Broadcast Electronics FM-5T, User Manual

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4-5
4-21. The transmitter controller performs several operations. The following text presents a de
scription of the major controller functions.
4-22.
AUTOMATIC RF OUTPUT LEVEL CONTROL.
The controller is designed to provide manual
and automatic RF output power level controller. When the unit is configured for automatic
power control (APC) operation, the controller monitors screen current, PA forward power,
PA reflected power, and the exciter forward power and automatically adjusts the PA screen
voltage via a dc servo motor-driven variable autotransformer to maintain a constant trans
mitter RF output. If excessive PA reflected power, excessive screen current, or low exciter
power is measured, the raise power" command will be inhibited to prevent an overload
from occurring. Manual screen control is assumed by switching the APC feature to off. In
the manual mode the raise and lower switches directly control the dc servo motor to vary
the screen voltage supply. In the APC mode, the switches control a reference voltage
stored in the microprocessor memory. This memory is maintained by the battery backup
system so that the transmitter can automatically return to the desired power level when
ever power is applied.
4-23. The dc servo motor control circuit uses a full-on/full-off scheme to drive the dc servo mo
tor. This feature, combined with a deadband, eliminates hunting in this servo loop. The
front panel RAISE and LOWER switches illuminate when the motor is driven by manual
or automatic operation.
4-24.
VSWR FOLDBACK PROTECTION.
When the unit is in the APC mode, PA forward power
is automatically reduced if output reflected power becomes excessive enough to overload
the transmitter. As the condition which caused the high VSWR returns to normal, RF
power will be proportionately raised until full power is again restored.
4-25.
SOFT START.
The controller monitors PA plate voltage and reduces the screen voltage to
zero upon its absence. When the plate supply is energized, as during initial turn on, the
controller will gradually increase the screen voltage until the stored power setting is
achieved. This prevents inadvertent VSWR overloads at turn on, such as during icing of
an antenna.
4-26.
MOMENTARY POWER INTERRUPTION.
In the event of a momentary power interruption,
proper transmitter operation will resume immediately after power returns. If an extended
power failure occurs, information maintained by the battery back-up system will enable
the controller to initiate a start cycle to automatically return the transmitter to operation
without assistance. If the transmitter internal interlock string opens during a power fail
ure, the automatic restart feature will be defeated and the transmitter will enter the off
condition when power is re-applied.
4-27.
OVERLOADS.
If an overload occurs, the transmitter will deenergize, allow the overload to
clear, then automatically return to operation. If four overloads occur within 60 seconds,
the transmitter will deenergize. The overload must be manually cleared and the trans-
mitter HIGH VOLTAGE ON switch/indicator depressed before operation will resume.
Single overloads of greater than 220 milliseconds duration will immediately deenergize the
high voltage and filament supplies.
4-28.
INDICATORS.
Four LEDs are provided on the front panel as overload status indicators.
The first overload that occurs will be latched into the controller and will illuminate the ap
propriate red VSWR, PLATE, SCREEN, or GRID LED and the yellow overload reset
switch/indicator. All further overloads are monitored by the controller but will not be dis
played by the LEDs.