Quantitative estimation for electromagnetic field noise from PCB with switching transistor

To estimate the electromagnetic noise by low frequency switching operation, the spatial distributions of the electric and magnetic field around PCB with switching transistor are calculated by FDTD method implemented the switching transistor and measured. The size of the PCB is 225 mm long, 60 mm wide and 0.51 mm thick. The switching transistor (2SC2671) is operated by 3 MHz clock pulse. The simulated and measured results of the distribution on the PCB are good agreement. Even if the output impedance of the switching transistor is varied by switching operation, the standing wave on the PCB is observed. The spatial distribution in the y-direction into the 15 cm from PCB is not uniform due to the resonance of the signal line. From the quantitative estimation of the electromagnetic noise by switching operation is dominant to low impedance at on state. The noise radiation when the switching transistor turned on from off is problem.