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3
FUNCTIONAL THEORY OF OPERATION
The following functional theory of operation provides an overview of the station circuitry. For a more thorough functional
description of a particular module, refer to the STATION MODULES section of the appropriate band-specific Instruction
Manual. The block diagram in Figure 3 supports the following functional theory of operation.
Transmitter Circuitry Operation
68P81096E36-H
06/28/05
Introduction
The Transmitter Circuitry comprises two modules, the Exciter Module and the Power Ampli-
fier (PA) Module. These modules combine to generate, modulate, and amplify the rf signal
which is transmitted via the site transmit antenna. Modulation sensitivity and power output
are adjusted electronically for each channel (through the Radio Service Software), under the
direct control of the Station Control Module (SCM).
Exciter Module Operation
The Exciter Module, which interfaces directly to the SCM, generates a modulated rf signal at
the desired transmit frequency and sends this signal to the PA for amplification. The circuitry
operates as follows.
The transmit synthesizer and VCO (voltage-controlled oscillator) circuitry on the Exciter
Module accept frequency programming data from the SCM (via the SPI bus) and generate an
rf carrier at the specified frequency. The VCO is directly modulated by transmit audio/data
from the SCM. The resulting modulated rf signal (at a level of approximately +12 dBm) is
then fed to the PA.
Power Amplifier Module Operation
The PA modules are designed for continuous-duty operation across all bands and power lev-
els. The actual circuit stages employed in a PA depend on the specific frequency band, power
output level and intermodulation requirements. All PA modules contain an Intermediate Pow-
er Amplifier (IPA) at the input, a low-pass filter/directional coupler at the output, and diag-
nostic and power control circuitry.
High power (100 W) PA modules employ a single internal circulator to protect the PA from
transmitter intermodulation and antenna mismatch (VSWR). The low power 30 W PA mod-
ule employs two internal circulators. All PA stages and circulators are broad-band devices
and require no tuning to operate at the station site.
The modulated rf signal from the Exciter Module is input to the IPA in the PA Module, and
amplified to within a range of 0 to 10 W (depending on power control signals from the SCM).
In PA Modules operating below 600MHz, the rf signal is fed to either a Butterfly Module
(30W/40W PA models) or a Dual Device Module (DDM – 100 W PA models). In PA Mod-
ules operating above 600 MHz, a 15 W driver is introduced between the Pre-driver stage and
the Final Module. The gain of the Pre-driver stage is controlled by a power control voltage
Description
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