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, and canonical filters | Microstrip Filters for RF Microwave Applications. Jia-Sheng Hong M. J. Lancaster Copyright 2001 John Wiley Sons Inc. ISBNs 0-471-38877-7 Hardback 0-471-22161-9 Electronic CHAPTER 10 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 extract-ed-pole filters and canonical filters. SELECTIVE FILTERS WITH A SINGLE PAIR OF TRANSMISSION ZEROS 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 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 S21 W . .-. V10-io- 1 F Q coshj n. - 2 cosh il cosh-1 where il is the frequency variable that is normalized to the passband cut-off frequency of the lowpass prototype filter e is a ripple constant related to a given return on - n nan ni il. -o cosh il. o j 315 316 ADVANCED RF MICROWAVE FILTERS loss Lr 20 log Sn in dB and n is the degree of the filter. It is obvious that il -a a 1 are the frequency locations of a pair of attenuation poles. Note that if fta the filtering function Fn ft degenerates to the familiar Chebyshev function. The transmission frequency response of the bandpass filter may be determined using frequency mapping as discussed in Chapter 3 . ft 1 . -2L _ FBW m0 w in which co is the frequency variable of bandpass .
