Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 1, NO. 1, MAY 1989

The design of filters is a specialized area of microwave engineering that is a daunting first experience. However, when carried out in a step by step basis, it can be handled in a straightforward manner. This article shows the steps to designing a waveguide band-pass filter using impedance inverters, along with practical results.

The Low-Pass Prototype

First, we calculate the number of sections required. Several parameters are used to characterize a filter s performance. The starting requirement most often specified is frequency response specification. To meet these requirements, we transform the required response to a normalized low-pass specification with a cutoff frequency of 1 radian/second.

This normalized response curve may be compared with curves for low-pass prototype filters, which also have 1 rad/sec cutoff. From these curves, a prototype filter of the proper order (number of sections) may be designated. Curves for the normalized frequency vs. amplitude response have been tabulated for many families of filters, each having auxiliary characteristics which influence the design choice. Reference 1 gives perhaps the clearest curves. The tabulated response families and their characteristics are:

• Butterworth, also known as maximally flat

• Chebyshev, equal-ripple amplitude response in pass-band

• Gaussian, linear or equal-ripple delay response

• Legendre, least-squares amplitude response

• Elliptic, equal ripple amplitude response in pass-band and stopband