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MORE COMPLICATED ELEMENTS OF FEED LINES 415

8.4 ANALOGUE FILTERS

8.4.1 Overview
Feed line filters are the most vital component of RF systems letting not only control signals’
shape but protect them against undesired and damaging interference. Filters may come in
many forms, sizes, and response types. Despite this and with rare exceptions, their
development is based on the equivalent classical circuits like illustrated in Figure 8.4.1. Real
equivalent circuits might be much more complex and include among everything else the
parasitic elements, mutual coupling between elements, etc. Our task here is to show how to
transform the synthesized lumped prototype into real microwave filter with minimal
parameter regression. An extensive literature is devoted to this and related topics; we refer
the reader to [8 - 10] for more details advising to use the references there and internet search
for deeper study. Meanwhile, the feed line filter technology practically reaches a level of
state-of-art perfection, and the numerous computer-aided tools let completely automate the
whole process starting from specification and sensitivity analysis to PCB layout and the
12
subsequent production. Note that Nuhertz Technologies offers several computer-based
tools linked to CST [11] as well as a free version of Filter Solutions. Such programs can
handle active and passive filters, lumped-element RF filters, and digital filters.

Figure 8.4.1 Lumped filters and their idealized amplitude transfer function

12 http://www.nuhertz.com/