Impedance Analyzer

Just about everything about Red Pitaya
Bulova
Posts: 115
Joined: Fri Jan 09, 2015 6:07 pm
Location: Babenhausen/Germany

Re: Impedance Analyzer

Post by Bulova » Fri Feb 10, 2023 2:03 pm

Hello Pavel,

I tried to do some measurements with the RP and the VNA app. But to be honest, I don't know how to get some serious results. I connected my DUT (first test with a simple 51 Ohm resistor and then with my parallel resonant circuit (resonance at about 70 kHz) at the IN1 and OUT1 of my RP. I started VNA, pressed connect and choosed impedance. But wherever I pressed Single, Load, Open or whatever, nothing plausable is generated. The red phase line makes something like a burst, the blue Z-line makes something like Himalya.
I did not find answers in the VNA-guide. I am sure if the description matches to the app. I don't know, what is meant with MSA, and I thought VNA reflection mode should be used after a look at the possible RLC circuits picture. How to perform a S11 scan? Where is RLC analysis? And click "Analysis" is wanted, which I cannot find. And then comes "4. The graphs automatically change to LC--Series L and LC--Series C for the series model ... " which I could not find at all either.
I really not sure if the description is for that!

Best regards

Dieter

pavel
Posts: 798
Joined: Sat May 23, 2015 5:22 pm

Re: Impedance Analyzer

Post by pavel » Fri Feb 10, 2023 2:31 pm

Since you wanted to measure impedance, I assumed you knew how to use a VNA. The linked documents are for other VNAs, but the usage is still the same.

I have just found a video showing how to use the VNA application for the Red Pitaya board:

https://www.youtube.com/watch?v=w97yiPHSQA0&t=372s

I think the most basic measurement can be done in the following steps:
  • connect infinite Ohm DUT, press Open
  • connect 0 Ohm DUT, press Short
  • connect 50 Ohm DUT, press Load
  • connect unknown DUT, press Single
The first three steps are called calibration and its results can be saved to a file for later reuse.

Bulova
Posts: 115
Joined: Fri Jan 09, 2015 6:07 pm
Location: Babenhausen/Germany

Re: Impedance Analyzer

Post by Bulova » Fri Feb 10, 2023 7:08 pm

Hello Pavel,

no, I am not a VNA specialist, that is for sure. But I do know, what it means to measure an impedance of a simple DUT. I thought without calibration measurements could be done, even with less accuracy.

Now I have made the calibration before, and then I got some results, which also cannot be the truth. But after a while I got better results. I think it is a question how the "open" state is made. First I let the input really open, but then I thought it could be better to ground the input for this to avoid disturbances as good as can be done. And at least simple resistors show correct results.

Best regards

Dieter

pavel
Posts: 798
Joined: Sat May 23, 2015 5:22 pm

Re: Impedance Analyzer

Post by pavel » Fri Feb 10, 2023 8:57 pm

Not sure, but from your previous two descriptions, I get the feeling that something is missing in the connection between the board and the DUT.

In the simplest configuration, I connect 50 Ohm termination resistors to the input and to the output of the Red Pitaya board.

Some simple schematics can be found in this forum post.

Here are the links to a couple of web pages with some examples of some very simple test fixtures:
https://www.scottyspectrumanalyzer.us/f ... ystal.html
https://destevez.net/2017/09/measuring- ... -lite-vna/

Bulova
Posts: 115
Joined: Fri Jan 09, 2015 6:07 pm
Location: Babenhausen/Germany

Re: Impedance Analyzer

Post by Bulova » Fri Feb 10, 2023 9:38 pm

Hello Pavel,

yes you are certainly right with the termination. That way, the input is always not totally open, as it is terminated. I have got good results too, but have to take care to use very short lines. For lower frequencies it seems to work ok, but with higher frequencies I have to use terminaton all the time. I will make this from now on.
I will try to become a bit familiar with Smith diagrams too. This should be a good tool to get experienced in that.

Best regards

Dieter

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redpitaya
Site Admin
Posts: 897
Joined: Wed Mar 26, 2014 7:04 pm

Re: Impedance Analyzer

Post by redpitaya » Tue Feb 14, 2023 8:55 am

For the marketplace to work, the Red Pitaya needs an internet connection.

We plan to update and rework the apps in a future OS update (approximately in the Q3 of 2023). Right now we are working on updating the Linux and all the other software to the latest versions, as well as combining all the images for different boards into one.

Bulova
Posts: 115
Joined: Fri Jan 09, 2015 6:07 pm
Location: Babenhausen/Germany

Re: Impedance Analyzer

Post by Bulova » Tue Feb 14, 2023 2:55 pm

Hello Pavel,

the VNA app is really comfortable. I would have attached some pictures, but there is still a problem with attaching files.

I have built a small box of aluminum with two short SMA cables to connect to the RP and a 3-position switch for providing open, short, and load condition, and there are two 4mm-banana socket for the DUT (which is disconnected for the calibration mode, of course).

The only small problem I am having with the VNA app is to force it to an individual frequency interval. The "Rescale" knob does not help each time.

Best regards

Dieter

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redpitaya
Site Admin
Posts: 897
Joined: Wed Mar 26, 2014 7:04 pm

Re: Impedance Analyzer

Post by redpitaya » Tue Feb 14, 2023 2:58 pm

Dear Dieter,

Could you please provide some pictures and data from your tests and your setup, using both OS version 1.04-7 (or another version that you get good results with) and the latest BETA version (1.04-28) (that is not working for you)?

We will use this data to debug the application and provide better documentation.

Bulova
Posts: 115
Joined: Fri Jan 09, 2015 6:07 pm
Location: Babenhausen/Germany

Re: Impedance Analyzer

Post by Bulova » Tue Feb 14, 2023 4:01 pm

Hello,

yes, I would like to do it, but I am still having the error by trying to make an attachment:

Sorry, the board attachment quota has been reached.

Although it is just a PNG with less than 20 kB.

Or shall I send the pics to an email address?

Best regards

Dieter

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redpitaya
Site Admin
Posts: 897
Joined: Wed Mar 26, 2014 7:04 pm

Re: Impedance Analyzer

Post by redpitaya » Thu Feb 16, 2023 3:08 pm

Thank you for the mail.

We will revise the Impedance analyzer when we update the applications in a future OS update (Q3 2023) and add some explanation to the documentation.

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