Ferrite thin film noise suppressor suitable for flexible printed circuit

Conducted and radiated noise suppression effects attained by the ferrite-plated thin film of 3 μm in thickness were evaluated using the mock samples of ferrite-plated flexible printed circuits (FPCs), in order to confirm a feasibility of its application to FPCs as a novel thin film noise suppressor. Transmission loss power ratio i.e. conducted noise suppression effect was evaluated from the measured S-parameters using the flexible microstrip line of 70 mm in length up to 6 GHz. The transmission loss power ratio attained by the ferrite film increased with increasing frequency, reaching approximately 4 dB attenuation enhancement at 6 GHz. This was almost the same as the commercially-available composite-type noise suppression sheet (NSS) of 50 μm in thickness. Radiated noise suppression effect was evaluated from 0.7 GHz to 5 GHz using the radiation suppression ratio, which was defined as the transmission parameter S₂₁ from the flexible open circuit of 1 m in length to the receiving antenna 3 m away from the circuit. The radiation suppression ratio suppressed by the ferrite film was approximately 10 dB in average at 0.7 GHz, which was effective up to 5 GHz. This was also equivalent to the composite-type NSS. These experimental results suggest that the ferrite-plated film is an effective noise suppressor for FPC, suppressing the noise currents and radiated emission up to several GHz.