Motorola ASTRO Digital Spectra Plus Detailed Service Manual page 135

Theory of Operation: Power Amplifiers 3-75
Control Voltage Limiter
R5807 and R5808 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
40-Watt UHF PA, this maximum value is 10 V. This voltage-control limit is set by the values of R5807
and R5808.
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 40-Watt amplifier is monitored by sensing the voltage across R5875.
CURRENT SENSE + connects to one end of R5875; 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 R5875 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 R5875, 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 final amplifier, a parallel pair of microstrip lines form a forward power-sensing directional
coupler. Because of increased coupling with frequency, C5903, L5902, C5904, L5903, and C5905
are used to compensate and filter out harmonics. CR5900 rectifies the signal. R5904, R5905, and
RT5904 provide thermal compensation. 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 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, the control voltage or the final device current exceeds safe levels.
6881076C25-E September 5, 2008