CHAPTER FIVE
Lowpass and Bandpass Filters
Conventional microstrip lowpass and bandpass filters such as stepped-impedance filters, open-stub filters, semi-lumped element filters, end- and parallel-coupled half-wavelength resonator filters, hairpin-line filters, interdigital and combline filters, pseudocombline filters, and stub-line filters are widely used in many RF/microwave applications. This chapter presents examples of the illustrative designs of these filters.
5.1 LOWPASS FILTERS
In general, design of microstrip lowpass filters involves two main steps. The first is to select an appropriate lowpass prototype, such as one as described in Chapter 3. The choice of the type of response, including passband ripple and the number of reactive elements, will depend on the required specifications. The element values of the lowpass prototype filter, which are usually normalized to make a source impedance g0 = 1 and a cutoff frequency Ωc = 1.0, are then transformed to the L−C elements for the desired cutoff frequency and source impedance, which is normally 50 Ω for microstrip filters. Having obtained a suitable lumped-element filter design, the next main step in the design of microstrip lowpass filters is to find an appropriate microstrip realization that approximates the lumped-element filter. In this section, we concentrate on the second step. Several microstrip lowpass filters will be described.
5.1.1 Stepped-Impedance L-C Ladder-Type Lowpass Filters
Figure 5.1a shows a general structure ...
