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RF testing of three port device with two port VNA.

Nancy12345

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Hi,

I need help regarding testing my RF IC. I have designed a phase shifter which has differential inputs and single ended output. The VNA in my lab is a two port VNA for single ended input and output.
Is there a way to buy an input balun or if I design one on the PCB, can I use that to input differential signals in my DUT for the testing?
The operating frequency of the DUT is 8 GHz.

Please if you have experiene in RF testing, help me with tis issue.

Thank you
 

BradtheRad

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Your situation sounds similar to the tv antenna cable formats... one old, one new. The old 300 ohm twin lead is differential. Later came 75 ohm coax which has the shield (ground) around a center wire (signal).

A converter unit is recommended to go from one format to the other. Not sure if it's a balun or transformer. Of course tv is VHF-UHF, not 8 GHz.
 

BigBoss

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You can simply connect two identical devices in cascade then extract s-parameters.
OR
Terminate one of the differential inputs by 50 Ohm and measure port-to-port then repeat the same for other port,After all, extract s-parameters by converting single ended to differential by aid of any RF simulator or Matlab.
 

Nancy12345

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You can simply connect two identical devices in cascade then extract s-parameters.
OR
Terminate one of the differential inputs by 50 Ohm and measure port-to-port then repeat the same for other port,After all, extract s-parameters by converting single ended to differential by aid of any RF simulator or Matlab.

Thank you so much for answering the question.
My device is a phase shifter which uses a RC poly phase filter, it uses two differential signals to generate four 90 degree apart signals. if one differential signal is not there, PPF wil not work as far as I know.
Can I just use an external balun? Are they availabe for 8 GHz?
--- Updated ---

Your situation sounds similar to the tv antenna cable formats... one old, one new. The old 300 ohm twin lead is differential. Later came 75 ohm coax which has the shield (ground) around a center wire (signal).

A converter unit is recommended to go from one format to the other. Not sure if it's a balun or transformer. Of course tv is VHF-UHF, not 8 GHz.

Thank you so much for answering the question.
Can I buy a external balun operating at 8 GHz and use it?
 

BigBoss

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Polyphase Filter is used to obtain 4 Single-Ended Signals 90 degree apart with a single Differential Input.But you may use one output or multiple outputs.Therefore Phase Relation is very important in this case.The best way to measure the Ports one-by-one the combine the s-parameters.
Balun will not work very well because Balun will also have a Phase Shift from ideal value.
 

Nancy12345

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Polyphase Filter is used to obtain 4 Single-Ended Signals 90 degree apart with a single Differential Input.But you may use one output or multiple outputs.Therefore Phase Relation is very important in this case.The best way to measure the Ports one-by-one the combine the s-parameters.
Balun will not work very well because Balun will also have a Phase Shift from ideal value.
Ok. So terminating one load with 50 ohm and taking measurement from other and then repeat the process with changing the ports is the best way for measurement?

Thank you.
 

dick_freebird

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You can get baluns but putting them in the
lineup adds the balun frequency response to
the mix.

I imagine you'd first characterize the balun
with the VNA, loaded as you'd use it, and then
pull the device data from which to de-embed
the balun's contribution.
 

Nancy12345

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You can get baluns but putting them in the
lineup adds the balun frequency response to
the mix.

I imagine you'd first characterize the balun
with the VNA, loaded as you'd use it, and then
pull the device data from which to de-embed
the balun's contribution.
Thank you so much for answering the question.

Do I need to buy some 50 ohm termination loads as well for proper calibration?
 

ktr

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You can do both and compare the results honestly. Solution Bigboss offers is a solid one. I would personally do 2 different measurements and compare them to each other.

1- Solution of Bigboss

2- get a https://www.minicircuits.com/WebStore/dashboard.html?model=TCM1-83X+ ; I used this balun up to 8 Ghz. It works nicely. Get some 50 ohm terminations; which should have Return loss better than at least -30db.

Compare the two results; with the difference between them equaling balun's added noise. You can also remove the balun's effect if you can add it to your calibration data; but that is a easier said than done in practice.
 

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