Abstract
In this paper, a new compact unit cell for a coplanar waveguide (CPW) lowpass filter (LPF) is proposed. By combining a pair of coupled parallel stepped impedance resonators (SIRs) and high impedance short stubs in the CPW line, a fifth-order elliptic lowpass filter unit cell (LUC) is designed. The extra transmission zero introduced by parallel coupled SIRs is used to extend the stopband and increase the roll-off rate. The characteristics of the proposed LUC is investigated to achieve a sharp roll-off and a wide stopband. The measured results are in accordance with the simulated results. It has an insertion loss less than 0.9 dB from dc to 6 GHz, and a wide -15 dB stopband from 7.5 to 18 GHz. In addition, the filter dimensions are as small as 4.9 mm × 8.7 mm, that is, 0.046λg2, where λg is the guided wavelength at the cut-off frequency. The filter structure is simple and easy to fabricate as well.
Highlights
Compact size and high performance lowpass filters (LPFs) are widely used in radio frequency (RF) and microwave front-end systems to suppress undesired harmonics and spurious frequency bands
The most conventional LPFs use a cascade of multiple unit cells to achieve a sharp roll-off and wide stopband response [1,2,3]
The seriesconnected parallel resonators in coplanar waveguide (CPW) technology can be implemented by open complementary split ring resonators (OCSRRs) [2,5], defected ground structure (DGS) [6,7] and by etching interdigital capacitances in the signal strip of the CPW with an electrical short in one of the fingers [1,8,9]
Summary
Compact size and high performance lowpass filters (LPFs) are widely used in radio frequency (RF) and microwave front-end systems to suppress undesired harmonics and spurious frequency bands. The design of an elliptic-function LPF unit cell (LUC) with sharp roll-off and wide stopband is the most effective method of obtaining a compact LPF with high performance. The slots on the signal strip and ground planes of the CPW were used to realize the third-order elliptic LUCs. The LUCs with different structures present the same cut-off frequency, but their performance suffers from narrow stopband and gradual roll-off. The EBG unit cell has the characteristics of an elliptic-function and the two transmission zeros in the stopband are used to enhance the rejection level of the stopband It presents relatively high radiation loss at high frequencies and gradual roll-off. An elliptic-function CPW LUC based on coupled parallel SIRs is proposed to obtain a compact size, sharp roll-off and wide stopband performance. The proposed LUC is fabricated on a low-cost FR4 substrate and experimentally verified with the measured results
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