Vector Network Analyzers' bandwidth  - Page 1
Forum section: electronics / beginners

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 29, 2022, 02:12:00 pm
content: I'm posting into this section because it's a beginner question.Why do VNAs like NanoVNA do have such high starting frequencies? I mean, they start around ~50 KHz and go up to a few GHz.Why can't they build something that has a different range, like oscilloscopes (e.g. a few Hertzes to some hundreds MHz..).Thanks.

author: radiolistener
author_rank: Super Contributor
timestamp: October 29, 2022, 02:27:36 pm
content: Quote from: balnazzar on October 29, 2022, 02:12:00 pmfew Hertz is not possible on NanoVNA, because it uses si5351 frequency synthesizer. As I remember it's min frequency is about 2-5 kHz. But also RF front-end circuit should support low frequency.My instance of NanoVNA works good from 10 kHz, but I'm using my own firmware. I tested frequency response of ultrasonic piezo-transducers which working at 24 kHz and can see it's resonant frequency with no issue. In most cases 50 kHz is just software limitation, it can be just hardcoded value or due to specific limitations of algorithms which are used in firmware. I hear reports that some NanoVNA clones has very bad sensitivity at low frequency and almost unusable at 10 kHz. So, in some clones it also limited with RF frontend hardware.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 29, 2022, 02:41:21 pm
content: Quote from: radiolistener on October 29, 2022, 02:27:36 pmNanoVNA starts from 10 KHz, while the V2 starts from 50, but gets up to 3 GHz rather than 1.5...Mmhh, is there something like NanoVNA, but in the range I'm interested in? Starting from ~100 Hz rather than DC would be OK.If not, would you share your firmware (I presume for V1) to get it working from 2 KHz?

author: radiolistener
author_rank: Super Contributor
timestamp: October 29, 2022, 03:10:48 pm
content: Quote from: balnazzar on October 29, 2022, 02:41:21 pmfor what reason you're needs so low frequency?I even cannot imagine what you want to do at that low frequency?Quote from: balnazzar on October 29, 2022, 02:41:21 pmI don't remember why, but 2 kHz is not possible on NanoVNA. The min stable and usable frequency is 10 kHz.

author: PartialDischarge
author_rank: Super Contributor
timestamp: October 29, 2022, 03:51:39 pm
content: Quote from: radiolistener on October 29, 2022, 03:10:48 pmThere are many reasons why this would be interesting (audio, power line frequency devices...), and VNAs with low frequencies are available but rare (HP 4195A, Anritsu MS4630...) and sometimes a dynamic signal analyser is a good option.

author: Bud
author_rank: Super Contributor
timestamp: October 29, 2022, 05:52:44 pm
content: VNAs are designed for and referenced to 50 Ohm or 75 Ohm impedance. There is not much use for it if any use at all at perhaps below 100kHz circuits. At these frequencies you just use an oscilloscope with Frequency Response Analysis feature or an Impedance Analyzer.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 29, 2022, 06:09:05 pm
content: Replying to all in a single shot:Indeed I'm after regular FRA, but getting it for cheapo is not a trivial thing (the bare minimum seems to be a 1104x-e with a siglent AWG, for not much less than 900 eur...), so I eyed such little things that cost just 60 eur, hoping they could serve my purpose.@radiolistener: I was making reference to your post above, where you said that with your firmware, the NanoVNA could get as low as 2-5 KHz..Another option is a low end Picoscope, but the their awg only gets to 100 KHz (!).

author: Bud
author_rank: Super Contributor
timestamp: October 29, 2022, 07:10:36 pm
content: Keysight EDUX 1002G if you can find one.I am not familiar with nanovna but if it does not have a configurable reference impedance, you are back to 50 Ohm domain. From practical perspective that means the more the measured impedance deviates from 50 Ohm the more the measurement errors are.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 29, 2022, 08:16:41 pm
content: Quote from: Bud on October 29, 2022, 07:10:36 pmEDUX 1002G? Of course you can find that one, as long as you can shell out the 800 euros required to buy itThanks for the 50 Ohm heads-up, I didn't think about it. Was somewhat under the impression that it was selectable like on scopes...

