Page 232 - Analog and Digital Filter Design

Impedance Matching Networks 229

Graphs of the normalized frequency response for several designs were given in
Chapter 3; these can be used to find the required filter order.

Each filter section in a diplexer requires high input impedance outside its pass-
band. This is in order not to absorb any incident power. All the incident power
should pass through only one filter section (ignoring passband edge effects). This
condition is necessary because it gives the minimum loss and also the correct
termination impedance for the source.

Diplexer filter sections also need to have high output impedance outside their
passband. The reason for this is to prevent incorrect load impedance from affect-
ing either the frequency response or the input impedance. This is important
because it is likely that at least one filter section will have a load that is not
matched outside its passband.

You now need to realize a diplexer filter section having both high input
impedance and high output impedance outside its passband. This must be an
odd-order filter with series connected components at either end. As an example,
a third-order lowpass/highpass diplexer will have a lowpass section with series
L, shunt C, and series L components; the highpass section will have series C,
shunt L, and series C components, as shown in Figure 8.5.

Lowpass
Rs L1 L3

I
Source Load 1

Figure 8.5 Load 2
Thire-Order Lowpass/Highpass
Diplexer I I I

Diplexers need filter sections designed for zero source impedance. The source
impedance is usually considered to be part of the filter, like inductors and capac-
itors. The frequency response of a filter usually depends on the resistance of the
source to achieve a correct output response. If the input impedance of a filter
rises so does the input voltage, because the source is no longer loaded and there
is no voltage drop across the source impedance. However, the response of the
filter is correct because the source impedance was taken into account in calcu-
lating component values.