Page 299 - Complete Wireless Design

Filter Design

298 Chapter Six

7. Ground each stub section directly to the ground plane through a via at the
indicated end.
8. Optimize for good S and S , as well as for the desired bandwidth.
21 11
Low-pass filter (Fig. 6.45). A low-pass filter at microwave frequencies can be
designed by using the distributed equivalent in microstrip. Design a lumped fil-
ter, and convert these lumped reactive values to distributed equivalent compo-
nents (see Sec. 1.3.2, “Microstrip as equivalent components”). However,
converting from a lumped LPF into an equivalent distributed LPF can be diffi-
cult because of the excessively low or high line impedances required to meet a cal-
culated lumped filter’s equivalent reactance, while remaining with a line length
shorter than 30 degrees for decent filter realization. This can be overcome by
designing a hybrid filter: a combination of lumped and distributed parts.

High-pass filter. A pure distributed high-pass filter is not easy to design. This
is because of adjacent coupling when a simple equivalent circuit is employed,
plus the added complication of having to use the series lumped capacitors that
are needed in such a structure.

6.2.4 Distributed filter issues
Element collisions occur when the distributed microstrip elements become too
close together in the filter’s distributed design, and can actually touch or over-
lap. A different layout or dielectric constant would then be indicated.
Many distributed filters will have odd and even bandpass returns (reen-
trance) that will reduce the stopband attenuation to very poor levels at vari-
ous frequencies. Some distributed filters that integrate lumped components,
such as the combline microstrip/capacitor filter, will have less of these har-
monic passbands within their stopbands. This same effect will also occur with
low-pass and stopband filter structures. Reentrance with half-wave filter
types occurs at every other harmonic.

Figure 6.45 Lumped low-pass filter structure and distributed equivalent.

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