L22—www.lynxstudio.comAsus Xonar Essence—www.asus.comM-Audio Audiophile 192—www.m-audio.com(The one I use, no longer available. Note: is PCI,not PCIe)These have no reconstruction filters which stop an 20kHz regardless of sample rate.Some may only be available used....

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 26, 2022, 10:40:40 pm
content: Quote from: maxwell3e10 on August 19, 2020, 03:41:34 amAnother option would be AD7768-1, seems purpose made for the job, in stock and not that expensive. I might get a few to play around with.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 26, 2022, 10:46:39 pm
content: Quote from: Berni on August 20, 2020, 05:11:17 amI do have a need for a DSA. The question is more about if I will do it as a one off or am I trying to build a "NanoDSA".If I am just building one for myself, I'd just wait for the AD4630-24 eval board to ship, plug it into my fpga dev board, write just enough code to have the project do what I want to do, and be done with it.On the other hand, if I am building a "NanoDSA", I'd go with a couple of AD7768-1, or even a stereo audio ADC, pair it with a STM32 (or clone, even), and build something down to a cost.I think I'll build some form of a DSA, because I genuinely have a need for one, just not sure if I have the will and motivation to do a "NanoDSA" project.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: October 27, 2022, 12:30:21 am
content: If you think of making it as a project, I would go for at least 1-2 MHz sample rate. This would put it clearly above the sampling rate of audio products, which otherwise are hard to compete with in terms of hardware performance. Software is another question. It would be useful to take input from an audio interface (ADC+DAC), which often have a USB connection and standard drivers, and implement all software functionality of an SR785.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2022, 01:28:23 am
content: Quote from: gigavolt on August 20, 2020, 03:26:56 pmExactly, and it is too specialized of an instrument. For most network analysis, 8 bits is plenty and a DSO with signal generator only needs appropriate firmware. There are some relatively low cost products intended for the audio market that operate up to 192 or 384 kS/s like theQA403. At higher cost,Cleverscopeoperates at much higher frequencies, but with lower resolution and an oscilloscope input stage will compromise performance.I have thought about designing a more general bidirectional 2-port low frequency VNA for low frequency network analysis, but the bidirectional design would compromise performance as a dynamic signal analyzer because of limited common mode rejection, and I am not sure that is a problem worth solving.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 27, 2022, 01:51:26 am
content: Quote from: David Hess on October 27, 2022, 01:28:23 amhttps://www.testunlimited.com/pdf/an/5988-6774EN.pdfAccording to page 20, a 2 channel DSA (one ch for ref, one ch for DUT output) with a noise source should make for a blazingly fast if not "real time" network analysis / bode plotting tool. I think if I want to include this function, I will indeed want to have at least 2Msps for the ADC so that the bode plot can cover lets say 100Hz to 1MHz in a single sweep.DC2390A eval board from ADI (https://www.analog.com/media/en/technical-documentation/user-guides/DC2390AF.PDF) would be the perfect solution paired with a FPGA board, with two LTC2500-32 and two fast DAC. Shame the connector on the board is a HSMC which doesn't plug into anything I own.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2022, 02:32:18 am
content: Quote from: TopQuark on October 27, 2022, 01:51:26 amAn impulse source is even faster. Or an edge can be used as the source and then the signal differentiated before the FFT. Limited only by the FFT execution time and the time to capture the acquisition, the display of the network response can be real time. I am actually disappointed that the recent oscilloscope bode plot generators cannot use this method.QuoteThere are a bunch of products which almost do it, but at least for me the software is not trivial either.I have been thinking more in terms of a VNA because it would be so easy to use as a super LCR meter and I have never seen one that operates in the way that I am thinking. Of course maybe there is a good reason for that. HP published an application note on the subject which discusses the advantages and disadvantages of the various designs, and I should reread it.I am not really interested in distortion measurement. There are too many products which already handle that well so making another distortion analyzer is less interesting.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 27, 2022, 02:47:30 am
content: QuoteWith the impulse method you still need to capture enough data points to have a high resolution FFT and most importantly measure lower frequency response no? I think a periodic random noise source and the pulse method both excites all the frequency bins within the analysis span, and does the same thing. Speed is limited by the lowest frequency component you wish to analyse, correct me if I am wrong.https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4338271/#msg4338271I have previously written a simple script that turns the bode plot data from a Siglent scope into detailed LCR (mostly C) impedance analysis. I think the FPGA solution would have more than enough horsepower to do every one of those graphs in real time, and generate all those analysis every data frame. So basically a new bode plot, impedance analysis and LCR analysis every few tenth of ms (or longer, again dependent on lower limit of analysis frequency).I think I am going to experiment with my Red Pitaya this afternoon and write some software to prototype the DSA idea.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 27, 2022, 04:36:29 am
content: Update.While window shopping on mouser and refreshing my shopping cart, I suddenly saw the AD4630-24 come back in stock and immediately ordered a few. Plan now is to whip up a breakout board for the AD4630-24 with 0.1 in header connections, so I can use it with FPGA or with MCU to see what kind of platform works well enough.If the DIY breakout board works fine, I'll cancel my order for the back ordered ADI official eval board, which has a FMC connector and so only works with high end FPGA dev boards.I am debating if I should add a frontend and Vref to the DIY breakout board design, or keep everything modular and external so I can easily swap things out for development. I have some goodies in my inventory that should work well with the ADC chip, like LTC6655-4.096, ADA4898, AD8429, ADA4523, OPA2189 etc.Anyways, I am quite excited to have the AD4630-24 on order ready to ship, and will probably spend my time designing the breakout board rather than playing with the Red Pitaya.

