An Efficient Procedure for Narrowband Bandpass Filter Design

Bandpass filters (BPF) may be constructed by cascading lowpass (LP) and highpass (HP) filters. This type of realization is suitable for devices with wide bandwidths but is not convenient for the realization of narrowband BPFs, that is, for bandwidths of less than about 10 percent [1]. This is related to the high degree (number of poles and zeros) needed for the filters to achieve good selectivity. The cascaded type of realization causes large insertion losses and poor amplitude flatness. Cauer and generalized Chebyshev [2, 3] LP prototype filters are very good starting points for the design of selective BPFs. This article discusses the BPF network transformations suitable for physical realization [4, 5]. These transformations are based on the insertion of redundancy ideal transformers in BPFs and the use of Norton’s equivalent networks [6]. A proper choice of transformer transformation ratio produces a network with a minimum spread of element values. A new and efficient numerically-based procedure for getting an exact solution of optimal parameter t in closed form is presented in [5]. The lumped element network ultimately obtained has no transformers and is very convenient for the design of microwave filters, diplexers and multiplexers using printed circuit technology. This article also discusses the efficient transformation for the design of a narrowband bandpass filter with transmission lines. The use of suspended substrate techniques [1, 7] allows us to use highly selective prototypes, which can achieve excellent performances. The suspended substrate stripline (SSS) narrowband BPF design procedure is introduced by an example filter with excellent characteristics.