How to test 75ohm part using VNA with 50ohm environment

[SIQ]

Hi,

As I do not have pure 75ohm VNA available, I would like to know may I use typical VNA (like 8364) with 50ohm environment (ie, 50ohm test port, cable, adapter etc.) to test 75 ohm part (coaxial interface). The S parameter should be finally renormalized to 75ohm?

My thoughts are as following:

1. Use standard 50 ohm calibration kit to do SOLT Cal., but setting the “system imp” to be 75ohm. Does the tested S parameter is renormalized to 75ohm?

2. Use the standard procedure as do 50 ohm SOLT Cal (ie, system imp still to be 50ohm), the tested S parameter then go to ADS for post-processing, renormalized to be 75ohm.

I am not sure if both methods work. If do, which method is better?

Many thanks!

 

[BigBoss]

Use minimum loss 75Ohm-50Ohm transformer that is available from HP..
Then you should do these calibrations with this adapter.

 

[PeterMartin]

Anything from HP will be expensive and you'll still not have a genuine 75 ohm cal.

Another way is to make up your own Calibration Kit, in BNC, TNC or whatever with a short, an open, a thru and a 75 ohm load. You can even make your own load with 2 x 150 ohm resistors.

As 75 ohms is only used for frequencies up to 1GHz or so this is quite easy to do and the errors are quite small.

 

[kspalla]

my take is
"both will work similarly and are correct."

 

[FvM]

"both will work similarly and are correct."

Method 1 (as far as I understand it) does not work. Changing the system impedance only alters the impedance reference,
but not the S-Parameters. As a simple example, if you calibrate for 50 ohm and change the system impedance to 75 ohm, 50 ohm
load will still be displayed in the Smith diagram center (S11 = 0). You have to calibrate with a 75 ohm load (as suggested by
Peter Martin) and should at best use impedance matching adapters to increase the accuracy of reflection measurements
(as BigBoss said).

 

[Element_115]

The only bad thing about the min-loss pad is that it takes 5.7dB to
go from 50Ω to 75Ω. So your S11 & S22 will most likely be 11.4dB
better than what they really are, and that could mask real problems.
Also your S21 will be 5.7dB lower, but you can just add 5.7db to
the S21 measurement.

You can always just rent a 75Ω Network Analyzer.

Cheers

 

[SIQ]

Thank you all for the answers.
According to bigboss suggestion, as I do not have 75 cal kit (only 50 ohm cal), it sounds to me the “new DUT” includes my “old DUT” + 75-50 transformer. I am not sure how big this transformer will affect the result. What benefits will come with this transformer? Also how could I get the renormalized 75ohm S parameter?
If using Perter Martin suggestions, it sounds applicable. I have no experience if this home-made Cal kit will be good enough.
Bottom line, does the methods I suggested will work?
Thanks!

 

[FvM]

your S11 & S22 will most likely be 11.4dB better than what they really are, and that could mask real problems.

I don't think that anything will look "better". You basically get a larger error in S11/S22 and a lower sensitivity in S21/S12 measurement.

I don't really understand the problem with performing a 75 ohm calibration. You need a 75 ohm terminator, but the VNA does
all calculations for you.

Besides the minimum loss "transformers" suggested by BigBoss that are actually attenuators, you have also the option to use an
almost lossless 1:1.22 transformer. The VNA calibration should be able to correct it's errors at least for a limited frequency range.

 

[sciferalpha]

Hi to all, I'm new here... just want to ask if it is necessary to use 75ohm impedance cables and connectors after the minimum loss pads installed after the 50ohm impedance VNA..
Thanks!

 

[BigBoss]

Hi to all, I'm new here... just want to ask if it is necessary to use 75ohm impedance cables and connectors after the minimum loss pads installed after the 50ohm impedance VNA..
Thanks!

Sure..
After minimum loss pad , from now on your characterictic impedance is 75 Ohm, so you should use convenient equipment in according with this impedance.

---------- Post added at 14:43 ---------- Previous post was at 14:40 ----------

The only bad thing about the min-loss pad is that it takes 5.7dB to
go from 50Ω to 75Ω. So your S11 & S22 will most likely be 11.4dB
better than what they really are, and that could mask real problems.
Also your S21 will be 5.7dB lower, but you can just add 5.7db to
the S21 measurement.

You can always just rent a 75Ω Network Analyzer.

Cheers

When you do a calibration, everything is calibarted so all measurement results are mapped onto this new characteristic impedance.
So, if you read S21=15dB, it's actually 15dB regarding to new Characteristic impedance, you don't need to add or extract something...

 

[Kohi_boy]

How about other calibration method? TRL for example?

 

[ropt728]

To all masters and pros out there,

For my condition , I have made a 75ohm test board for my testing of 75ohm part using VNA with 50ohm environment

From what i've done,
I have made my conductor width and gap become narrow to meet the 75 ohm , is this correct ?
For DC in ( which is soldered on board), do i have to made the conductor Width and gap also become narrow ?
Does it have any effect on the 75 ohm ?

Thank you

Very appreciated yours response