Power Control Circuitry; Power Set; Current Limit; Thermal Protection - Motorola GTX/LCS 2000 Service Manual

Power Control Circuitry

Power Set

Current Limit

Thermal Protection

This network forms a low-pass filter to attenuate harmonic energy of the
transmitter to specifications level.
A forward-power detector follows the harmonic filter. This forward-power
detector is a microstrip printed circuit, which couples a small amount of the
forward energy and sends it to diode CR6506 where it is rectified. This rectified
signal (VFORWARD) forms the forward voltage which the power control circuit
holds constant. Holding this voltage constant (which is proportional to the RF
rectified energy appearing across the diode) ensures the forward-power out of the
radio is held to a constant value.
In the PA section, 50k thermistor R6519 senses temperature in the area of the
power module. This signal is fed back into the power control circuit to protect the
power amplifier against over-temperature conditions. Resistor R6520, in series
with the A+ line supply, feeds voltage to the power module. The voltage across
R6520 is monitored and these two inputs are channeled to the power control
circuit. The power control circuit monitors the voltage drop across this resistor,
which is determined by the magnitude of the drain current in U6501. It uses this
as a limiting mechanism whereby the power control circuit limits the magnitude
of current that can be drawn by U6501. This protects the device from over
dissipation.
Bias voltage reverse polarity protection for the transmitter is provided by diode
CR6508. Under reverse polarity conditions to the radio, this diode conducts and
protects the radio from damage. This diode also provides transient over-voltage
protection by breaking down when the supply voltage to the radio exceeds 24 V.
The power control circuitry consists of three mechanisms.
The control loop compares the power output level to the pre-set value. The
VFORWARD voltage from the forward power detector is amplified by U0701
(pins 5 and 7) and compared to D/A U0702, outputs 2 and 4. Should a difference
be detected, an error current is generated into the loop integrator (U0702, pins 8
and 10) until equilibrium is obtained. The use of 2 D/A outputs for power set (pins
2 and 4) enables fine tuning where pin 2 is the coarse tune through resistor R0703
(10 kΩ) and pin 4 is fine tune through resistor R0706 (100 kΩ).
The current to the power module is monitored using R6520 and compared to a pre-
set limit (D/A pin 11, and U0701 pins 1-3). When the limit is exceeded, an error
current is generated through diode CR0721 (pins 1 and 3) to the loop integrator,
which lessens the PA_CNTRL line and reduces the power amplifier output.
When the power module overheats, the resistance of thermistor R 6519 drops.
Because of the voltage divider formed by R0725, R0721, and R6519, this
resistance drop is sensed and amplified through U0701 pins 12-14. This causes a
current error through diode CR0721 pins 2 and 3 to flow into the loop integrator
and reduce transmitted output power.
The PA_ENABLE line is for emergency shut down of transmission. When the line
is driven high, transistor Q0731 is driven into saturation, causing the power
control line to be driven to ground voltage. The K9.1_ENABLE line generates the
K9.1 voltage when high enabling transmission.
Theory of Operation 7-5