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Theory of Operation: Transmitter
Protection Mechanisms
Final-stage temperature is sensed in the RFPA resulting in VTEMP, which is proportional to
temperature. VTEMP is compared against a reference voltage TEMP_1 (U0959, pin 9) via U0957-1.
When VTEMP exceeds TEMP_1, the U0957-1, pin 1, voltage increases and forward biases one of
the D0951 diodes, which cuts back power. Power continues to cut back with rising temperature until
the voltage level at the junction of R0978 and R0983 is high enough to forward bias D0952, thus
clamping the cut back so that the radio meets its duty cycle specification while providing protection
against high-temperature conditions. The clamping level is set via TEMP_2 (U0959, pin 10) and
U0957-2. U0957-3 is used to sense if a high A+ battery voltage condition exists and, if it does, the
Q0955 gate is biased on, which increases the clamp voltage allowing for additional power cutback
for a high A+, high temperature condition.
Final-stage current is also monitored via VCURRENT, which is proportional to current. VCURRENT
is compared against a reference CURR_LIM_SET (U0959, pin 1) which is tuned after power
characterization. If VCURRENT exceeds CURR_LIM_SET, then U0957-4, pin 14, voltage rises and
forward biases one of the D0951 diodes, which limits power.
Finally, control voltage is limited by U0956-4 and D0950. RFPA_CNTRL can rise to the control
voltage limit set by R0942-4. U0965 provides protection against supply voltage transients. When
transients on the A+ voltage exceed 24 volts U0965 pin 1 is pulled high thus turning on Q0954 via
TX_DISABLE, R0932, U0958 disabling the control voltage to the PA first stage and momentarily
turning off the transmitter. In addition, Q0900 is turned on, forward biasing one of the D0950 diodes
and thus reducing the control voltage. When the voltage transient drops back below 20 volts, U0965
pin 1 goes low thus enabling the control loop and turning the transmitter on again.
3.12.3 UHF Range 2 (450-520 MHz) Band
3.12.3.1 45-Watt Transmitter
The following text discusses the 45-W transmitter.
3.12.3.1.1 RF Power Amplifier (RFPA)
The RFPA consists of three gain stages, which are shown in
TRANSMIT
2mW
Q5501
TX_INJ
From
FGU
Figure 3-28. 45-Watt RF Power Amplifier (RFPA) Gain Stages (UHF Range 2)
First Stage
The RFPA first stage provides gain that is determined by the control voltage, RFPA_CNTRL. This
control voltage is generated in the power control section and is a function of the final-stage output
power, temperature, and current, as well as the control and A+ voltage levels. See
Power Control"
6815854H01-A
FIRST
BUFFER
STAGE
0.5mW
U5501
C65
C65
K9.1V
RFPA_CNTRL
on
page 3-39
for a detailed explanation of the power control section.
Figure
DRIVER
STAGE
250mW
Q5502
1518
K9.1V
DRV_9.3V
VGBIAS3
3-28.
FINAL
VGBIAS2
STAGE
51W
3.5W
Q5503
1570
RFPA_OUT
To Antenna
A+
VGBIAS1
"3.12.3.1.3.
June 15, 2005
3-37
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Troubleshooting
