Page 366 - Complete Wireless Design

Support Circuit Design

Support Circuit Design 365

Figure 8.35 A lumped 90 degree directional coupler.

Figure 8.36 A 50-ohm resistive wideband splitter/combiner.

8.7 Power Supplies
8.7.1 Introduction
Most wireless communications equipment runs off DC power supplies that
obtain their energy from the AC mains. This is because batteries will furnish
current only for a limited period of time, and so are reserved for equipment
that will not normally be near AC—such as most portable devices. AC main
voltage is converted by a power supply into the required DC levels needed by
the system in order to provide power at a constant and regulated voltage.
The basic power supply is shown in Fig. 8.37. It consists of a two- or three-
pronged plug, a transformer, a rectifier, a low-pass filter, and a regulator. The
transformer changes the AC main voltage of 120 VRMS into any desired volt-
age, either up or down; the bridge rectifier circuit turns the AC into a pulsat-
ing DC; the low-pass filter converts the varying DC into a steady DC; the
three-terminal regulator maintains the output voltage within tight specifica-
tions. C suppresses any output oscillations, and helps in regulation, with R
3 B

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