Broadcast Electronics FM-5T, Руководство пользователя

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1-7
1-42. The controller is equipped with circuitry which rectifies and calibrates the PA directional
coupler forward and reflected power signals. These signals serve as control inputs. In
addition to the forward and reflected power signals, PA screen current, and IPA forward
power allow automatic control of the PA screen voltage using a dc servo motor. If excessive
PA reflected power, excessive screen current, or low IPA power is measured, the raise
power" function will be inhibited to prevent an overload condition. The absence of plate
voltage will inhibit the raise function and signal the controller to adjust the screen voltage
to minimum. Excessive transmitter RF output or a high PA reflection will first inhibit the
raise function. If the condition exceeds the limits, the circuit will initiate a sequence which
lowers power proportionately in response to the condition.
1-43.
VSWR FOLDBACK.
1-44. In the automatic power control mode, PA power will be automatically reduced if PA re
flected power becomes excessive enough to overload the transmitter. As the condition
which caused the high VSWR returns to normal, RF power will be proportionally raised
until full output is restored. A similar circuit for PA forward power will reduce power if the
output is excessive.
1-45. A dead-band window is used to prevent the controller from hunting. If reflected power is
below the lower limit of the dead-band, the controller will perform no correction. If re
flected power increases beyond the lower limit, the raise feature will be inhibited to pre
vent the forward power control function from raising power to avoid a transmitter over
load. If the PA reflected power continues to rise and is within the dead-band, no lowering
of power will occur. If the upper limit of the dead-band is reached, a lower command will
be applied.
1-46.
SOFT START.
1-47. Soft start operation is when APC is enabled, the controller monitors the plate voltage and
reduces the screen voltage to zero upon the absence of plate voltage. When the plate sup
ply is energized such as during power-on, the controller will perform the following:
1. Gradually increase the screen voltage until the APC power level reference is
achieved unless limited by low IPA drive, excessive screen current, or a high VSWR
condition. This prevents inadvertent cycling of the VSWR overload at turn-on if
the load is not optimal such as during an ice storm.
2. When the plate voltage sample decreases below the fixed level, the following events
will occur:
A. The raise function will be inhibited.
B. The controller will output a lower command to: 1) lower the screen voltage and
2) stop lowering the screen voltage at a minimum level. Once the minimum
level is achieved, the lower command will remain.
C. When the HIGH VOLTAGE ON switch/indicator is depressed, the plate voltage
sample from the plate meter multiplier circuit board will rise above the fixed
reference. The raise inhibit will be removed along with the power lower signal
and the APC mode will be allowed to re-establish the transmitter RF power.