Low frequency network analyzer recommendations?

[hirada]

What options does one have when looking for an affordable NF (~1 - 1MHz)* network analyzer to display the frequency response of a circuit? So far I am aware of the Analog Discovery 2, but is there anything else that might be worth looking at? Let's say sub 6 - 700 quid? Used or new?

PC based would be acceptable, since I already do own a low end picoscope, that is just unfortunately lacking the software, but stand alone would be preferred.

Thanks

* 1MHz probably does not really qualify as NF any more, but since most analyzers on ebay start only about a third of that and go up into the GHz rage, it  is still acceptable. And if I do not reach quite that mark that could be ok as well, if the rest fits the bill. 

[uski]

Take a look at the NanoVNA v2 Plus 4 :
https://www.tindie.com/products/hcxqsgroup/4-nanovna-v2-plus4/

$129 for 10kHz to 4 GHz

Most VNAs will not go down to 1 Hz. For 1 Hz it is more in the domain of audio analyzers.
Do you really need to go that low ?

I used a Rohde & Schwarz UPL which does DC to 110 kHz but it is an expensive, discontinued and very specialized piece of equipment.
The successor is a UPV and this is >$10k on the used market, not exactly cheap

[hirada]

Wow, that was fast, thanks very much. Yes, I do need to go that low, as audio (no high end though) is a part of what I want to do, so 10kHz as a starting point is way to high, but I would like to have some headroom for the higher frequencies and I do not need those  ultra tight specifications that audio analysers usually offer - in trade off for range. 

But I do have the QA401 on my radar, probably rather than the Digilent, as latter duplicates to much of my picoscope functionality and I am not too fond of its hardware with the BNC daughterboard hanging out in the wild. But it does offer a wider frequency range and is cheaper. And maybe there are alternatives.

[Mechatrommer]

(~1 - 1MHz)* network analyzer to display the frequency response of a circuit?

what application do you need such low frequency network analyzer? i think you can make setup from FG and your existing scope. do you really need S11? or we talking only S21?

Take a look at the NanoVNA v2 Plus 4 :
https://www.tindie.com/products/hcxqsgroup/4-nanovna-v2-plus4/
$129 for 10kHz to 4 GHz

those GHz VNA are not trustable at very low freq, i'm guessing below 10MHz, if we talk about S11. some limitation on directional bridge, but i havent checked closely the wheatstone bridge they are using.

[uski]

You could get an audio analyzer for the very low frequencies, and a VNA from Siglent (for instance SVA1015+) that goes 9kHz+ to 1.5GHz

The QA401 stops at a few hundred kHz from what I see in its product brief.

Alternatively, some oscilloscopes from Siglent and Rigol have an integrated signal generator and offer an automatic bode plot function, which could possibly cover your entire frequency range. But I never used these features myself and I am not sure if they are good or suit your needs. If they do however, you could get away with a very cheap solution. Worth taking a look at it.

[hirada]

Hello Mechatrommer,

thanks for your reply, I would like to answer your question, but I am afraid, I have no idea what the practical meaning of FG, S11 or S12 are - besides being some form of specialized measurement after a quick online search.

Well, being still a beginner, basically all I am looking for, as far as I have understood so far, is a bode plot. Just feed a device a frequency or amplitude sweep and display the response over that range. Nothing too special or fancy, I believe.
 
What would be great, though, is to being able to have phase information as well. How does the phase of two signals in relation to each other change over frequency.

[ch_scr]

If all you need is a Bode plot (that includes by definition amplitude and phase information) an oscilloscope that with frequency generator (FG) and the necessary software to display it is your best bet for the give frequency range.
As the signal will go through the thing you are testing (from it's "port 1" to it's "port 2") when measuring, it might be called a S21 measurement.
I believe Keysight, Rohde&Schwarz and Siglent make applicable scopes.

[hirada]

@uski: Thanks, I do believe, the sigilent is way above my budget and I still have hopes, I can get along with a single device, as I do not have that high demands. As of yet?

But it looks like an audio analyzer is the way to go. Either a QA401 or by building a frontend to a soundcard like the one from Pete Millett, as I already do own a pretty decent one.
At least to start with. I would not have expected for Digilent to have a de facto monopoly for general, non audio specialized bode plots, that still go low down (and somewhat up).

@ch_scr

and the necessary software to display it

That seems to be the sole problem. The software needs to play with the hardware. My el cheapo picoscope could do all this, it just lacks that one  option in the software. For a reason, I suppose. Unless I am missing something and if there is a generic software, I am all for it.

I sure would not mind some slightly higher specs, somewhere in the middle between the pico and a full fledged audio analyzer, but I seems so far I am the only one and it would make at least for a good start.

