Chapter 3 Lab Exercise: Electric-Field Coupling in a Ribbon Cable

spectrum analyzer connected to a current probe around the center of an unconnected wire

Two circuits sharing a 50-cm length of ribbon cable exhibit measurable levels of all three types of EM coupling. This lab exercise could be combined with the common-impedance and magnetic-field coupling exercises that use almost the same set-up.

Preparation:  Students should be provided with the dimensions of the ribbon cable and parameters of the test set-up before coming to the lab. They should calculate the expected value of the electric-field coupling before making any measurements. (This could be done as part of an earlier homework assignment.)

Equipment Required:

  • waveform signal source (100 Hz to 10 MHz)
  • 2-channel oscilloscope
  • 50-cm length of ribbon cable with at least 3 wires

Procedure:

Step 1: Connect the RF source to Channel 1 of the oscilloscope through wires 1 and 2 of the ribbon cable. Wire 1 should be connected to the grounded side of both instruments. This time, set the input impedance of Channel 1 to be 1 MΩ instead of 50 Ω.

Step 2: Channel 2 of the oscilloscope should be connected to measure the voltage induced on wire 3. The ground side of Channel 2 should be connected to wire 1 and the signal side to wire 3. Set the input impedance of Channel 2 to be 50 Ω.

Step 3: Leave the opposite side of wire 3 unconnected. Explain why doing this prevents Channel 2 from picking up any common-impedance coupling or magnetic-field coupling.

Step 4: Starting at 100 Hz, increase the amplitude of the source until there are measurable sinusoidal waveforms on both oscilloscope channels. Channel 1 will have a much higher amplitude than Channel 2. 

Step 5: Determine the ratio of the Channel 2 voltage to the Channel 1 voltage. How does this compare to calculated value of the electric-field coupling? 

Step 6: Repeat the measurement above at 1 kHz, 10 kHz, 100 kHz, 1 MHz and 10 MHz. Compare to the calculated values of electric-field coupling at those frequencies. Is the agreement good? Is the coupling proportional to frequency? If not, why not? 

Step 7: Change the shape of the signal from a sine wave to a square wave. Describe the signal observed in Channels 2. Is this consistent with the expectation for electric-field coupling?

Notes: 

If it is difficult to measure the electric-field coupling at 100 Hz, try using the 1-MΩ input for Channel 2.

If the ribbon cable has more than 3 wires, these measurements can be repeated with wires that increase the shared loop area. For example, instead of using wires 1,2 and 3, the measurements can be made using wires 1, 8 and 9. Since electric-field coupling depends on the mutual capacitance and not on the loop area, the results will be nearly identical.