Page 226 - Analog and Digital Filter Design

8
CHAPTER

IMPEDANCE MATCHING NETWORKS

Impedance matching networks are used to ensure that circuits have the correct
load. This is particularly important for transmission lines carrying radio fre-
quencies because an incorrect load will cause some of the signal power to be
reflected towards the signal source. Reflected signals combine with forward
transmitted signals to produce standing waves along the transmission line. The
peak amplitude of these waves divided by their minimum amplitude gives the
Voltage Standing Wave Ratio (VSWR). Ideally there will be no reflected power
and the VSWR equals 1.
Impedance matching is also important for active and passive components
in a system. For example, passive filters should have the correct load imped-
ance. Otherwise the filter will not have the correct frequency response.
Also, active components must have the correct load impedance to prevent
instability.

Power Splitters and Diplexer Filters

Wideband radio frequency (RF) sources often pass through a power
splitter, such as when several receivers share a common antenna. Splitters
can be transformer-coupled or resistive. Transformer-coupled splitters use an
impedance matching step-up transformer combined with a center-tapped
autotransformer. Resistive splitters comprise three resistors in a star or delta
arrangement.

Both transformer-coupled and resistive splitter designs have the disadvantage
of producing a loss in each signal path. The insertion loss is 3 dB in the case of
a transformer-coupled two-way splitter, and 6dB in the case of a resistive split-
ter. Both types of splitter allow a wide band of frequencies to be transmitted.
The transformer-coupled splitter is usually limited to about a decade frequency
range. The resistive splitter works over a wider range frequency range, which
can be from DC to beyond a Gigahertz.