Use of EM analysis to study shielding effects in microstrip circuits

This paper describes results obtained using electromagnetic (EM) analysis to investigate shielding effects in microstrip circuits. Questions considered include the following: 1) under what circumstances do the effects of a package with metallic top and side walls significantly affect the behavior of microstrip transmission line characteristics and discontinuity behavior?; and 2) are currently used "rules of thumb" sufficient for avoiding shielding effects? Shielding effects are not yet adequately accounted for in the linear nodal analyses of widely used microwave CAD software such as HP-EESOF Software's Libra or Compact Software's Microwave Harmonica. The common approach by designers is to try to avoid shielding effects by making sure that the package's cover and side walls are distanced from the circuit by three to five substrate thicknesses. The approach taken in this paper is to use the electromagnetic analysis software package "em" (available from Sonnet Software) as a tool in an attempt to answer the questions posed above. Confidence in em is gained by comparing em results to measured and independently obtained numerical data for an edge coupled bandpass four resonator filter. Shielding effects are then examined in selected microstrip examples.