Service manual

CIRCUIT DESCRIPTION
5-20
April 2001
Part No. 001-9800-203
on the input of Q506 is provided by C529, C537,
L505, and C530. The input level is set by R528 and
R530. Biasing is provided by R520 and R525, and
C525/C526 and C520/C521 decouple RF signals.
Impedance matching on the output is provided by
L502, C570, C531, L506, and C532. Second amplifier
Q507 is similar in design to Q506. These stages
together produce about 20 dB of gain.
The 8-volt supply to Q506 and Q507 is switched
on in the transmit mode by Q505 and Q504. This
switch is controlled by the microcontroller through the
Q7 output (pin 11) of shift register U801. This output
is high in the transmit mode and low in the receive
mode. This signal also controls the antenna switch
circuit on the PA board described in Section 5.9.4.
This transmit 8V supply is not delayed which
allows Q506 and Q507 and the transmitter frequency
to stabilize before power is produced. The delayed
PTT signal is applied to the RF board on J201, pin 2.
This signal controls the power control circuit
described in Section 5.9.6. The emitters of Q503 and
Q505 are grounded through Q517. That transistor is
turned off when the logic is in an undetermined state
such as during Flash programming. This ensures that
the transmitter is turned off during these times.
470-512 MHz Models
The input signal to the exciter is the transmit
frequency from buffer amplifier Q801 in the synthe-
sizer. It is at a level of approximately 0 dBm and is
applied to first amplifier Q509. Impedance matching
on the input of Q509 is provided by C540, C552,
L503, C550, C551, and L504. Biasing is provided by
R531 and R535, and C526/C527 and C521/C523
decouple RF signals. Impedance matching on the
output is provided by L501, C541, C555, and C556.
Second amplifier Q510 is similar in design to Q509.
These stages together produce about 20 dB of gain.
Power to Q509 is switched by Q506 similar to the
430-470 MHz version just described.
5.9.2 THIRD AMP AND PREDRIVER
430-470 MHz Models
From Q507 the signal is fed to third amplifier
Q508 which provides approximately 10 dB of gain.
Impedance matching with Q507 is provided by L503,
C571, C533, C538, L504, L507, C579, and C539.
Biasing is provided by R5542, R555, and R532.
Impedance matching between Q508 and Q509 is
provided by L501, several capacitors, and two sections
of microstrip. Microstrip is a form of transmission line
with distributed series inductance and shunt capaci-
tance. The characteristic impedance is determined by
the width of the microstrip and the PC board material
and thickness (distance from ground plane). Resistor
R523 lowers the Q of L501 to make it less frequency
selective. Q508 is powered by the switched battery
supply. AC signals are decoupled from this supply by
C517-C519 and ferrite bead EP500.
Predriver Q509 is an RF power MOSFET. The
gate is biased at approximately one-half the drain
voltage by R519 and R522. Capacitors C512-C516,
C522, and C523 provide decoupling of AC signals.
R524 and R527 lower the Q of the input matching
circuit which improves stability.
Supply voltage to Q509 is from the power control
circuit described in Section 5.9.6. This circuit varies
the supply voltage to change the power output of Q509
in order to maintain constant transmitter power output.
RF choke L500, ferrite bead EP501, and several
capacitors isolate the power control circuit from RF
signals. Several capacitors and sections of microstrip
on the drain of Q509 provide an output impedance of
50 ohms to the power amplifier board. This stage
provides a gain of approximately 10 dB, resulting in a
power input to the PA board of up to approximately 8
watts.
470-512 MHz Models
From Q510 the signal is fed to third amplifier
Q511 which is a power MOSFET that provides
approximately 10 dB of gain. Impedance matching
with Q510 is provided by L502, C542, C557. L505,
and C543. Resistors R532 and R534 lower the Q of
the input matching circuit which improves stability.
The gate of Q511 is biased by R527 and R528. Decou-
pling of RF signals is provided by C528/C529 and
C516/C517.
Supply voltage to Q511 is from the power control
circuit described in Section 5.9.6. This circuit varies
UHF TRANSMITTER DESCRIPTION