You betcha! Once upon a time I was in charge of EMC design for a large scale military digital telecommunications system. This system had to handle both red and black analog and digital data with the proviso that never the twain shall meet. In the lab where all the engineering tests were taking place everything went great until the cables were hung on the system with care. Then all through the lab radiated RF noise got much-much worse, so a special set of shielded cables were procured in hopes of solving the curse. Read this as: we spent a lot of money . . . management was unhappy! And sure enough things got worse. There are times when you just wish Murphy would stay home.
With some experimentation we were able to determine that the presence of the cable’s shields was the culprit that made things worse. High frequency common mode loop current from unbalanced differential mode signal harmonics were driving the cable shields making them behave like the antennas that they were. This is illustrated in Figure 1.
Briefly, the ground plane has impedance because of material resistivity and inductance, i.e. Z = R + XL. CM current flowing through the ground plane impedance produces a voltage drop Vab between the points a and b where the cable shields are connected to the grounding system. This voltage appears across the cable shields forming an antenna. This is illustrated in Figure 2.
This is not an easy problem to solve but there are some things that can be done to dramatically reduce the emissions.
1. Don’t generate any CM currents. . . yeah right! We already know how difficult it is to reduce these let alone eliminate them.
2. Reduce the amplitude of the CM current. Use lower power circuit designs, low power logic, LVDS, etc, and try hard not to have the CM source in the middle of a cable arrangement. In the configuration shown in Figure 1 the connection from c to b is in parallel with any unbalanced parasitic capacitance from the DM signal and its return. Eliminating this connection will reduce the common mode currents at the lower frequencies. If that’s not enough, the entire system can be floated but this must be designed in from the beginning.
3. Reduce the impedance of the ground plane. If the rack, frame, chassis, or enclosure forming the ground plane is made from steel, using higher conductivity materials such as aluminum and copper will certainly reduce the resistance. This will reduce the voltage drop at the lower frequencies. Unfortunately as the frequency increases the inductance becomes the dominant factor. Even a ground plane made from aluminum or copper will have increasing impedance with frequency but nowhere near that of steel or other materials with high permeability.
4. Move points a and b closer together. This will reduce both ground system R and XL by creating a single point ground (SPG) system and band limiting the CM currents at the lower frequencies.
5. At the higher frequencies conductor impedance can be higher than parasitic capacitance and the CM is reduced using low pass filters, generally in the form of series ferrites or CM chokes. The filter configuration depends on where the filters are placed.
Keep in mind that in the normal course of performing their intended function, power, control, and signal leads can and do act as transmit or receiving antennas for both CM and DM and they unintentionally couple signals through radiation to other collocated systems and equipment.
– Ron Brewer
