VNA calibration results. Are these normal?

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bitterbierce
Posts: 7
Joined: Fri Nov 16, 2018 6:18 pm

VNA calibration results. Are these normal?

Post by bitterbierce » Sun Nov 18, 2018 11:31 pm

Is this a normal VNA calibration Smith chart?
Captura de pantalla (5).png
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pavel
Posts: 531
Joined: Sat May 23, 2015 5:22 pm

Re: VNA calibration results. Are these normal?

Post by pavel » Mon Nov 19, 2018 7:44 am

Is this a normal VNA calibration Smith chart?
I'm afraid that the provided information is insufficient to answer this question.

Could you provide more details about your calibration and measurements?

Have you done the open, short and load calibration?

What DUT are you using to obtain the Smith chart?

bitterbierce
Posts: 7
Joined: Fri Nov 16, 2018 6:18 pm

Re: VNA calibration results. Are these normal?

Post by bitterbierce » Mon Nov 19, 2018 4:49 pm

I did all three calibrations in order. The first, the smith chart, is the result of loading it with the 50 ohm load. The dut is the 50 ohm load and a single shot. I saved the results as vna.ini an reloaded them. The pictures provided are those of the calibration process results. These are my first steps with a VNA.

Thanks for the reply

pavel
Posts: 531
Joined: Sat May 23, 2015 5:22 pm

Re: VNA calibration results. Are these normal?

Post by pavel » Mon Nov 19, 2018 9:53 pm

Thanks for the details.

If you're using the same 50 Ohm load for the load calibration and for the DUT measurement, then the Smith chart should show a single dot at the center of the chart.

So, the Smith chart that you obtain doesn't look normal.

Another question. How is the DUT connected to Red Pitaya?

bitterbierce
Posts: 7
Joined: Fri Nov 16, 2018 6:18 pm

Re: VNA calibration results. Are these normal?

Post by bitterbierce » Tue Nov 20, 2018 11:08 am

Hello, I didn't want to answer yesterday evening before testing other things.
To answer your last question first, The DUT is the 50 ohm load plug included in the kit screwed directly into the DUT output. The plug was tested to have 50R impedance with an ordinary multimeter.
First, I tried checking the calibration.
Pantallazo-2018-11-20 10-23-55.png
The results looked to me way off the average, so i tried going back to factory defaults and the VNA calibration output was even worse. It didn't even make it to 50 ohm on average and the Smith chart output under the 50 ohm plug is similar: A signal bouncing back and forth ending up in a central pink dot, spot on over the center of the Smith Chart. Wrote back the old calibration parameters. A test with the available scope + signal generator app at different frequencies (above 1500Hz) does not reveal nothing wrong, no significant distortion in the the square wave and none at all in the sine wave.
Both inputs are set to low, power supply has well over 2A capability.
I currently have no fan over the heatsink, because I haven't found one yet, but the board is not in a box and it's pretty cold at home.
I am using your (pavel's) SD-card image directly from the GitHub repository.
Also, my board is the 125/14 V1.1
Should I return the VNA unit or both the Red Pitaya and the VNA?
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pavel
Posts: 531
Joined: Sat May 23, 2015 5:22 pm

Re: VNA calibration results. Are these normal?

Post by pavel » Tue Nov 20, 2018 11:39 am

Thanks for more details about your setup.

So, if I'm not mistaken, here is the setup you're using:
  • Red Pitaya VNA module
  • SD card image red-pitaya-alpine-3.8-armhf-20181022.zip
  • VNA client vna-win32-20181022.zip
The RF input calibration data isn't used by the VNA application. So, it's strange that changing the calibration constants affects the VNA measurements.

I've just checked the very first screenshot that you posted. Looks like the Z values are OK starting from 1000 KHz. So, it's possible that the problem is with the measurements at low frequencies.

I think that the measurements at low frequencies could be improved by setting the bandwidth parameter to lower values.

Could you try to redo the open/short/load calibration and the DUT measurements with the bandwidth parameter set to 1000 Hz?

bitterbierce
Posts: 7
Joined: Fri Nov 16, 2018 6:18 pm

Re: VNA calibration results. Are these normal?

Post by bitterbierce » Tue Nov 20, 2018 12:54 pm

I performed the very first calibration according to the default parameters as If opened the the app, the first time. So, if I'm not mistaken, your sugestion is to narrow the calibration from 1 KHz to 2KHz (lower to upper BW) and a narrower sweep in testing?
Thanks

pavel
Posts: 531
Joined: Sat May 23, 2015 5:22 pm

Re: VNA calibration results. Are these normal?

Post by pavel » Tue Nov 20, 2018 1:40 pm

The VNA client interface has an input field called Bandwidth. My suggestion is to set the value of the Bandwidth input field to 1000 and to redo the open/short/load calibration and the DUT measurement.

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