CIRCUIT DESCRIPTION
5-21
logic board, and the voltage across this divider is
monitored by A/D converter U21. If the PA tempera-
ture increases above limits set in software, the power
is first cut back. Then if it continues to rise, the trans-
mitter is turned off.
5.10.6 POWER CONTROL
Introduction
The power control circuit maintains a constant
power output as changes occur in temperature and
voltage. It does this by sensing the forward power and
then varying the output of Q505 to maintain a constant
output power (see Section 5.10.2). Although current to
final amplifier Q509 is also sensed, power output is
affected by this input only if current becomes exces-
sive. Power output is then cut back to approximately
25% of full power.
The power output level is set in 127 steps by D/A
converter U501 which is controlled by the microcon-
troller. This allows power to be adjusted using the
PCTune software and computer and also different
power levels to be programmed. In addition, it allows
the microcontroller to cut back power if the power
amplifier temperature is excessive. Temperature is
sensed via thermistor RT501 (see Section 5.10.5).
U502A Operation
The forward power signal from the directional
coupler is applied to pin 2 of amplifier U502A. This is
a DC signal that increases proportionally to forward
power. The other input to U502A is a DC reference
voltage from a D/A converter formed by shift register
U501 and several resistors. The voltage from this D/A
converter sets the voltage on pin 3 which sets the
power output of the transmitter.
U502A is a difference amplifier which amplifies
the difference between the reference voltage on pin 3
and the forward power signal on pin 2. The turn-on
time of U502A is controlled by the time constant of
C525 and R527. This circuit operates as follows:
Assume the output power attempts to increase. The
DC voltage applied to U502A, pin 2 then increases
which causes the output voltage on pin 1 to decrease.
Transistor Q505 then turns off slightly which increases
the attenuation provided by the attenuation circuit (see
Section 5.10.2). The output power then decreases to
maintain a constant power output.
Delayed PTT
Transistor Q504 is used to delay power output for
a short time after the transmitter is keyed. This allows
the synthesizer and exciter to stabilize so that the
transmitter does not transmit off-frequency. The signal
which controls Q504 is from pin 14 of micro-
controller U9 on the logic board. In the receive mode
this output is low, so Q504 is off. Pin 2 of U500A is
then pulled high by the 7.2-volt supply applied
through R530 and CR505. This causes the output on
pin 1 of U502A to go low which shuts off Q505 and
produces maximum attenuation. Then when the trans-
mitter is keyed, the Q504 control signal goes high after
a short delay. Q504 then turns on and diode CR505 is
reverse biased. Only the forward power signal is then
applied to pin 2 of U502A.
Over-Current Shutdown
Current to final amplifier Q509 is monitored by
sensing the voltage drop across R550. Pins 3 and 6 of
U505 are connected across this resistor. As current
increases, the output voltage on U505, pin 8 decreases.
This signal is applied to Schmitt trigger U502B. When
the voltage on pin 6 rises above the reference on pin 5,
the output on pin 7 goes low. This lowers the power
control voltage applied to U505 which lowers the
power output to approximately 25% of full power.
5.10.7 RF AMPLIFIER (Q503)
The receive signal from the antenna switch is
applied to bandpass filter Z501. This is a three-pole
filter with a center frequency of 860 MHz and a band-
width of 18 MHz. This filter attenuates frequencies
outside the receive band such as the first injection,
image, and half IF frequencies.
The signal is then applied to RF amplifier Q503
which improves and stabilizes receiver sensitivity and
also recovers filter losses. A section of microstrip and
C515 provide impedance matching on the input.
CR504 protects the base-emitter junction of Q503
from damage caused by high level input signals.
800 MHz 35W PA BOARD (Cont’d)
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