Vector network / spectrum analyzer usage help

[lfarrand]

Hi folks,

I have a Siglent SVA1032X (actually it's a Siglent SSA3021X+ that I hacked). I also have a coil that I'd like to find the resonant frequency and any other attributes that might be helpful.

I would be eternally greatful if someone could give me a quick overview of how I can go about testing my coil. Will I need some extra parts like a bridge or attenuator etc?

I'm a novice with this particular tool, but I know what things I would like to measure. Just not so sure how to go about it.

Thanks,

Lee

[Itsu]

Hi Lee,

that looks like a nice piece of equipment there (SVA1032X) as it includes a VNA.

But you are in for a steep learning curve to master such a VNA.

Most important to know is that you should calibrate your VNA before each measurement you make by using the Open, Short, Load (50 Ohm) and thru plugs so that your VNA "knows" from what point it should be measuring (reference plane).

I found a video showing this on the SVA1032X here:  https://youtu.be/S0mjcRuFtKw?t=2

Then when making impedance measurements, its good to know what configuration is the best to use for low Impedance (<50 Ohm), 50 Ohm impedance and high Impedance (>50 Ohm).
This website shows the 3 configurations of your Device Under Test (DUT) and a graph with the error percentage for each configuration which i found useful:
https://www.mwrf.com/technologies/test-measurement/article/21849791/copper-mountain-technologies-make-accurate-impedance-measurements-using-a-vna

Furthermore, there are many tutorials to find on the web.

I would suggest using the "Series measurement configuration" to look for your resonance frequency of your coil.

By the way, there are more simple ways to measure the resonance frequency of a coil using a Function Generator and scope, so use that to find out your coil's resonance frequency and check that against your VNA results.

Itsu

[Itsu]

Here an example, having this coil:



Which measures 52MHz resonance frequency by using the FG and scope (the single wire going to the FG, so loosely coupled), and the scope attached to the coil leads and sweeping the FG for max. amplitude on the scope.


When using the VNA (my nanoVNA) in series measurement configuration over a 10kHz to 200MHz range freshly calibrated using the open, short, load and thru plugs i get this result (log Mag S21):



So this way you can confirm the resonance frequency results from the FG / scope, but it can provide additional info about this coil like impedance over this range, phase, etc.

Itsu

[lfarrand]

Thanks Itsu, that's really helpful.

I had a play around with the Bode plot function on my scope (Rigol DHO924S). It's a bit limited in what it shows, and the resolution isn't great. Nonetheless, I was able to roughly identify the resonant frequency of my coil. If I want any more fine grained detail then I'll have to bite the bullet and invest some time in learning how to use the VNA.

Could you please elaborate a bit further when you said 'the single wire going to the FG, so loosely coupled'? I see two wires leading back to your FG (red & black) in the photo.

[Itsu]

Lee,

it's quite straight forward, the single loop wire through the toroid / coil just injects some signal into the coil without too much influencing it and the toroid / coil reacts when the signal reaches its resonance frequency.
The scope shows when this happens (resonance) when the amplitude peaks.

With the FG (red and black leads) attached to the single loop wire you sweep through the frequency range to find the resonance (max signal) on the scope.

You can use either a sine or a square wave signal from the FG as the resonance frequency signal will always be a sine wave.


See the below diagram for further details.




Itsu

[Verpies]

I had a play around with the Bode plot function on my scope (Rigol DHO924S). It's a bit limited in what it shows, and the resolution isn't great. r FG (red & black) in the photo.

Please post a video of that Bode plot function.  I am thinking about buying this scope because it is easily battery-powered for 90min with an ordinary USB-C power bank used for charging mobile phones.
Easy galvanic isolation....

Actually, I would buy the cheaper DHO914 model and "upgrade" it to the DHO924.

The advice from Itsu about VNA errors and  measuring ferrites was good.  I would only improve it with two openable H-field probes and a mechanical holder so they can be calibrated in the same position without the ferrite in them, as with the ferrite ...without moving.

[lfarrand]

Please post a video of that Bode plot function.  I am thinking about buying this scope because it is easily battery-powered for 90min with an ordinary USB-C power bank used for charging mobile phones.
Easy galvanic isolation....

Actually, I would buy the cheaper DHO914 model and "upgrade" it to the DHO924.

The advice from Itsu about VNA errors and  measuring ferrites was good.  I would only improve it with two openable H-field probes and a mechanical holder so they can be calibrated in the same position without the ferrite in them, as with the ferrite ...without moving.

I'll take a video of the Bode plot function shortly. Apparently the Bode plot function is only available on the 'S' models, i.e. 914S & 924S. I bought it for exactly that reason - to be able to work isolated from mains and also have an element of portability to it. It's small enough that I can have it on my home office desk, which is invaluable otherwise it would have to live in the garage like my old scope.

I spent some time with my Siglent SA/VNA and I think I've grasped the concept now. To identify resonance am I right in assuming that I'm looking for a 0 degree phase angle? I know I can also look at the frequency response curves to identify the signature trace, but I'm thinking about how to know when the system is in resonance using something that I can sense with my Arduino.

The Siglent is orders of magnitude better than the Bode plot on the Rigol, which is to be expected since the Bode plot is an add on to the Rigol and the Siglent is an expensive bit of kit. The Rigol is limited to 300 points max. whereas the Siglent had no problem with 100,000+ points. The refresh rate admittedly did slow down at that resolution, but it was useful despite the slowness.