Out-of-Band Multi-Notch Generation in RF Filters Through Parallelization

A simple technique for frequency-static multi-notch/transmission-zero (TZ) creation in RF filters with single- and dual-band bandpass transfer functions is presented. It is based on the direct connection of two identical inline filtering units without stopband TZs in an in-parallel configuration by means of unequal input/output transmission-line segments for each branch. Unlike in typical transversal-signal-interference and channelized filter architectures, the overall length of these connecting transmission-line segments for both branches is the same. Thus, the multi-notch/TZ generation in the whole filtering action is accomplished by differently distributing this total electrical length between the input/output transmission-line segments for the two branches. As such, enhanced selectivity in the filtering response with several embedded notches when compared to the filtering transfer function of an isolated branch can be obtained. To experimentally demonstrate the generality of this approach, two proof-of-concept filter prototypes corresponding to a 2/3-GHz microstrip dual-band bandpass filter (BPF) and a 62-GHz waveguide BPF are built and tested.