In this paper, the suppression of induced voltage on a printed wiring board through impedance loading by inserting impedance devices such as ferrite beads is focused on. How the suppression effect changes according to the insertion position of such devices is also investigated. Electromagnetic-field simulations were used to determine the distribution of voltage and current induced in wiring when a printed wiring board is exposed to an external electromagnetic field. Then, on the basis of these distributions, electromagnetic-field simulations were performed, and experiments were conducted to investigate the relationship between the insertion position of impedance devices and their suppression effect. It was verified that induced voltage can be large when a mismatch occurs between the impedance at the two ends of printed wiring, and that the suppression effect can differ significantly according to where an impedance device is inserted. A large effect was obtained by inserting an impedance device at a point 1/4 wavelength in distance from the end of a wire where voltage is being induced. In addition, comparing the use of resistors with the use of chip ferrite beads as impedance devices revealed similar tendencies in both. The above behavior was confirmed by numerical analysis.

Attempts have been made to evaluate and investigate the radiated emissions from fiber optical modules that are currently available in the market. Far electric field strength measurements show that the radiated emission has a peak at a high-order harmonic wave of the fundamental pulse frequency and reaches a level exceeding the limiting values of the CISPR noise specifications. Near magnetic field distribution measurements show that the source of the interference noise lies between a light emitting diode (LED) module and an LED driver. These measurements are compared with those of electromagnetic field calculations based on a high-frequency equivalent circuit. As a result, it was established that both the peaking effects of deformed pulse waves transmitted between an LED module and an LED driver and the radiation characteristics of the optical transmitter circuit act as factors for increasing the radiation level of the peak frequencies in the radiated emission from fiber optical modules.