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. 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 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. These transformations are based on the insertion of redundancy ideal transformers in BPFs and the use of Norton's equivalent networks.
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