CONVENTIONAL PARALLEL-COUPLED LINE FILTER
Parallel-coupled line bandpass filter was first proposed by Cohn in 1958  and has been considered as one of the most useful bandpass filters over the decades. Nowadays, although numerous advanced filter design techniques have been presented by the researchers, parallel-coupled line filter is still one of the most important filter topologies in many practical applications due to its matured design procedure and well-documented design parameters. In recent years, full-wave simulation tools were successfully developed to allow the automatic analysis and design of the parallel-coupled line filters with the user-defined specifications. In this chapter, the design procedure for the parallel-coupled line filters will be reviewed. In order to show a full picture of the filter development, we would like to start with the classic lumped-element lowpass filter prototype. After that, lowpass to bandpass transformation will be introduced. Finally, the lumped bandpass network is linked to the conventional parallel-coupled line implementation. The design formulas are all explicitly derived step-by-step to clearly describe a systematic design procedure.
3.2 LUMPED-ELEMENT LOWPASS FILTER PROTOTYPE
Most of the conventional lowpass, highpass, bandpass, and bandstop microwave filters are evolved and designed from an initial lowpass prototype filter through the standard procedure of transformation, impedance, and frequency scaling. ...