Quasi-Reflectionless Microstrip Bandpass Filters With Improved Passband
Flatness and Out-of-band Rejection
- Xiaohu Wu ,
- Yingsong Li ,
- Xiaoguang Liu
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.