Can I measure 50 to 75 Ω impedance matching devices with a network analyzer?

If you need to measure a DUT with different reference impedances on each port, you must use the adapter removal/insertion characteristics and impedance conversion characteristics kit of the network analyzer. If the DUT is a two-port device with a characteristic impedance that can be converted from 50 Ω to 75 Ω, you must prepare a 50-75 Ω minimum loss pad and 50 Ω and 75 Ω calibration kits according to the connector type.

Step 1. Perform a full 2-port calibration using a 50 Ω calibration kit to calibrate the port where the adapter insertion/removal is performed (Figure 1).

Figure 1. Performing a full 2-port calibration using a 50 Ω calibration kit.

Step 2. Modify the 75 Ω calibration kit.

1. Select the desired 75 Ω calibration kit from Cal > Cal Kit.

2. Redefine the 75 Ω “Load” standard as “Arbitrary”.
   Cal > Modify Cal Kit > Define STDs > Broadband > STD Type > Arbitrary

Step 3. Perform adapter insertion.

1. Open the Adapter Removal menu.
   Cal > Calibrate > Adapter Removal

2. In Select Port, select the port where you want to insert the minimum loss pad.

3. From Cal Kit, select the 75 Ω calibration kit (modified in step 2) that you want to use to characterize the minimum loss pad. Use the calibration kit on the flat surface where the adapter is inserted.

4. Connect the minimum loss pad to the selected test port.

5. Connect the open, short and load of the 75 Ω calibration kit and click/press Open, Short and Load respectively (Figure 2). Use the ENA to measure the calibration kit standards, calculate the adapter characteristics and insert the adapter.

6. Click Done to complete the testing process.

Figure 2. Inserting the adapter using the 75 Ω calibration kit.

Note: Do not change the system characteristic impedance of ENA, which should be 50 Ω.

Step 4. Convert the reference impedance of the port connected to the minimum loss pad.

1. Open the Fixture Simulator menu.
   Analysis > Fixture Simulator

Figure 3. Measurement setup for 50-75 Ω impedance matching device.

1. Click Port Z Conversion and specify the reference impedance for the selected port. If the minimum loss pad is connected to Port 1, you need to change Port 1 Z0 Real from 50 Ω to 75 Ω.

2. Click Port Z Conversion to set the port impedance conversion function to ON.

3. Click Return.

4. Click Fixture Simulator and turn it ON.

Figure 4. Transition in reflection coefficient (S11) when connected to a 75 Ω load standard through an 11852B minimum loss pad.

Note: Because standard format files should be based on a reference impedance, when saving data in a standard format file, you must stop using the Port Z conversion and convert the port reference impedance back from 75 Ω to 50 Ω.