MMR-BASED UWB BANDPASS FILTERS
The traditional filter theory was systematically established under the assumption of narrow passband, and it has been found very powerful in the design of microwave filters with various filtering performance. For a wideband filter, it was usually constructed by cascading a few transmission line resonators via enhanced coupling. However, this traditional approach requires extremely high-degree coupling of coupled-lines between two adjacent resonators, resulting in a narrow stopband between the desired passband and its first harmonic passband. Since the FCC releases the ultra-wideband (UWB) spectrum for unlicensed commercial applications in 2002, tremendous attention has been paid on proposal and implementation of a variety of advanced filters with desired UWB specifications. But, unfortunately, a lot of critical issues still puzzle researchers in many aspects, such as good in-band transmission, highly rejected skirt, wide out-of-band attenuation, multiple-notched bands, and so on.
The concept of a multiple-mode resonator (MMR) with stepped impedance configuration was originally presented in Reference 1. As discussed in Chapter 4, the first two resonant modes were excited and utilized together to form a passband with about 70% fractional bandwidth (FBW). Later on, in 2005, this work was extended to make a UWB bandpass filter with a FBW of about 109.5% . This initial UWB bandpass filter was built up using the microstrip ...