Power Amplifier Board - BW Broadcast TX5 FM Broadcast Transmitter Technical Manual

Technical data

3.23 POWER AMPLIFIER BOARD

The RF signal from the exciter section of the combo board arrives at the MCX RF input connector CON5 via a
coaxial cable. From here the 50 ohm input impedance is matched to the gate of the FET transistor T1 by the
impedance matching network formed by C8, C9, C30, L1 and L7. R26, R22, VR2 and C10 provide bias control to
the gate of T1.
FB, L2 and L2 provide 24 volts to T1's drain from the 24 volt line as well as providing some impedance matching
to the output of T1. The RF output is fruther fed into a low pass filter composed of C17 to C24 and inductors L3 to
L5. These components reduce the level of any harmonic products generated by the power amplifier. The output of
the low pass filter feeds into the VSWR sensor which generates our forward and reverse RF power measurements
for metering and for VSWR fault protection and alarms. The RF line passes through ferrite ring RFT1. IC1 and
associated resistors R2-R12 buffer the sensors outputs and provide a level suitable for the control/LCD boards
microcontroller and protection circuitry.
The RF output sniff for the front panel is formed by a potential divider from the RF output line. This potential divid-
er is formed by resistors R1 and R2. A coaxial cable connects the potentially divided RF output to the front panel.
A temperature sensor circuit is located close to the main RF transistor T1. This temperature sensor is mounted
into the heatsink and consists of the sensor itself TS1 which is located under the board and an op-amp circuit for
level adjustment. The op-amp IC2 and associated components convert the output signal from the sensor itself to a
level more suitable for the microcontroller and protection circuitry on the Control / LCD board.
CON2 connection block provides an interface to the board for the power connections from the two power supply
modules and an interface to the LCD control board. This connector carries readings for the forward and reverse
power, temperature, PA voltage, 18 volts from the regulator off the AUX power line to power the LCD control
board and the combo board, and last but not least the PA power modules control line which controls it's output
voltage.
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BW Broadcast technical manual