Functional Theory Of Operation; Transmitter Circuitry Operation - Motorola MTR2000 Installation And Operation Manual

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 Pow-
er Amplifier (PA) Module. These modules combine to generate, modulate, and am-
plify 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 amplifi-
cation. The circuitry operates as follows.
The transmit synthesizer and VCO (voltage-controlled oscillator) circuitry on the Ex-
citer 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 levels. The actual circuit stages employed in a PA depend on the specific fre-
quency band, power output level and intermodulation requirements. All PA mod-
ules contain an Intermediate Power Amplifier (IPA) at the input, a low-pass
filter/directional coupler at the output, and diagnostic 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 module 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 Mod-
ule, 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 – 100W
PA models). In PA Modules operating above 600 MHz, a 15 W driver is introduced
Description
5