Analysis of three and two coupled lines and design of a planar coupled-line bandpass filter and of baluns suitable for MIC and MMIC

Abstract

Coupled line structures in inhomogeneous media are analyzed and applied in three circuits, a novel three-coupled line bandpass filter having sharp cutoff and two planar baluns composed of two sections of broadside-coupled lines. These are very suitable for microwave integrated circuits (MIC) and microwave integrated circuits (MMIC). Two quasistatic analyses, the spectral domain analysis and a variational technique, are applied to the three-coupled line structure for the first time and numerical data are presented. The spectral domain analysis is utilized to obtain results for two lines. A quasi-TEM analysis is applied to the three- coupled line structure and specific port terminations are applied to the resulting six-port impedance matrix to obtain the ABCD matrix parameters for a bandpass filter. By a similar quasi-TEM analysis, the generalized four-port scattering matrix of a two-coupled line structure is derived for the first time. The scattering matrix for two planar baluns are derived using the generalized scattering matrix. A design procedure for the planar Marchand balun based on the derived scattering parameters is presented. Experimental results from circuits designed with this theory are presented and are seen to agree well with prediction.