A COMPACT TRIPLE-BAND PASSBAND FILTER USING MULTI-STUB LOADED RESONATOR AND ASYMMETRICAL SIRS

Abstract

This letter presents a compact triple-band passband filter with high frequency selectivity. The first and second passbands are formed by a multi-stub loaded resonator. The bandwidth of the second passband can be independently adjusted by tuning the coupling strength between the two loaded open stubs. The third passband is implemented using two asymmetrical stepped impedance resonators (SIRs) with a tapped line structure as external coupling form. The asymmetrical SIRs also serve as one part of the external coupling structure of the multi-stub loaded resonator so as to achieve a compact circuit size. Additionally, a source-load coupling is introduced, thus, multiple transmission zeros are generated to improve the frequency selectivity. After a detailed introduction of the operation principle, a triple-band passband filter with centre passband frequencies of 1.5, 2.6 and 3.6 GHz is designed and fabricated. The measured results verify the effectiveness of the proposed filter. As key components of the multi-band wireless service system, the multi-band passband filters have been extensively studied for past decades. They should have a compact size, flexible design freedom, high frequency selectivity and sufficient suppression of the parasitic harmonic modes. Many approaches have been utilized to implement multi-band filters. The most common method is based on SIRs (1-3), whose first two resonant modes can be controlled by properly selecting the relevant impedance or strip width ratio. But this method often suffers from a large circuit size and limited design freedom. The stub loaded multi-mode microstrip resonator (4-7) and the degenerate modes of perturbed ring resonators (8- 14) can also be utilized to construct multi-band filters. In (15), the multi-stub loaded resonator was used to implement a dual-band passband filter with independently controlled bandwidth by inserting a half-wavelength resonator between the loaded open stubs so as to introduce two signal paths. The transmission zeros generated by the direct source-load coupling improved the isolation between the passbands greatly and suppressed the parasitic harmonic modes to achieve a wider upper stopband. Following (15), in this letter, a compact triple-band passband filter with high frequency selectivity is proposed, whose first two passbands are implemented using the multi-stub loaded resonator in (15). The third passband is formed by using two asymmetrical electronically coupled SIRs. The SIR has a tapped line structure as the external coupling form. Simultaneously, as one part of the input/output structure of the multi-stub loaded resonator, the SIR not only accomplish the external coupling of the multi- stub loaded resonator, but also can introduce source-load coupling to improve frequency selectivity. Thus, the multi-stub loaded resonator combined with two SIRs achieves triple-band passband filtering function, while the circuit size does not increase considerably. Structure and design of the filter are in detail introduced in the remainder. A triple-band passband filter with center frequencies of 1.5, 2.6 and 3.6 GHz is fabricated and measured. The measured results verify the effectiveness of the proposed filter.