RF and Microwave Circuit and Component Design for Wireless Systems

CHAPTER SIX

Filters and Couplers

6.1 FILTERS

A passive microwave filter is a circuit component consisting of lumped elements (inductors, capacitors, and resistors) only or distributed elements (waveguide sections or microstrip or finline or any other medium) or both, arranged in a particular configuration (topology) so that desired signal frequencies are allowed to pass with minimum possible attenuation while undesired frequencies are attenuated. The microwave filters that are described in this section are two-port reciprocal, doubly terminated, passive, linear, reflective, and lossy. In the design of a filter, the important specifications one looks into are frequency range, bandwidth, insertion loss, stopband attenuation and frequencies, input and output impedance levels, voltage standing-wave ratio (VSWR), group delay, temperature range, and transient response. This section describes filter basics, design methods and design examples.

6.1.1 Filter Parameter Definition

A filter may be viewed conceptually as shown in Figure 6.1. At the input plane of the filter, the power may be broken into three components: P_in, the incident power from the generator; P_R, the power reflected back toward the generator; and P_A, the power absorbed by the filter. The power passed on to the load R_L is P_L. Conservation of energy demands that

where P_L = P_A if the filter is lossless, and P_L = P_in if the ...