CIRCUIT DESCRIPTION
5-20
A shunt path is provided around CR518 by R583,
C537, and R585. Pin diode CR519 is at its maximum
forward biased condition when Q505 is off, and
connects R583 to AC ground through C538. This
maintains a constant 50-ohm impedance. Then as
Q505 turns on, CR518 becomes forward biased and
provides less attenuation. Likewise, CR519 becomes
less forward biased which increases the impedance of
the path to ground. From the attenuator the signal is
coupled by C542 to a 1 dB, 50-ohm pad formed by
R544-R546 and then applied to PA module U504.
5.10.3 POWER AMPLIFIER MODULE (U504),
FINAL (Q509)
Power amplifier module U504 provides approxi-
mately 19 dB of gain. Pins 2, 3, and 4 are the supply
voltage inputs to three separate gain stages. The
supply voltage on pin 2 (VS1) is switched by Q508
and limited to 12 volts by CR508 and R549. Switch
Q508 is controlled by the same signal used to control
the high power antenna switch (see Section 5.10.4).
The supply voltage applied to pins 3 and 4 (VS2/
VS3) is the unswitched battery from the power jack.
Therefore, power is applied to these pins even when
transceiver power is turned off.
The output signal on U504, pin 5 is then applied
to final amplifier Q509 which provides about 5 dB of
gain. Current to this stage flows through R550, and
transmitter current is monitored by sensing the voltage
drop across this resistor (see Section 5.10.6). The
output impedance on U504, pin 5 is 50 ohms, and it is
matched to Q509 by a section of microstrip, C556,
C557, and C558. Class C biasing of Q509 is provided
by L507. The unswitched battery supply applied to
Q509 is isolated from RF by ferrite bead EP503,
inductor L508, and several capacitors. Impedance
matching is provided on the output by C559, C561,
C562, C566, and a section of microstrip.
5.10.4 HIGH POWER ANTENNA SWITCH
The high power antenna switch consists of pin
diodes* CR501, CR502, CR503, and other compo-
nents. This switch effectively switches the antenna to
the power amplifier in the transmit mode, and the
receive RF amplifier path in the receive mode (see
preceding illustration).
Transistors Q506 and Q501 controlled by the Q7
output of shift register U501 after it is double inverted
by Q516 and Q517. This signal is high in the transmit
mode and low in the receive mode. Therefore, Q506
and Q501 are on in the transmit mode which forward
biases CR501, CR502, and CR503. One current path is
through Q501, R503, R504, CR501, L508, CR502,
and L508, and the other is through Q506, R559,
CR503, and R560.
Since a forward biased pin diode has a low
impedance, the RF signal passes through CR502 to the
low-pass filter. The signal is blocked from the RF
amplifier by two discrete grounded quarter-wave lines.
One line is formed by L508/C507 and the other by
L502/C514. Diode CR501 is effectively AC grounded
by C507, and CR503 is AC grounded by C514. When
one end of a quarter-wave line is grounded, the other
end presents a high impedance to the quarter-wave
frequency.
In the receive mode, all three diodes are reverse
biased. Therefore, the receive signal from the antenna
is blocked from the power amplifier by CR502 and has
a low impedance path through the quarter-wave lines
which are no longer grounded. Resistors R505 and
R506 improve the isolation provided by CR501 and
CR502 when they are reverse biased in the receive
mode.
5.10.5 DIRECTIONAL COUPLER, LOW-PASS
FILTER, TEMP SENSE
A directional coupler is formed by adjacent sec-
tions of microstrip near C566. The forward component
of output power is rectified by CR509 and developed
across R557 and then fed to the power control circuit.
Reverse power is not detected in this transceiver.
From the directional coupler the transmit RF
signal is coupled by C511 to a low-pass harmonic
filter formed by C501-C505 and several sections of
microstrip. This filter attenuates harmonic frequencies
occurring above the transmit band. Resistor R501
dissipates static buildup on the antenna.
The ambient power amplifier temperature is
sensed by thermistor RT501. The resistance of a ther-
mistor decreases as temperature increases. The ther-
mistor forms a voltage divider with R147 on the audio/
800 MHz 35W PA BOARD (Cont’d)
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