Miniaturisation and electronic tuning techniques for microstrip spurline filters

A new technique for the miniaturisation and electronic tuning of microstrip spurline filters is presented. The technique is based on connecting a reactive component across the end of the coupling gap of the spurline. The circuit is analysed by using a general coupled-line model, leading to the conclusion that, if a capacitance is used, the filter can be miniaturised or electronically tuned. The analysis leads to a set of simple design equations for these applications and a range of simulations are used to investigate the technique further. The validity of the techniques is demonstrated with three different practical circuits. For a miniaturised spurline filter operating at 1.8 GHz, 50 length reduction is achieved. Two varactor-tuned designs are described: a single-section and a cascaded spurline filter. The single-section spurline can be tuned across almost one octave bandwidth, from 580 MHz to 1.06 GHz whereas the cascaded design achieves tuning range of 320-723 MHz.