Digital Circuit Boards: Mach 1 GHz

5.16 Power Line Interference

Power line filters can keep some of the common-mode and differential interference from entering the hardware. In most cases, available filters do not function well below a few megahertz. The filtering reference conductor is usually the equipment grounding conductor that carries filter current back to the service entrance. If the equipment grounding path is inductive (long) then the filter cannot be effective. It is important to note that even with a good filter installation, some of the equipment ground current uses the racks, the signal conductors, and the cables that interconnect hardware to return to the service entrance. These currents represent field energy in the facility.

In a single phase power service, the neutral or grounded power conductor carries power currents for all the loads on a feeder and its branch circuits. This means that this conductor is not at the same potential as the equipment grounding conductor, which does not carry load current. The voltage impressed across a power transformer primary has some common-mode content caused by this neutral voltage drop. The content depends on the attenuation provided by the line filter. Filtering effectivity depends on whether every power conductor is filtered and how the filters are installed.

If there is a power transformer, there is a capacitance from turns on the primary coils to the secondary coils. Common-mode interference on the primary coil will flow through this capacitance to the circuits ...