Motorola ASTRO Digital Spectra Service Manual page 130

3-80 Theory of Operation: Power Amplifiers
Power Module Control Voltage Limiter
R9562 and R9563 connect in series to the emitter of Q9500. The ratio of R9563 and R9562 feed a
portion of the control voltage (U9850, Pins 2 and 3) to U500, pin 4. When pin 4 exceeds 3.2 V, the
output of the control op-amp (U500, pin 42) is reduced. Eventually, this reduces the control voltage
available to the power module (U9850).
The input RF power to the 45-Watt amplifier Q9880) must stay below 17 Watts. Power is coupled
from the inter-stage 50-ohm transmission line to a 100 ohm transmission line and rectified by
CR9930 on the PA, producing a DC voltage on U500, pin 4. IF this voltage exceeds 3.2 V, the output
voltage on U500, pin 42, is reduced, lowering the control voltage and reducing U9850's gain until its
RF output power is approximately 17 Watts.
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 45-Watt final amplifier (in the 35-Watt PA only) is monitored by sensing
the voltage across R9875. CURRENT SENSE + connects to one end of R9875 and 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 R9785 is reduced. If U500, pin 40 is
programmed for 4.5 V, then the difference of potential between Pins 37 and 38 must exceed 3 V
before current limiting begins. The voltage across R9875, 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 base drive to Q9500, reducing drive to the final amplifier to, effectively, control the final
amplifier's maximum current.
Forward Power Limiter
The parallel pair of microstrip lines after the final amplifier, form a forward power sensing directional
coupler. Because the coupling increases with frequency, the compensation network of L9806 and
C9901 is used. CR9900 rectifies the signal, C9900 filters it, and R9905 and R9904 form a voltage
divider. 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 input to the directional coupler
buffer.
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 amp'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, either the inter-stage power (in 35-Watt models only), the control voltage, or the final
device current exceeds safe levels.
July 1, 2002 68P81076C25-C