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Testing 4 port device with 2 port PNA

Question asked by rickhart on Oct 2, 2007
Latest reply on Oct 2, 2007 by Dr_joel

We have a rather interesting problem and may be why they make 4 port Analyzers. Here are the specifics:

1) We are testing a 4 port hybrid coupler in a temperature/vacuum chamber environment.

2) We are testing with a 2 port PNA.

3) DUT P1 & P2 are the inputs, P3 & P4 the outputs

4) We need to test the following:
   a) P1 to P2 for Isolation
   b) P1 to P3 for Input & Output Return Loss
   c) P1 to P4 for Input & Output Return Loss
   d) P2 to P3 for Input & Output Return Loss
   e) P2 tp P4 for Isolation

5) Since we don't have direct access to the DUT (it's under vacuum) we need 5 different cal's (P1/P2, P1/P3, P1/P4, P2/P3, P3/P4. So far so good.

6) We connect up all 4 ports of the DUT to a bulkhead on the chamber wall. On the other end of the bulkhead we have access to the 4 RF paths. 2 of the RF paths will connect to the PNA & the other 2 will have 50 ohm loads.

Our problem is is that I'm getting about a 3 to 4 dB change in Isolation when comparing a termination connected directly to the DUT vs connecting to the DUT via the bulkhead (due to, I assume the uncalibrated cables in between the bulkhead and the DUT.

Question: Is it possible to configure 2, 2-port  Network Analyzers to test a 4 port device?