Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 10, NO. 3, APRIL 1998

A bandstop filter using multilayer microstrips was designed to operate around f _c=1840 MHz and reject unwanted carrier frequencies in the IF processing unit of a Digital Communication System (DCS). Its design involves a double-layer microstrip resonator coupled to a microstrip line. This coupler was modeled and the filter s performance was predicted. The filter was then constructed and its performance measured. This article describes the design method and the results of those experimental measurements.

Parallel Stop for Bandstop Effect

Figure 1 shows a microstrip line with a parallel open stub, which brings about a stopband effect around some frequency f _c . In MICs, where space is restricted, one might prefer to have a somewhat more compact structure. One possibility is to rotate the open microstrip and place it on top of the access line, as shown in Figure 2, with an additional layer of substrate between them. Broadside coupling has been used for obtaining tight coupling between microstrips, with a third cover layer for improved directivity [1]. Directivity is not critical in case of the bandstop filter in Figure 2. The structure in Figure 2 was selected for use as a bandstop filter. Using Malherbe s model [2] and first assuming a homogenous cross-section as in Figure 3 for this coupler, a...