CHAPTER NINE

Advanced RF/Microwave Filters

There have been increasing demands for advanced RF/microwave filters other than conventional Chebyshev filters in order to meet stringent requirements from RF/microwave systems, particularly from wireless communications systems. In this chapter, we will discuss the designs of some advanced filters. These include selective filters with a single pair of transmission zeros, cascaded quadruplet (CQ) filters, trisection and cascaded trisection (CT) filters, cross-coupled filters using transmission-line inserted inverters, linear-phase filters for group-delay equalization, extracted-pole filters, canonical filters, and multiband filters.

9.1 SELECTIVE FILTERS WITH A SINGLE PAIR OF TRANSMISSION ZEROS

9.1.1 Filter Characteristics

The filter having only one pair of transmission zeros (or attenuation poles) at finite frequencies gives much improved skirt selectivity, making it a viable intermediate case between the Chebyshev and elliptic-function filters, yet with little practical difficulty of physical realization [1–4]. The transfer function of this type of filter is

(9.1) Numbered Display Equation

where Ω is the frequency variable that is normalized to the passband cut-off frequency of lowpass prototype filter, ɛ is a ripple constant related to a given return loss LR = 20log |S11 in dB, and n is the degree of the filter. It is obvious that Ω = ±Ωaa> 1) are the frequency ...

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