Motorola ASTRO Digital Spectra Service Manual page 117

Theory of Operation: Power Amplifiers
Temperature Sensing
The temperature-sensing circuit of the PA works with the RPCIC to protect the PA devices from
exclusively high temperatures. On the PA board, this circuit, formed by resistors R3878 thru R3880
and thermistor RT3877, provides a temperature-dependent voltage to the RPCIC via P0853, pin 7.
As the PA temperature increases, the resistance of RT3875 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 amplifier 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 degrees F
(60 degrees C).
The temperature sense circuitry can easily be tested by placing an ordinary leaded 4.7k ohm across
RT3875, PA output power should drop significantly if this circuit is working properly.
NOTE: Under severe environmental conditions, more than one circuit may be 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.
68P81076C25-C
3-67
July 1, 2002