author: Berni
author_rank: Super Contributor
timestamp: October 27, 2022, 05:22:51 am
content: Quote from: TopQuark on October 27, 2022, 01:51:26 amI have an old Agilent 10MHz VSA boatanchor that i use as a DSA and low frequency network analyzer.Despite the old ADC tech it has an impressively wide dynamic range and low distortion, so it can be used for measuring THD in audio and similar.It also has the ability to show a live bode plot (updates multiple times per second). The way it does it is emitting a frequency sweep chirp out of the signal generator output that is synchronized to the capture window, then just does a FFT on the whole thing.As for the impulse method, that also works, but is more practical when done on a square wave. Those are easier to generate correctly and suffer less dynamic range issues. One edge of the square wave is a step function, that can then be put trough a differentiator to turn it into a impulse response that can then be fed into FFT to get the frequency response. Some oscilloscopes can do this using math functions. But in general sweeps tend to make better use of the ADC than this.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 27, 2022, 01:35:57 pm
content: Quote from: Berni on October 27, 2022, 05:22:51 amHmm... If real time bode plotting is something I wish to pursue, I wonder if it makes sense to push the sampling rate to let's say 20MSPS to get 10MHz frequency response range. The FPGA should still have enough horsepower to do real time FFT at that rate. Going from minutes to get a bode plot with the siglent scope to potentially fraction of a second per update does sound really appealing

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2022, 11:21:18 pm
content: Quote from: TopQuark on October 27, 2022, 02:47:30 amThat is right. The lowest frequency of interest determines the amount of time that the acquisition must be taken over. The same issue comes up with low frequency noise measurements; if you want to include noise down to 0.1 Hz, then you have to make at least 10 seconds worth of measurements.The problem with using noise as I understand it is that multiple acquisitions are necessary to account for its random nature. From the link below:Note that the random noise source requires that we use the averaged FFT so we can see the mean response at each frequency. This is the only way to obtain statistically significant data from the white noise source.Quote from: Berni on October 27, 2022, 05:22:51 amHere is an old EDN articlewhich discusses the three methods. The swept sine method naturally has the highest dynamic range.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 29, 2022, 05:17:08 am
content: https://www.eevblog.com/forum/testgear/siglent-sds2000x-hd-12bit-(published-for-chinese-domestic-market-only)/msg4489057/#msg4489057Gave the DSA bode plot method a shot with my 12-bit scope, wrote a bit of software to try out the method. In my experiments noise seems to work best, as my function gen can't produce burst sweep chirp with 0v on either end, it is either continuous sweep or burst at fixed frequency.I currently have 3 directions I can pursue for the DSA project.1. Go for the AD4630-2. Two channel 2 Msps @ 24 bits will make for a really high dynamic range DSA under 1MHz. Need to design my own breakout board, but chips is on the way. Sampling rate is low enough that a MCU might be good enough, might not need FPGA.2. Go for ADC3683EVM. Two channel 65 Msps @ 18 bits. Good dynamic range, can look at signal under 30MHz and oversample for lower frequencies. Plug and play FMC card. Firmly in FPGA territory. I have a board on the way, might use it for the DSA project, or use it elsewhere.3. Red pitaya. 125 Msps @ 14 bits. Can swap the ADC chip to LTC2185 for 16 bits and improve SNR by 3-4dB. I already have the board, everything is wired up well, DAC included. FPGA on board.For my DSA needs, I only need to look at signal under 1MHz. But if I am to spend the trouble and effort to develop the project, I would want to have it work as a bode plot machine as well, and so I would want to have it work up to 10MHz. So currently I am leaning towards options 2 and 3.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 31, 2022, 06:02:00 am
content: Look what arrived in the mail todayThe ZU15EG FPGA SoC probably has more horsepower than most entry level scopes, and the ADC3683 is one hell of an ADC. Not cheap, nor small, but at least the hardware has the potential of being a kickass DSA.

author: KE5FX
author_rank: Super Contributor
timestamp: October 31, 2022, 04:48:21 pm
content: Cool ADC, but wow, what a brain-damaged clocking architecture. If I'm reading the data sheet right, the LVDS output is synchronous to the sampling clock, butyouhave to supply the clock. Oh, yes, and it has to run at rates like 4.5x SCLK in some of the more useful modes.If they hadn't bungled the digital side so badly, that chip wouldn't even need an FPGA.

author: TopQuark
author_rank: Frequent Contributor
timestamp: October 31, 2022, 05:41:33 pm
content: Yea it is pretty silly. Current thinking is to clock the ADC sampling clock input with a low jitter source. Data frame clock output (same freq as sampling clock, with delay and likely more jitter) from the ADC will be used to sync the data frame transfer and create the bit timing clock with a PLL on the FPGA.