Radiated emission from a PCB with an attached cable resulting from a nonzero ground plane impedance

In this paper, radiated emission from a PCB with an attached cable resulting from a nonzero ground plane impedance is studied experimentally and with FDTD modeling. Frequency responses of the CM current, the electric field near the PCB, and the electric far field are investigated. First, the effect of a thin wire to mimic an interconnected ground wire is discussed. Comparing the cases with and without the thin wire, the difference is only at the resonance frequency, and there is no change on the magnitude of the CM current. Second, the effect of a termination on EMI coupling-path that results from the nonzero ground plane impedance is discussed. When the terminating resistor is smaller than the nonzero impedance, EMI antenna is driven as current-driven. Therefore, for relatively low impedance load such as 51 Ω, EMI antenna is driven as voltage-driven at lower frequency. As the terminating resistor is larger, the CM current becomes larger at lower frequencies (10∼100 MHz). This indicates that a high impedance load such as a logic IC results in larger radiation at lower frequency.