Service manual

July 1, 2002 68P81076C25-C
3-58 Theory of Operation: Power Amplifiers
Control Voltage Limiter
R3807 and R3808 form a voltage divider that connects to control voltage drive. The output of this
voltage divider is connected to the control-voltage-limit input (pin 4) of the RPCIC. If the voltage at
this input reaches 3.2 V, then the control voltage will be clamped to a maximum value. For the
high-power VHF PA, this maximum value is 9 V. This voltage control limit is set by the values of
R3807 and R3808.
Current Limiter
U204, the processor on the VOCON board, sends data to U502, the digital-to-analog converter, to
properly set the voltage on U502, pin 15, which is the TX CURRENT LIMIT control line to the RPCIC
(U500, pin 40). Sixteen different voltages, ranging from 1.5 to 4.5 V, can be programmed from U502.
The collector current of the 110-Watt amplifier is monitored by sensing the voltage across R3849.
CURRENT SENSE + connects to one end of R3849; CURRENT SENSE - connects to the other end.
These lines connect to the command board on U500 pins 37 and 38, respectively. If the TX
CURRENT LIMIT is set for 1.5 V, then the voltage difference between U500 pins 37 and 38 must be
0.1 V before the current through R3849 is reduced. If U500 pin 40 is programmed for 4.5 V, then the
difference of potential between pins 37 and 38 must exceed 0.3 V before current limiting begins. The
voltage across R3849, where current sense occurs, can be determined by multiplying the voltage on
U500 pin 40, by 0.067. When current is being limited, the output of the op-amp (U500, pin 42) begins
shutting down the conduction of Q503 and Q504, reducing PA control voltage, and reducing drive to
the final amplifier to, effectively, control the final amplifier's maximum current.
Forward Power Limiter
After the harmonic filter a parallel pair of microstrip lines form a forward power sensing directional
coupler and detector. The output of this directional coupler/detector is a DC voltage that is
proportional to the forward RF power from the final amplifier. During normal transmission, the DC
voltage from the forward detect line to the RPCIC ranges from 2 to 5.0 V. This voltage connects to
U500 pin 9, the directional coupler buffer input.
The directional coupler's buffered output, U500 pin 8, is summed to pin 2 with the digital/analog
buffer's output through R509 and R507, respectively. In typical operation, the closed loop operation
of the circuit attempts to keep the voltage at U500 pin 2 a constant value of 3.2 V. The control amp
will maintain this condition by increasing or decreasing the control amp output voltage. This control
amp output voltage is routed to the LLA via transistors Q503 and Q504. The output of Q504 is
designated "control voltage drive" and is routed to J1 pin 2 of the PA board.
Since control voltage drive controls the gain of the LLA, it determines the drive level to the following
stages and thus the output power of the final amplifier. The output power of the final stage is
detected by the directional coupler and is routed back to U500 pin 2 via the buffer and R507. Thus
the loop is complete and forward power is maintained a constant value. The voltage at pin 2 will drop
below 3.2 V during low line voltage conditions where the PA cannot produce rated power. Current
limit and voltage control limit circuits will also affect the voltage at pin 2 as described in the following.