Design of the Hilbert fractal defected ground structure low-pass filter

The conventional micro-strip low-pass filter has the shortages of large size, narrow stop-band and wide transition band. To solve these problems, the Hilbert fractal defected ground structure (DGS) micro-strip low-pass filter is designed in this paper. The filter is cascaded by the Hilbert fractal DGS units and shunt branches. The Hilbert fractal DGS unit has a smaller size and can reduce the pass-band reflection loss, and the shunt branch can restrain the higher order harmonic components effectively so the stop-band performance is improved greatly. The tested results show that the cut-off frequency is up to 4.5GHz, the insert loss is less than 0.5dB at the pass-band, and the VSWR is less than 1.5 at the frequency range of 4GHz below. The stop-band suppression is less than −20dB from 5.5GHz to 14GHz, the reduction is more than 10dB, and the filter size is 24.4mm 7.43mm. The goals of miniaturization, high selectivity and wide stop-band low pass filter have been achieved.