[Someone]

since I already do own a low end picoscope, that is just unfortunately lacking the software

Yeah, nah: https://bitbucket.org/hexamer/fra4picoscope/wiki/Home

[Mechatrommer]

being still a beginner..

ahh. this is "Test Equipment" section btw. i asked because i wonder if there is ee discipline i'm missing. FG = function generator. S11, S21 is specific term for "network analyzer" i'm guessing what you are after (esp you mentioned audio) is spectrum analyzer, not network analyzer. audio analyzer is one kind of spectrum analyzer with special purpose for audio and unusually lower noise floor and jitter compared to normal joe spectrum analyzer. these are to see your device's output purity or any distortion. otoh S11 is return/reflection loss coefficient, i havent heard this is a concern in audio industry, if one doesnt know this, it means its not his business. S21 is the forward gain/loss, this is most interest in many areas, that is Bode plot is one of it. you want to see what effect of output compared to input (magnitude or phase or both).

so if you are interested in output purity alone, what you are after is spectrum/audio/signal analyzer, but if you care about signal amplification or attenuation, ie output with respect to input (S21), you are looking for spectrum analyzer (SA) with tracking generator (TG), or DSO with FG (function generator) that can give you bode plot, or a VNA (network analyzer). VNA can give you bode plot magnitude and phase, somewhat low noise floor, but only work at higher frequency than you requested. SA+TG usually can show magnitude only, but low noise floor, normal SA wont cut your 1Hz requirement, even the mentioned audio analyzer by uski only can go 10Hz and expensive due to audiophool nasty requirement (low noise). DSO+FG can give you nice bode plot, but not very low noise floor (limited dynamic range). so, what was your specification is?

[Mechatrommer]

@uski: Thanks, I do believe, the sigilent is way above my budget and I still have hopes, I can get along with a single device, as I do not have that high demands. As of yet?
...
That seems to be the sole problem. The software needs to play with the hardware. My el cheapo picoscope could do all this, it just lacks that one  option in the software. For a reason, I suppose. Unless I am missing something and if there is a generic software, I am all for it.

i built a PC bode plotter/impedance analyzer sometime ago (snapshot attached)  but only works for my DSO (DS1052E) and FG DDS3x25, but if you dont have programming skill, you may look around (google) Picoscope seems to have some writing maybe applicable to you?

https://www.picotech.com/library/picoapp/frequency-response-analyzer-with-bode-plots
https://bitbucket.org/hexamer/fra4picoscope/wiki/Home

if you want cheap/audio spectrum analyzer, some forum members swore by PC softwares that utilize your sound card with better dynamic range than a scope. you can google as well "pc/computer/windows spectrum analyzer" ymmv.

[hirada]

Edit: The tool is still under active developement, I have just read the wrong copyright message.

@someone:

You made my day! Have of course not dug deeper into it yet, but by its requirements at first glance it may eventually even be able to run on linux! Of course developement has halted, but one never knows wether it does not get picked up again one day. And by upgrading to a slightly higher spec'd picoscope, it also may grow along.


@Mechatrommer

Thanks again for your time and in depth explanations.

I indeed have wondered, wether I should ask in Beginners or here. But since we are talking about my quest for new test equipment, I threw a coin and went here.

But I am not looking for a spectrum analyzer, I already do have this one my picoscope. So S21, if I understand your explanation correctly, is exactly what  I am looking for. S11 is way above my head, but I suspect it is, what draws those nice wormholes on the screen.

To jump right to your last sentence, DSO+FG are perfectly fine, I do not need an ultra low noise floor, I am not in the high end business at all, but I do need a FG/DSO combination with a software, that supports creating this report. Or rather a standalone device, that has been my hope.

While there seems not to be such a hardware device, someone has found the software! And you as well. Needs a test and a deeper peek at the thread over at picoscope, as it seem out of developement, but at least a very promising start.
And in case I still have a XP VM with a passed though USB Card, that'll do the trick as a fallback, if the software should become incompatible with a new SDK. 

if you want cheap/audio spectrum analyzer, some forum members swore by PC softwares that utilize your sound card with better dynamic range than a scope

And somebody even has made a frontend for this method:
http://www.pmillett.com/ATEST.htm
And as mentioned before, I do own a pretty decent soundcard, somehow I just feel uncomfortable in using it as a mesuring device. Gotta really hate psychology. And software like mataa for Gnu Octave or Matlab seem to be quite powerfull, but come in at a rather steep learning curve.

However, there are some interesting discussions about this soundcard solution vs. the QA401, but generally, they  are all way above my humble requirements. May change in future. 

So for now I'll give the Fra4Picoscope software a go and see from there, where I will eventually be heading.

Thanks again all for your input! Big help

[Kibabalu]

You could get an audio analyzer for the very low frequencies, and a VNA from Siglent (for instance SVA1015+) that goes 9kHz+ to 1.5GHz

...

Just the spectrum analysis mode starts at 9 kHz.  In network analysis mode the lower limit is 5 MHz, because of the Tracking generator

[precaud]

Hard to beat the ol' HP 3577A for price/performance. I also wins the kg/$ race too 

Low-frequency network analysis is often referred to as "Frequency Response Analysis" aka FRA. Keep an eye out for used units from Omicron, AP Instruments, and others. These require a PC to control them, though.

[Kibabalu]

Some scopes also have this function. E. g. the Keysight DSO-X 1000 and 3000 series. For some reason not the 2000 series. Unfortunately i have one of those :-(

[switchabl]

There is also the ADALM2000, which is similar to the Analog Discovery with somewhat lower specs and a lower price. The software (Scopy) doesn't look quite as nice as Waveforms but it should do what you want.

Both seem to be backordered in most places right now and the Analog Discovery has apparently become quite expensive. Although I see that amazon.de appears to have a few Analog Discovery 2 in stock for ~215€, which would be a pretty good deal. It's a really versatile little tool, especially with the 14bit ADC. May not be as intersting for you, depending on what Picoscope you have.

[SilverSolder]

Of course, don't forget that the Digilent Waveforms software (for the Analog Discovery) is free to download -  and it works great with a regular PC sound card, using audio type frequencies obviously!

There is no better way to get started than 100% free, right?