Size Reduction and Harmonic Suppression of Parallel Coupled-Line Bandpass Filters Using Defected Ground Structure

A novel miniaturized parallel coupledline 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 of the DGS unit. These features offer the classical parallel coupled-line bandpass filter simultaneous compactness and wide stopband performance. Using the proposed DGS unit, does not require the filter parameters to be recalculated and, this way, the classical design methodology for microstrip parallel coupled-line filters can still be used. As an example, a 2.0 GHz parallel coupledline filter has been designed and measured in order to show the validity of the proposed DGS. Compared with the conventional parallel coupledline 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 is reduced by 20% compared to that of the conventional parallel coupled-line filter.