Broadcast Electronics AM-2.5E, Руководство пользователя

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4-23
4-117.
LIGHTNING DETECTION CIRCUIT BOARD.
Lightning potentials present at the
transmitter output are detected by the lightning detection circuit board. The circuit
board is equipped with an RF sampling circuit and an optically operated transistor
circuit.
4-118. The RF sampling circuit consists of a parallel capacitor circuit. The circuit is designed to
provide a constant RF voltage sample for application to the directional coupler circuit
board. The optically operated transistor circuit is designed to detect lightning potentials
present at the transmitter output. The circuit operates in association with the lightning
detector spark-gap. When lightning is present at the transmitter RF output, the
spark-gap will: 1) be biased on to conduct the potential to ground and 2) emit a light
pulse to optically operated transistor Q401. Q401 will be biased on to output a lightning
detect signal to the controller circuit board.
4-119.
RF OUTPUT POWER CONTROL CIRCUITRY.
The transmitter output power is controlled
by circuitry on the controller and power supply circuit boards. The transmitter power
level is controlled by a Pulse-Width-Modulated (PWM) signal generated by the controller
circuit board. The power control PWM signal is routed for application to the power
supply circuit board(s). The power supply circuit board(s) respond by routing the
required dc voltage to the modulator circuit boards and the RF amplifier circuit boards.
The following text presents a description of the RF power control circuitry.
4-120. RF Output Power Control - Controller Circuit Board. A transmitter RF output power level
is selected by the power control 1 through power control 5 switch/indicators. The power
control switch/indicators route a LOW control signal to a priority encoder circuit. The
priority encoder circuit determines which control pulse is routed to the power control
circuitry. The circuit allows transmitter off commands to be assigned a high priority.
With no transmitter off commands present, the power control signal is applied to
multiplexer U39. U39 selects a reference voltage from the power control potentiometers
on the controller circuit board. The potentiometers are designed to establish a preset
transmitter power level such as 1 kW. The voltage is used as the reference for
digital-to-analog converter U42. With no fault conditions present, the voltage is applied
without change to power trim digital-to-analog converter U43. With no power trim
commands present, the reference voltage is applied to a driver circuit which converts the
dc control voltage to a power control PWM signal. The power control PWM duty cycle
responds to the changes in the voltage reference level. When the voltage reference
increases, the power control PWM signal duty-cycle increases. When the voltage
reference decreases, the power control PWM signal duty-cycle decreases. The power
control PWM signal is routed for application to the power supply circuit board(s).
4-121. The transmitter output power level is trimmed to a precise level by the power up and
down switches. The switches output a LOW control signal to a control logic circuit. The
control logic circuit generates the required control signals to drive an up/down counter
circuit. The circuit is designed to: 1) count up if power is required to be increased or
2) count down if power is required to be decreased. The up/down counter circuit output is
converted to a dc potential by digital-to-analog converter U43. The output of U43 is
summed with the dc potential from power control switch digital-to-analog converter U42
to generate a dc power control signal. The power control signal is applied to the driver
circuit which converts the signal to a power control PWM signal.
4-122. Automatic Power Control Circuitry. The transmitter power control circuitry is equipped
with several monitor circuits designed to determine if power control correction is required
during adverse operating conditions. Circuitry on the controller circuit board monitors
the transmitter components and the RF output for: 1) lightning, 2) high forward power,
3) high reflected power, and 4) over-temperature. If a lightning, high forward power,
high reflected power, or over-temperature, condition occurs, a signal is routed to the
fault detection circuit.