Broadcast Electronics AM-2.5E, User Manual

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4-7
4-21. The modular design of the RF power modules allow the modules to be removed from the
transmitter for maintenance. The remaining power modules will provide power to
maintain on-air operation.
4-22. The PWM signal from the exciter circuit board is applied to the modulator circuit board.
The modulator circuit board is designed to amplify and convert the PWM signal to a dc
voltage which varies at an audio rate. The output of the modulator circuit board is
applied to the RF amplifier circuit boards. Four indicator circuits monitor and display the
status of the: 1) PWM drive signal, 2) B+ dc supply, 3) modulator fault conditions, and
4) blown fuse conditions.
4-23. The RF amplifier circuit boards are designed with Class E amplifier technology and
MOSFET power transistors. The circuit board operates from: 1) the varying dc voltage
from the modulator circuit board and 2) the RF drive signal from the exciter circuit board.
The RF drive signal from the exciter circuit board and the varying dc voltage from the
modulator are applied to a push-pull power MOSFET transistor amplifier circuit. The
amplifier circuit is designed to output approximately 687.5 watts of RF power. A fault
detection circuit monitors amplifier operation for fault conditions.
4-24.
RF COMBINER.
4-25. Power from each RF amplifier circuit board is applied to a star combiner network. The
combiner components are located on the rear panel of each power block. The star
combiner network consists of individual transformer and inductor networks for each
amplifier circuit board. The combiner accepts RF power from each RF power module to
produce the rated RF output power.
4-26. In addition to combining the RF power from the RF power modules, the combiner design
allows one or more RF power modules to be removed from the transmitter for
maintenance. The remaining RF power modules will continue to operate to maintain
on-air operation. This is accomplished without the use of dummy modules or bypass
switches.
4-27.
OUTPUT NETWORK.
4-28. The output network is a modular assembly designed to match the transmitter impedance
to the antenna. The assembly consists of a: 1) bandpass filter, 2) directional coupler
circuit board, 3) T-matching network, 4) lightning protection circuit board, and 5) a
lightning detection circuit board.
4-29. The bandpass filter is an eight element LC filter designed to attenuate all harmonic
frequencies to FCC, DOC, and CCIR levels. The directional coupler circuit board consists
of a circuit designed to sample the transmitter RF output. The circuit board generates
both forward and reflected power samples for application to the controller circuit board.
The T" matching network consists of an LC network. The network is designed to match
the transmitter impedance to the antenna.
4-30. A lightning protection circuit board is provided to protect the transmitter circuitry from
direct lightning potentials. The circuit protects the transmitter by shunting lightning
potentials to ground. The lightning detector circuit board is designed to mute the
transmitter RF output during the presence of a lightning potential. The lightning
detector circuit board is controlled by a spark gap. The circuit will respond to potentials
of 1500 volts in AM-2.5E models and 2100 volts in AM-5E models. This prevents the
transmitter from muting during near-by lightning activity.