High-speed, highly sensitive devices are packaged in a shielded module against electromagnetic interference (EMI). However, the highly reflective metals used to shield external incident EM power cause internal EMI through multiple reflections, and densely packed devices such as high-gain amplifiers and signal generators, as well as impedance mismatching in printed circuit board (PCB) design, radiate undesired EM power. In this study, the internal EMI was analyzed using in-situ measurements with paired 6 GHz-band 30 dB SiGe low-noise amplifier (LNA) modules incorporated in an aluminum die-cast (ADC) mold, a type of metal package. The measured S-parameter transmission coefficient, S₂₁, revealed an extraordinary phenomenon due to resonant oscillation caused by multiple reflections of the radiated EM power from the LNA output, and S₂₁ fluctuations caused by radiated EM power from another LNA. The resonant oscillation frequency depended on the mechanical dimensions of the ADC package and on the metal lid reflectivity. The resonant oscillation phenomenon was theoretically analyzed using equations for the signal power and phase changes through multiple reflection passes. The authors designed a triple-layer sheet of EM wave shielding (EMWS) /Metal (M) /EMWS, and experimentally confirmed its complete effectiveness in shielding both internal and external EMI.
Chinen et al. (Sun,) studied this question.