BW Broadcast technical manual
Page 34
Technical data
IC2 is a dual op-amp whose purpose is to control the output power of the transmitter. Side A of the op-amp is con-
figured as a unity gain buffer for the PWM and was described previously. VR2 provides an adjustable DC level to
side B of the op-amp. Side B is configured as a DC amplifier with the gain set by R17, R18, R19, R20 and R25.
The larger the DC signal provided by side A and the larger the DC signal at the output of Side B. This DC signal
is used to turn down the output of the Power amplifiers power supply and with it the RF power output level.
LED3 and R20 provide a connection from the reverse RF power sensor into the input the non inverting input of
side B of the op-amp. LED3 only allows DC through above a certain level. Any DC above this level will increase
the output voltage from the op-amp and the RF power level will decrease. This forms the VSWR protection for
the transmitter. LED1 and LED2 also provide the same function but with the DC level this time being supplied
by the temperature sensor on the PA board. Too high a temperature will cause the DC level to exceed the turn
on point of the two diodes and voltage will once again turn up the op-amps voltage and turn down the RF power
level which should lower the temperature of the transmitter. The output from side B is fed to the switched mode
power supply of the power amplifier via R21 and CON1. R22 and C15 provide supply decoupling for the op-amp.
C14,15,16,17,18 provide further decoupling and feedback for the power control feedback loop formed around side
B of the op-amp.
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