CIRCUIT DESCRIPTION
3-21
February 2001
Part No. 001-9800-001
3.9.5 DIRECTIONAL COUPLER, LOW-PASS
FILTER
The transmit signal is fed to a directional coupler
formed by adjacent sections of microstrip. The
forward component of output power is rectified by
CR602 and developed across R607 and fed to the
power control circuit. Reverse power is not detected in
this transceiver.
From the directional coupler the transmit signal is
fed to a low-pass harmonic filter formed by L602-
L604 and several capacitors. This filter attenuates
harmonic frequencies occurring above the transmit
band. R603 dissipates static buildup on the antenna.
The ambient power amplifier temperature is
sensed by thermistor R601. The resistance of a ther-
mistor decreases as temperature increases. R601 and
R143 on the audio/logic board form a voltage divider,
and the voltage across this divider is monitored by an
A/D converter input of the microcontroller (pin 63). If
the PA temperature increases above limits set in soft-
ware, the power is first cut back. Then if it continues
to rise, the transmitter is turned off.
3.9.6 POWER CONTROL (U500A/B, Q500-Q503)
Introduction
The power control circuit maintains a constant
power output as changes occur in temperature and
voltage. It does this by varying the supply voltage to
predriver Q509 (unrevised 430-470 MHz) or third
amplifier Q511 (all others). This changes the power
output of that stage which in turn controls the power
output of the transmitter. The power control circuit
senses forward power to control power output. The
current to final amplifier Q601 is also sensed, but it
affects power output only if it becomes excessive.
Gradual power shutdown then occurs.
The power output level is set in 127 steps by D/A
converter U801 that is controlled by the microcon-
troller. This allows power to be adjusted from the front
panel using the test mode and also different power
levels to be programmed for each system. In addition,
it allows the microcontroller to cut back power when
power amplifier temperature is excessive as just
described.
U500A, Q500/Q502 Operation
The forward power signal from the directional
coupler is applied to pin 2 of amplifier U500A. This is
a DC signal that increases in proportion to forward
power. The other input to U500A is a DC voltage from
a D/A converter formed by shift register U801 and
several resistors. This stage is similar in design to D/A
converter U305 described in Section 3.4.4. Program-
ming of U801 is described in Section 3.7.8. The
voltage from this D/A converter sets the reference
voltage on pin 3 which sets the power output of the
transmitter.
U500A is a difference amplifier which amplifies
the difference between the reference voltage on pin 3
and the forward power signal on pin 3. The turn-on
time of U500A is controlled by the time constant of
C502 and R508. Negative AC feedback to prevent
oscillation is also provided by C502. This circuit oper-
ates as follows: Assume the output power attempts to
increase. The DC voltage applied to U500A, pin 2
then increases which causes the output voltage on pin
1 to decrease. Transistors Q502 and Q500 then turn off
slightly which decreases the supply voltage to
predriver Q509 (or third amplifier Q511). The output
power then decreases to maintain a constant power
output. R510 and R513 limit the voltage gain of Q500
and Q503 to approximately two.
Delayed PTT
Transistor Q503 is used to delay power output for
a short time after the transmitter is keyed. This allows
the synthesizer and the exciter to stabilize so that the
transmitter does not transmit off-frequency. The signal
which controls Q503 is from the Q2 output (pin 17) of
latch U111 on the audio/logic board. In the receive
mode this output is low, so Q503 is off. Pin 2 of
U500A is then pulled high by the 8-volt supply
applied through R505 and CR503. This causes the
output on pin 1 of U500A to go low which shuts off
power to Q509 (or Q511). Then when the transmitter
is keyed, the Q503 control signal goes high after a
short delay. Q503 then turns on and diode CR503 is
reverse biased. Only the forward power signal is then
applied to pin 2 of U500A.
UHF TRANSMITTER DESCRIPTION
