Compact multi-band bandpass filter with independently controlled passbands using CRLH-TZRP

In this paper, a compact multi-band bandpass filter is introduced based on the composite right/left-handed transmission zero resonator pairs (CRLH-TZRP) concept. The proposed filter is designed by allocating transmission zeroes (TZs) produced by the concept to allow emergence of the bandpass. Instead of a conventional λ/4 uniform-impedance resonator, a composite right/left-handed (CRLH) resonator is utilized to produce TZs, allowing for the proposal of different types of filters. Each CRLH resonator induces two inherent TZs and one transmission pole (TP). By combining two of CRLH resonators, a CRLH-TZRP is achieved, which is composed of four TZs and three TPs. The proposed filter is attained with two CRLH-TZRP together with proximity coupling. The performance of each passband can be independently adjusted by carefully choosing transmission zeroes. The designed filter has good out-of-band rejection and high selectivity. The proposed method can be applied to a microstrip structure. This structure has a small size, low cost, and feasible fabrication. For the validity of this concept, a microstrip filter has been designed and investigated for Wi-Fi, WiMAX, and 5G applications at 2.4, 3.5, and 4.9 GHz, respectively. It is implemented on a substrate with a relative dielectric constant of 10.8 and thickness of 0.64 mm, with an overall compact size of 10.4 × 10.8 × 0.64 mm3. The filter prototype has been fabricated and tested. The experiment results agree well with the simulation results.