Abstract
High-order quasi-reflectionless bandpass filters with improved passband flatness and good impedance matching both in-band and out-of-band are proposed in this work. The proposed design consists of conventional coupled-lines bandpass sections loaded with the presented absorptive stubs at the input and output. Analysis shows that the absorptive stub is equivalent to a 2-pole bandstop filter. Compared to the prior art, the higher-order nature of the presented absorptive stub enables a flatter passband and better out-of-band rejection. The overall filter stopband attenuation can be readily improved by increasing the number of coupled-lines sections without altering the passband responses. Furthermore, cross-coupling between the two absorptive stubs can be used to improve the out-of-band rejection by introducing two transmission zeros without affecting the absorption characteristics. The proposed design concepts are experimentally validated by the design and fabrication of a set of 2.4-GHz 1-, 2-, and 3-pole microstrip quasi-reflectionless bandpass filters. Measured frequency responses of these filters closely match those of the simulation.
Highlights
As fundamental building blocks in radio frequency (RF) and microwave systems, filters are used to better define the system bandwidth or reject undesired signals [1]
ABSORPTIVE COUPLED-LINES we provide a detailed analysis of the proposed absorptive coupled-line which consists of a conventional coupled-line section and an absorptive stub loaded at one of the open ends of the coupled-line
High-order distributed bandpass filters with all-band quasireflectionless and flat passband transmission have been demonstrated in this paper
Summary
As fundamental building blocks in radio frequency (RF) and microwave systems, filters are used to better define the system bandwidth or reject undesired signals [1]. Based on the reflectionless lowpass prototypes, distributed first-order and higher-order reflectionless filters are derived in [19], [20] and [4], respectively In these designs, only the order of the absorptive section circuit is increased whereas a single bandpass resonator is used in the main channel. High-order quasi-reflectionless filters with improved passband flatness and out-of-band rejection, as shown in Fig. 1 (c), are presented using the proposed absorptive stub section [Fig. 2 (c)]. Analysis shows that this structure absorbs the stopband reflection and the close-to-passband signals, improving the passband roll-off and close-in rejection.
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