Lowpass and Bandpass Filters Conventional microstrip lowpass and bandpass filters such as stepped-impedance filters, open-stub filters, semilumped 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. It is the purpose of this chapter to present the designs of these filters with instructive design examples. | Microstrip Filters for RF Microwave Applications. Jia-Sheng Hong M. J. Lancaster Copyright 2001 John Wiley Sons Inc. ISBNs 0-471-38877-7 Hardback 0-471-22161-9 Electronic CHAPTER 5 Lowpass and Bandpass Filters Conventional microstrip lowpass and bandpass filters such as stepped-impedance filters open-stub filters semilumped 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 mi-crowave applications. It is the purpose of this chapter to present the designs of these filters with instructive design examples. LOWPASS FILTERS In general the design of microstrip lowpass filters involves two main steps. The first one 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 il . are then transformed to the L-C elements for the desired cutoff frequency and the desired source impedance which is normally 50 ohms 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 realizations will be described. Stepped-Impedance L-C Ladder Type Lowpass Filters Figure a shows a general structure of the stepped-impedance lowpass microstrip filters which use a cascaded structure of alternating high- and low-impedance transmission lines. These are much shorter than the associated guided- 109 110 LOWPASS AND BANDPASS FILTERS a a General structure of the stepped-impedance lowpass .
