Motorola ASTRO Digital Spectra Service Manual page 112

3-62
The collector currents of the 25/10-Watt 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. 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 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, C3903, 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 1.5 to 5.0 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 amplifier'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.
Temperature Sensing
The temperature-sensing circuit of the PA works with the RPCIC to protect the PA devices from
excessively high temperatures. On the PA board, this circuit, formed by resistors R3878, R3879, and
thermistor RT3876, provides a temperature-dependent voltage to the RPCIC via P0853, pin 7. As
the PA temperature increases, the resistance of RT3876 decreases, causing the voltage at pin 7 to
increase. This voltage is routed to the RPCIC, U500, pin 13, which is the input to the thermistor
buffer. The buffer's output on pin 12 is connected to pin 2 via resistor R508. Note that pin 2 is the
control amp input and is a summing point for temperature, forward-power detect, and power set
signals. If the PA temperature becomes high enough so that the voltage at pin 7 exceeds 3.2 V, the
thermistor buffer starts supplying current to the node at pin 2. Due to the fixed output current of the
power-set buffer, the control loop can maintain 3.2 V at pin 2 only by reducing the forward-power
detect voltage and, therefore, reducing the PA output power. Since power output is reduced, the
generated heat is reduced to a safe level. If temperature decreases, the power output of the PA
gradually increases to its nominal value. Temperature cutback should occur at about 140°F (60°C).
The temperature sense circuitry can easily be tested by placing an ordinary leaded 4.7k ohm resistor
across RT3876. PA output power should drop significantly if this circuit is working properly.
NOTE: Under severe environmental conditions, more than one circuit may he attempting to reduce
power output at the same time (i.e., during high VSWR conditions, the current limiter may
initially reduce power, but eventual heat buildup will cause further power reduction by the
thermal cut-back circuit).
July 1, 2002
Theory of Operation: Power Amplifiers
68P81076C25-C