author: Bud
author_rank: Super Contributor
timestamp: October 29, 2022, 08:38:52 pm
content: 1002G, not 1052G. The first generation of EDUX.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 29, 2022, 08:45:18 pm
content: The NanoVNA is AC coupled at various stages and my not like working down to DC. For lower frequencies, my original NanoVNA seems to perform the best of the low cost VNAs I have looked at.If you don't require the phase, maybe a spectrum analyzer and tracking generator would be a better choice. If you are looking for old products, maybe the HP 3589A would fit your needs.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 29, 2022, 10:18:36 pm
content: Thanks! I have just put both the HP 3589A and the 1002G on my ebay notification list...

author: joeqsmith
author_rank: Super Contributor
timestamp: October 30, 2022, 12:34:56 am
content: I own a 3589A if you have any questions about it. You can measure S11 with it using an external coupler, or they offered a test set for it. The input can be set to 1Meg and is DC coupled. Consider it being 30 years old now when considering the price. The only problem I have had with mine was the battery had to be replaced.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 30, 2022, 12:58:02 am
content: Quote from: joeqsmith on October 30, 2022, 12:34:56 amFinding one won't be easy

author: Calvin
author_rank: Regular Contributor
timestamp: October 30, 2022, 04:48:50 am
content: Hi,maybe a soundcard based analyzer (up to 100kHz and 24Bit) might just be what You could use.Either as internal or as external (USB) Soundcard and running commercial (Arta) or free software (REW, Holm Impulse, etc).The Quant Asylum A401 to A403 would be a neat little onefitsall soundcard device.Up to ~10MHz and 14Bit resolution Digilent's Analog Discovery2 could fill the bill.Offering scope, voltmeter, signal generator functionality as well as VNA, impedance analyzer and some digital functionality (of course with high impedance inputs, no 50R).regardsCalvin

author: jasonRF
author_rank: Regular Contributor
timestamp: October 30, 2022, 07:06:02 am
content: Quote from: balnazzar on October 29, 2022, 02:12:00 pmVNAs are designed to measure things like S-parameters, which are useful in microwave engineering where you need to take into account the wave-nature of the signals in the electronics. This becomes important when circuit elements (including wires or transmission lines) are some fraction of a wavelength long. At 10 kHz, the wavelength is 30 km, so we wouldn’t normally think about S-parameters unless we had a transmission line that was at least 300m long.We have a lot of expensive VNAs at work - I would be surprised if any of them went down into the audio band, but of course I have only used them at MHz and GHz frequencies. Not that I use them that much - I am not at all a hardware engineer - but when we need to do a lot of field testing I take my turn.Jason

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 30, 2022, 11:22:08 am
content: Quote from: Calvin on October 30, 2022, 04:48:50 amGood idea. Some of them get almost to 200 KHz. Will look into those Quant Asylum, thanks!

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 30, 2022, 11:23:41 am
content: Quote from: jasonRF on October 30, 2022, 07:06:02 amThe strange thing is that a VNA capable of looking into 3 GHz signals is sub-100$, while if you need low frequency FRA you have to spend 10X more..

author: jasonRF
author_rank: Regular Contributor
timestamp: October 30, 2022, 12:36:45 pm
content: There has been a thread on this recentlyhttps://www.eevblog.com/forum/testgear/cheap-and-small-dynamic-signal-analyzer/msg3189774/#msg3189774

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2022, 02:12:42 am
content: There are three reasons besides the application:1 - AC coupling if used limits the low frequency response.2 - Transformers or directional couplers also limit the low frequency response.3 - Often the close in phase noise of the oscillator is high making low frequency measurements noisy.Low frequency VNAs which operate down to DC do exist and are sometimes just called network analyzers. Nothing prevents implementing an input/output port which operates down to DC, however doing this limits high frequency performance.