[solved] VNA suggestion for non-steady-state organics analysis? - Page 1
Forum section: electronics / projects

author: Whales
author_rank: Super Contributor
timestamp: November 16, 2022, 02:51:43 am
content: ContextI work in insect research, primarily species & gender discrimination of live mosquitoes but also we're starting to dabble in fruitfly. Our current methods are optical (shine an LED on a flying specimen, its wings block the light as they beat, record the average intensity of its shadow over time, throw at trained machine model).Our optical pathways are self-contained so our data is great compared to methods like acoustic mosquito detection (we don't have to deal with separating insects from changing background noise). Nonetheless we always want more data to feed the machine learning gods.Some other researchers have had success with "capacitive sensor" ICs for identifying insects. From reading capacitive sensor IC datasheets: they seem a bit like a basic VNA with one excitation frequency and outputs in arbitrary units.ProblemOne of our budgets is expiring in a few days time (several thousand AUD) and we want to try throwing a VNA at a few different insects to see if any unique impedance patterns/signatures show up.Imaginary setup: Leg-to-leg insect impedance analysis (mag+phase over frequency range) or leg-to-wing impedance analysis as it walks over or brushes against two metal contact pads. EDIT: hundreds of insects being fed over the two metal pads so eventually some end up in good contact positions for long enough for measurements to be made.Major difference to how most VNAs are used: this is not a steady state system like some cabling.We need the frequency sweeps to be relatively fast (eg 10 per second ormore) AND log/record all of these sweeps (without inter-sweep averaging) to a computer because the insects are actively moving.I have never touched any VNA other than a nanoVNA and I'm expecting some VNA models will have lots of hidden problems like waveform averaging and data I/O limitations (including crappy PC software). We don't know what frequency ranges are useful until we try, and although apparatus repeatability gets harder at higher freqs (eg GHz) these higher freqs mightmightstill be useful.I'm throwing myself at datasheets and trying to decipher what VNAs might work for us. Sadly it doesn't look like there is a sigrok for VNAs. Does anyone have any suggestions?

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: November 16, 2022, 04:56:05 am
content: For something that's changing so fast it sounds like you need a "realtime VNA" - in a similar vein to realtime spectrum analysers are used to look at a signal spectrum and catch time-domain transients.Unfortunately I don't think realtime VNAs have been invented yetAt least.. I haven't heard of them and that normally means they're terribly expensive.You could maybe emulate it with a continuous RF noise source (vaguely white noise with frequency) and a realtime spectrum analyser but RTSA are quite expensive. Noise sources are cheap though! A noise source and an SDR to look at the spectrum would fit your budget and data-gathering rate.If you have more interest in spending your budget than solving the problem I'll put on a different hat and be right back with some different ideas

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 16, 2022, 08:30:43 am
content: Quote from: Whales on November 16, 2022, 02:51:43 amDisclaimer: I have the luxury position of working in a high-end lab that throws millions of EUR per year at RF test equipment. I'm spoiled.All of this sounds prefectly doable for high-end VNAs. The problem will be the budget of a few thousand AUD - the calibration kits and cables for our high-end VNAs are already (well) above that budget, let alone the actual VNA. I don't know how far down you can go in the VNA hierarchy and retain these abilities. It also depends on the frequency range you need and the precision you need.

author: Whales
author_rank: Super Contributor
timestamp: November 16, 2022, 10:00:01 am
content: I will have to compromise a lot compared to how VNA's are normally used, but am I wrong to think several thousand AUD worth of VNA will get me notably more impedance data than a cheap capacitive sensing IC? I guess in some ways the IC might win out in the time domain because it doesn't have to sweep its stimulus frequency.Eg as a very vague and hopefully cheap starting point: 100 samples per sweep from (lowestSupported)Khz to 100MHz @ 10 sweeps per second = 1000 samples at different frequencies per second (2000 S-domain samples per second if my understanding is correct). That should be doable if the VNA lets me get the data out AND averaging can be turned off? Or am I completely misunderstanding the limitations that would make the equipment far more expensive in my situation?ThePicoVNA 106 is a little less than 10K AUD(ouch) and claims 5000 S-domain samples per second. That might work?Alas I'm at the whim of the software 's limitations (which AFAIK I can't test before buying). If anyone knows any particular maker's software that might be suitable then that would help, I can start looking at their VNA hardware.

author: Whales
author_rank: Super Contributor
timestamp: November 16, 2022, 10:03:22 am
content: Quote from: Geoff-AU on November 16, 2022, 04:56:05 am(EDIT)SDRs were a backup thought but I thought they would be even slower. I didn't think of using a broadband noise source to avoid having to sweep frequencies.If you don't sweep any siggens in the SDR's then you get:- frequency range limited to only the capture bandwidth, unless you sweep (defeating the purpose of the broadband noise source)- much more "real-time" style capture- ability to change the effective "sample rate over time" of the spectra in post by changing the FFT bitdepthI've only touched SDRs with a few MHz of sample bandwidth (ie probably not enough magnitudes of 10 to see notable DUT impedence variations) but I will have a look now.Also I think an SDR will only measure |Z| (not Zreal and Zimaginary) because it is only a single-port measurement device. That might be fine, I don't know until I march mosquitoes across it. Sounds like a good backup plan if a VNA becomes infeasible

author: Whales
author_rank: Super Contributor
timestamp: November 16, 2022, 10:15:16 am
content: A possible misunderstanding: I'm not measuring just one contact event from one insect. I can usher a (somewhat single file) line of hundreds of insects over the sensor pads and hope to eventually get some interesting pattern impedance-vs-freq curves, then repeat with a different species of insect. This means anything resembling 10Hz or better sweep+save cyclic rate will probably work.

author: bson
author_rank: Unknown
timestamp: November 17, 2022, 07:19:28 pm
content: Why not sedate one and measure it statically? It would seem to me if static probing doesn't yield anything interesting, then measurements taken from active specimens isn't going to be productive either...

author: Whales
author_rank: Super Contributor
timestamp: November 18, 2022, 01:26:28 am
content: Ty for the suggestion bson.We'd have to do that cautiously. Too much (eg CO2) asphyxiation and they keel over. Also its effects on them change over time.Overall update:we've decided not to buy a VNA, we're going to look for some other researchers who already own VNAs and ask if we can bring some mosquito apparatus into their labs for some initial tests. That might be the best first attempt option for us, and as long as we keep our apparatus a good distance away on sacrificial cables then we shouldn't be too much of a risk to their equipment.Thankyou to everyone for you advice, greatly appreciated.

author: Bud
author_rank: Super Contributor
timestamp: November 18, 2022, 02:29:03 am
content: This is the most bizarre application for a VNA I ever heard of ....