Service manual

Description: Functional Theory of Operation 1-7
1.3 Functional Theory of Operation
The following functional theory of operation provides an overview of the base station/repeater
circuitry.
1.3.1 Transmitter Circuitry Operation
1.3.1.1 Introduction
The Transmitter Circuitry comprises two modules, the Exciter Module and the Power Amplifier (PA)
Module. These modules combine to generate, modulate, and amplify the RF signal which is
transmitted via the site transmit antenna.
1.3.1.2 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 Voltage-Controlled Oscillator (VCO) circuitry on the Exciter Module
accept frequency programming data from the SCM via the Serial Peripheral Interface (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.
See Figure 1-5 for the Exciter block diagram.
1.3.1.3 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 frequency band. All PA modules
contain a driver and final RF amplification stage, a low-pass filter/directional coupler at the output,
and diagnostic and power control circuitry.
The PA modules employ a single internal circulator to protect the PA from transmitter intermodulation
and antenna mismatch (VSWR). They are broadband devices and require no tuning to operate at the
base station/repeater site.
The modulated RF signal from the Exciter Module is delivered to the PA Module and amplified to the
specified output power via the driver and final amplification stages. The gain of the driver stage is
controlled by a control voltage which is derived from power control signals from the SCM and high
VSWR/thermal protection circuitry on the PA output board.
A combination of hardware and software controls are used to regulate the power output level. To set
the power and current limits, the SCM provides software control through a D/A converter connected
to the SPI bus. This control relies on various monitored PA signals which are fed back to the SCM via
an A/D converter (also connected to the SPI bus).
The directional coupler is essentially a calibrated wattmeter which feeds a DC voltage proportional to
the output power to the power control circuitry to serve as the feedback signal in the power control
loop. Under normal operating conditions, the power control circuitry compares this DC voltage from
the directional coupler to a reference voltage from the D/A converter which represents the desired
output power. Based on the comparison, a power control voltage is generated to control the output
power from the PA Module.