Motorola ASTRO Digital Spectra Plus Detailed Service Manual page 126

3-66 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 50-Watt
VHF PA, this maximum value is 8 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 amplifier is monitored by sensing the voltage across R3875; CURRENT
SENSE + connects to one end of R3875; 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 R3875 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 R3875, where current sense occurs, can be determined by multiplying the voltage on
U500, pin 40, by 0.067 V. When current is being limited, the output of the operational amplifier,
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 final amplifier, a parallel pair of non-symmetrical microstrip lines form a forward
power-sensing directional coupler. Because of increased coupling with frequency, C3902 is used to
compensate and filter out harmonics. R3905, R3906, C3902, and L3903 provide DC bias to
CR3900, which rectifies the signal. During normal transmission, the DC voltage from the
forward-detect line to the RPCIC ranges from 2 to 4.5 V. This voltage connects to U500, pin 9, the
directional coupler buffer input.
The directional coupler's output, U500 pin 8, is summed to pin 2 with the digital/analog buffer's
output through R509 and R507 respectively.
Closed loop operation reduces the control amplifier's output pin 42, reduces the power module's
gain, and reduces power output to maintain the coupler buffer output U500, pin 2 at 3.2 V regardless
of the D/A voltage level. If the D/A voltage is high (4.5 V), little detected voltage is needed to keep pin
2 at 3.2 V, and the power, consequently, is low. If the D/A voltage is low (1.5 V), a large forward
detected voltage is needed to keep pin 2 at 3.2 V and power, consequently, is at maximum value.
The voltage at pin 2 drops below 3.2 V under proper operation during low line voltage conditions
where the PA cannot produce rated power, or if, under any conditions, the control voltage, or the
final device current exceeds safe levels.
September 5, 2008 6881076C25-E