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Calibration of VNA for measurements on a non-insertible device (Read 5287 times)
dm
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Calibration of VNA for measurements on a non-insertible device
Sep 21st, 2011, 4:31am
 
Hello,

I have a DUT (directional coupler) where three of the ports are standard SMA 3.5mm, and the last one is a APC-7 7mm. I want to measure the S-parameters of the for port 1 and 2 when the coupled ports are terminated with 50 ohms, however I have difficulties figuring out how to calibrate the VNA in order to get accurate measurements.

I know I have to use an 7mm-to-3.5mm adapter on one side, but since the phase thru the coupler is whats interests me the most, I want to remove the effect of this adapter, but I can't figure out how to do that properly.

I have both standard 3.5mm SMA cal kits available (85052D) and 7mm APC-7 cal kits from Focus Microwaves available

Does anyone with experience from similar problems have any suggestions to how I solve this? And/or any links to decent literature on the subject?

Any help is greatly appreciated.

Best regards,
dm
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rfidea
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Re: Calibration of VNA for measurements on a non-insertible device
Reply #1 - Sep 21st, 2011, 11:56am
 
I did not understand which phase you would like to measure but I assume that the problem is when measure from a 3.5mm port to the 7mm port.

One idea is to calibrate the VNA with the 3.5mm calibration kit. (Do the calibration broad band.) Then attach the 3.5/7mm adapter to one of the VNA ports.

You then attach the short reference from the 7mm calibration kit to the adapter. When measuring S11 in that port with Smith diagram you will see an arc at unity circle. In the VNA settings it is possible to add electrical length to the port. When you do that you will see the arc moving around. Try to add as much length as there is in the adapter and you will see the arc collaps into a point at the short point in the Smith diagram. Then you have removed the electrical lenght of the adapter. Now you will be able to measure the phase from the 3.5mm reference plane to the 7mm reference plane of your DUT.
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dm
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Re: Calibration of VNA for measurements on a non-insertible device
Reply #2 - Sep 22nd, 2011, 2:36pm
 
Thanks for your reply, and sorry for the confusion. To try to clear it up, I want to measure the S-parameters for port 1 and port 2 (input and output), while the coupled ports are terminated with 50 ohms, e.g

                              P3     P4 (both P3 and P4 are 3.5mm)
                               |       |
(7mm - input) P1--------------------P2 (3.5mm - output)

Your idea seems good for measuring the phase thru the DUT, but unfortunately I also need the magnitudes of the S-parameters (but as mentioned in the first post, the phase is the most critical of the two). But I'll give it a try anyway and see how good accuracy I will get; in theory the loss thru the DUT (P1 to P2) should be at an absolute minimum as the signal path between the ports is a 50 ohm airline, so I expect minimal reflections, especially at the port with the 7mm connector, but you never know.
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Re: Calibration of VNA for measurements on a non-insertible device
Reply #3 - Sep 23rd, 2011, 8:35am
 
It was a long time ago I was struggling with those issues. But I think I remember that some VNA:s has support for this. Are you using an Agilent one? Their application engineers are usually helpful.

Since you have both cal kits and an adapter you can do two full 12 error term correction calibrations, one with the adapter on the 7mm side and one with it on the 3.5mm side. Then I think there is some magic button inside the VNA that you can press to merge those two calibrations. Maybe that is some specific option you need to get. But I think there is enough measurements to remove the adapter effect from the DUT measurments.
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dm
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Re: Calibration of VNA for measurements on a non-insertible device
Reply #4 - Sep 25th, 2011, 11:11pm
 
Thanks for your advice on the merging, after looking more carefully through the user manual it seems like our PNA supports something similar at least, but I have to read a bit more about it to see how I actually carry it out.

Again thanks for your time and help.

Best regards,
dm
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