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Measurements >> RF Measurements >> VNA calibration
https://designers-guide.org/forum/YaBB.pl?num=1386991526

Message started by mixed_signal on Dec 13^th, 2013, 7:25pm

Title: VNA calibration
Post by mixed_signal on Dec 13^th, 2013, 7:25pm

Hi,
I am using a cal kit for my vna. After 1 port calibration,
1. marker is at centre when loaded
2. marker is at right end with little rotation when open
3. marker is at left end with little rotation when short

I can get rid of the rotation if i enter offset using delay 12 ps. Is it oK to calibrate this way.
Thanks!

Title: Re: VNA calibration
Post by rfidea on Dec 15^th, 2013, 2:15am

Your 12ps difference can be ok. The connector standard you are using have a reference plane that corresponds to the left and right points in the Smith-chart. But usually the open and short standards in your calkit has NOT the open and short placed at that reference planes due to practical reasons. But the VNA knows where the open and short are positioned so it can recalculate all parameters to the correct reference plane during calibration.

A good way to check the calibration is to attach an open transmission line of 10-20mm length, for example an adapter. Then measure s11 over the widest possible band, maybe 10M-6GHz. Then you should get a flat response with only some loss. If you get ripple and positive s11, then your calibration is wrong.

Title: Re: VNA calibration
Post by mixed_signal on Feb 5^th, 2014, 6:38pm

Dear rfidea,
Thanks for the reply but its short of unclear why its not perfect. I saw a youtube video where the guy gets the marker perfectly at left, center and right when short, load, open.

Title: Re: VNA calibration
Post by aaron_do on Feb 6^th, 2014, 1:52am

Quote:

I can get rid of the rotation if i enter offset using delay 12 ps.

did you remember to de-embed the "thru" connector? That's something I always missed out.

Title: Re: VNA calibration
Post by mixed_signal on Feb 6^th, 2014, 9:13pm

Hi aaron_do,

I did only 1 port calibration and hence did not use through connector.

I later tried doing full 2 port cal (sort, open, load, through) but still the problem exists. What do u mean by "demebed through"?

Title: Re: VNA calibration
Post by aaron_do on Feb 7^th, 2014, 1:30am

Hi mixed_signal,

since you say its a 1-port cal, I guess you aren't experiencing the problem I was referring to. I just re-read your post and see that you mentioned it was a 1-port cal...

The first several times I did my VNA cal, I would do a short, open, and load on each port, and then I would use the thru connector (some people spell it this way...not sure why) for the last step in the cal since both ports had a male connector at the end. I originally thought that the thru connector was specific to the cal kit, and that its data was stored in the VNA. Turns out I was wrong, and you need to calibrate out (de-embed might have been the wrong word) the thru connector as a final step. If you don't, as I remember, you will get some ripple in the frequency response.

Seems like rfidea was saying that your cal kit isn't perfect, but that the VNA knows the imperfection. I no longer have access to the VNA I was using, but I remember I could get a very good calibration as at the time we were looking at loss on the order of just points of a dB. Perhaps your VNA itself needs to be calibrated, or the cal kit is no longer valid (you have to be careful with those cal kits).

Aaron

Title: Re: VNA calibration
Post by mixed_signal on Feb 8^th, 2014, 4:36pm

Hi aaron_do,
Thank you for the reply.

I have a basic doubt: the calkit was already installed by somebody. Sometimes vna will have calkit installed by default or you have to install it. If I want to install a new kit, what are the data about the kit that has to be entered? Electrically what those relate to?

Title: Re: VNA calibration
Post by rfidea on Feb 8^th, 2014, 9:46pm

I thinks you are on the right track. If you are using Agilent you should be able in the VNA tog choose the calkit modell, just look at the box. Otherwise you ned till get hold of the calkit parameters. They where delivered with the calkit. Then you can enter them as custom calkit.