Cheap and Small Dynamic Signal Analyzer?

[gigavolt]

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.

[Vgkid]

Maybe a quantasylum qa401.

[gigavolt]

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.

[maxwell3e10]

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.

[David Hess]

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.

[gigavolt]

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.

[SilverSolder]

Some audio cards can be modded for better LF response, even DC.

[David Hess]

Some audio cards can be modded for better LF response, even DC.

You 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.

[2N3055]

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...

[Berni]

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.

[KE5FX]

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.

[Berni]

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.

You 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.

[exe]

What are recommended sounds cards for the job?

[electrolust]

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.

Maybe 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.

[electrolust]

Surprisingly there doesn't seem to be a new instrument-grade low-frequency spectrum analyzer.

eh?

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.

[maxwell3e10]

Signal Hound USB-SA44B (1Hz)
Bode 100 (1Hz)
Keysight E5061B (5 Hz)

I 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.

[Berni]

Surprisingly there doesn't seem to be a new instrument-grade low-frequency spectrum analyzer.

Signal Hound USB-SA44B (1Hz)
Bode 100 (1Hz)
Tek RSA7100B (1Hz)
Keysight E5061B (5 Hz)

You 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=eng
I 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.

What are recommended sounds cards for the job?

I 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.

[KE5FX]

+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.

[avlijas.sladjan]

https://www.picotech.com/oscilloscope/4262/picoscope-4262-overview

It is 16bit 5MHz oscilloscope but for low frequency can oversample to 20bit.

[maxwell3e10]

+1 to that.  The 89410A / 89441A is one of the most underrated boxes around. 

Looks promising and maybe a little cheaper than a used SR780, plus a color CRT, that's progress!

[Berni]

+1 to that.  The 89410A / 89441A is one of the most underrated boxes around. 

Looks promising and maybe a little cheaper than a used SR780, plus a color CRT, that's progress!

Not only that. It even supports LAN connectivity (But you will need some rather ancient networking gear to make that work)

[electrolust]

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=eng

sure, i mean if you need a local gravity source.

the question i was responding to was why aren't there new, 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.

[Berni]

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

[electrolust]

Still someone could make a modern miniaturized version of a DSA using modern ADC/DAC chips and a powerful MCU, much like the NanoVNA

Isn't "Multi-Instrument" that? and PicoScope 4262? Or do you (or OP) specifically mean self-contained like NanoVNA.

[maxwell3e10]

I'd want a self-contained instrument, not USB, with a DC-1 MHz frequency range, a SAR ADC and 100dB SNR.