TABLE OF CONTENTS This chapter is the first of several which apply the principles covered in earlier chapters to the development of practical filter structures for specific requirements. When necessary, new theories or techniques are introduced which are specific to the structure being considered.
The L-C lowpass is a direct application of synthesized prototypes and poses the fewest implementation difficulties of all filter structures. Ideally the same would be true for the distributed lowpass because the synthesis is based on the conversion of L-C filters. Also there is the potential for tighter tolerance on element values. However, difficulties are introduced by the unique characteristics of distributed elements such as reentrance, discontinuities and the realizable range of line impedance.
In this chapter, distributed lowpass filters are studied. The effects of these limitations are considered along with potential methods of mitigating these difficulties.
The stepped-impedance lowpass is a cascade of alternating high and low impedance transmission lines. The high impedance lines act as series inductors and the low impedance lines act as shunt capacitors. This filter structure was used as an example to introduce CAE techniques in Chapter 6. The schematic and a microstrip pictorial are given in Figure 6-2. Chapter 6 contained an example of a lowpass with a 2.25 GHz cutoff and 0.177 dB passband ripple.
In this section a microstrip stepped-impedance 7th order Chebyshev lowpass with a cutoff frequency of 900 MHz and 50 ohm terminations is considered. The nominal substrate dielectric constant is 6.0 with...
