Chapter 7 lab Exercise: Mode Conversion Due to Changes in Electrical Balance 2

spectrum analyzer with tracking generator driving a balun connected to a twisted wire pair connected to a differential trace pair

Common-mode current on cables is a major source of conducted and radiated emissions. Differential-mode signals can be converted to common-mode noise when their path of propagation experiences a change in electrical balance.

Preparation:  Students need to know become familiar with the definitions of differential-mode and common-mode as described in Chapter 7.

Equipment Required:

  • Vector network analyzer
  • a coaxial cable (50-100 cm)
  • a twisted or untwisted wire pair cable (50-100 cm)
  • RF current probe (30-100 MHz)
  • balun or isolation transformer that works up to 100 MHz.
  • test board with matched 50-Ω microstrip and 100-Ω differential pair traces
  • snap-on ferrite cores (optional)

Procedure:

Step 1: Connect Port 1 of the VNA to one section of the coaxial cable. Connect the other end of the cable to the test board's microstrip trace. 

Step 2: Connect Port 2 to the RF current probe and place the probe around the first section of coaxial cable at a fixed and repeatable position. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (This will be a small value.)

Step 3: Disconnect the microstrip trace and connect the coaxial cable to the differential trace pair. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. The imbalance change at the connection creates a common-mode voltage that drives the board relative to the cable. A significant common-mode current should be detected.

Step 4: Place the balun on the Port 1 output of the VNA and use it to drive a section of the wire-pair cable. Connect the other end of the cable to the differential trace pair. 

Step 5: Connect Port 2 to the RF current probe and place the probe around the wire-pair cable at a fixed and repeatable position. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (This will be a small value.)

Step 6: Disconnect the differential trace pair and connect the wire-pair to the microstrip trace. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. The imbalance change at the connection creates a common-mode voltage that drives the board relative to the cable. A significant common-mode current should be detected.

Step 7: Try clamping a snap-on ferrite core around the cable at various positions. How much reduction in the common-mode current can be achieved this way?

Notes: 

This exercise is similar to the previous exercise using a test board instead of a second cable section. It has the advantage of illustrating the importance of maintaining balance (or imbalance) when connecting cables to boards. It has the disadvantage of requiring a specially made test board. 

The common-mode current is proportional to the differential-mode voltage at the connection as well as the change in the imbalance. Both the balun and the different termination impedances alter this voltage between test set-ups. This voltage difference is easily calculated and can be accounted for when comparing the common-mode currents in the different test configurations.