Size reduction and harmonic suppression of parallel coupled-line bandpass filters using defected ground structure

A novel miniaturized parallel coupled-line bandpass filter with suppression of second, third and fourth harmonic frequencies, is demonstrated in this paper. The new filter is based on the slow-wave effect of the defected ground structure (DGS) to achieve size minimization, while the spurious responses are eliminated by the band-rejection property. These features offer the classical parallel coupled-line bandpass filter simultaneous compactness and wide stopband performance. Using DGS does not require the filter parameters to be recalculated and, this way, the classical design methodology for coupled-line microstrip filters can still be used. The simulations and measurements of a 2.0 GHz prototype bandpass filter are presented. The measured results agree well with the simulation data. Compared with the conventional parallel coupled-line bandpass filters, the second, third and fourth measured spurious responses are suppressed to -45, -43 and -34 dB, respectively. In addition, the size of the prototype filter circuitry can be reduced up to 20%.