Broadcast Electronics FM-5T, User Manual

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4-16
4-80. A pair of directional couplers located in the output transmission line provide RF output
voltages proportional to the PA forward and reflected power. The RF output voltages are
rectified and amplified to provide power and VSWR indications on the OUTPUT POWER
meter and samples for the transmitter controller. An additional port in the transmission
line provides a point to connect a station modulation monitor.
4-81.
PA METERING.
Seven meters on the FM-5T and six on the FM-5TS are used to indicate
transmitter operating parameters. The plate current, multimeter, and the filament voltage
meters measure samples from a PA metering circuit board which is mounted on the side of
the RF enclosure. Additional samples from this circuit board are routed to the controller
for overload and diagnostic features. The PA metering circuit board also contains fuses
which protect the filament meter wiring. Plate voltage metering is obtained from a high
voltage meter multiplier circuit board. Power output metering is derived from circuitry
within the controller. A filament time meter indicates total elapsed time of filament circuit
operation. The filament voltage meter is an iron vane type and accurately measures the
filament voltage at the cavity feed-thru terminals. On FM-5T models, monitoring of ac
input potentials is provided by a primary ac voltage meter.
4-82.
AUTOMATIC POWER CONTROL.
The transmitter controller monitors a number of trans
mitter parameters to function as part of a closed loop which maintains a constant RF out
put level from the transmitter (see Figure 4-4).
4-83. PA forward and reflected power samples from the transmitter low-pass filter are applied to
individual rectifier/amplifier circuits in the controller. The outputs from the rectifier/am
plifier circuits are routed to analog-to-digital (A-to-D) circuitry. The A-to-D circuitry
converts the signals to a digital format and routes the signals to the microprocessor. The
microprocessor uses the signals: 1) to output forward power and VSWR samples to the out
put power meter, 2) to output forward and reflected power samples to the remote meter
terminals, and 3) for automatic RF output power control operations. The controller moni
tors several parameters such as the forward and reflected power, screen current, and excit
er forward power to determine if power control and correction is required. When automatic
power control is enabled and power correction is required, the controller will use the ad
justable screen supply autotransformer to obtain the desired power level.
4-84. When APC operation is enabled and as RF output power varies, the controller will act to
maintain the established RF output level. If inadequate exciter drive exists for normal op
eration, PA reflected power increases, or if screen current is high, any power increase will
be inhibited. If the PA reflected power increases to a point which may damage the RF cir
cuitry of the transmitter, the controller will reduce the RF output to a safe level and the
transmitter will continue to operate. Full power will be automatically re-established when
the VSWR condition is corrected.
4-85. As an additional function, the controller will reduce the PA screen potential to minimum
whenever the plate voltage is off. Whenever the plate voltage is energized, the controller
will gradually increase the PA screen voltage until the rated transmitter RF output is es
tablished unless limited by low exciter drive, a high VSWR condition, or high screen cur
rent.