Broadcast Electronics AM-2.5E, User Manual

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1-4
1-9. Exciter Circuit Board. The ECU exciter circuit board is a modular plug-in exciter assem
bly. Instrumentation amplifiers provide balanced left and right channel transformerless
audio inputs. The exciter carrier frequency is established by a digital frequency synthesiz
er. The synthesizer is a phase-locked-loop circuit which provides extremely accurate and
reliable carrier frequency operation. A PWM (pulse-width-modulation) circuit is used to
generate an RF drive signal for application to a modulator circuit board in an RF power
module. If a stereo circuit board failure is encountered or when the stereo circuit board is
removed from the ECU chassis, the exciter circuitry is designed to automatically configure
to monophonic operation.
1-10. Controller Circuit Board. All transmitter control operations are directed by the ECU con
troller circuit board. The controller circuit board consists of CMOS logic control and moni
toring circuitry. The circuitry is designed to interface to all popular remote control systems
such as the Broadcast Electronics VMC-16 remote control system.
1-11. The transmitter power is controlled by a power control circuit. The circuit allows the
transmitter to be operated at five power levels. A power trim circuit allows the transmitter
output power to be adjusted to a precise level. An antenna interlock circuit is provided to
prevent the transmitter from operating into an incorrect antenna. A reflected power detec
tion circuit operates in association with the power control circuit to foldback the transmit
ter power during high VSWR conditions. In addition to the reflected power detector, a
lightning detector circuit is provided to mute the transmitter when high voltage is present
at the transmitter output during a lightning storm.
1-12.
OUTPUT NETWORK ASSEMBLY.
Matching of the transmitter impedance to the antenna is
accomplished by the output network assembly. The assembly is equipped with an
LC T network to match the transmitter output to the antenna. A bandpass filter is pro
vided to attenuate all harmonic frequencies to FCC, DOC, and CCIR levels. The assembly
also contains the lightning detection circuit board, a lightning protection circuit board, and
a directional coupler assembly.
1-13.
RF POWER MODULE.
The AM-2.5E and AM-5E transmitters are equipped with RF pow
er modules. A power module is a modular plug-in assembly containing two RF power am
plifier circuit boards and one modulator circuit board. Each power module is equipped
with MOSFET transistors to produce approximately 1375 watts of RF power.
1-14. The RF power modules are designed using Class E amplifier technology. A Class E ampli
fier exhibits high efficiency and provides superior audio performance. In addition to the
superior efficiency and audio performance, the power modules are designed to be removed
from the chassis for maintenance. The remaining power modules will provide full power to
maintain on-air operation.
1-15.
POWER SUPPLY.
A modular switching power supply provides operating potentials for two
RF power modules. The power supply design uses an SCR controlled bridge to rectify the
ac line voltage into a dc potential. The supply is filtered and routed to the RF power mod
ules for control and regulation. A fault detection circuit monitors power supply activity for
failure conditions. A separate modular switching power supply provides operating poten
tials for the ECU circuitry.
1-16.
COMBINER ASSEMBLY.
The AM-2.5E and AM-5E are equipped with a star combining
system. The system combines the outputs of the RF power modules to provide: 1) a 2.5
kW output in AM-2.5E models and 2) a 5 kW output in AM-5E models. If a power module
is removed from the chassis, the remaining power modules will continue operation to main
tain on-air operation.