Service manual

CIRCUIT DESCRIPTION
6-11
Month 2000
Part No. 001-2004-601
6.2.7 BUFFER AMPLIFIER
A cascode amplifier formed by Q406 and Q407
provides amplification and also isolation between the
VCO and Synthesizer U403. A cascode amplifier is
used because it provides high gain, high isolation and
consumes only a small amount of power. The input
signal to this amplifier is coupled from VCO A007,
pin 6 by C433. C433 also provides DC blocking.
Bias for the amplifier is provided by R450, R451,
R453, R454 and R455. L403 is an RF choke. RF
bypass is provided by C430, C431 and C479. The
output of Q406/Q407 is coupled to U403, pin 11 by a
non-polarized capacitor formed by C429/C499.
6.2.8 LOCK DETECT
When the synthesizer is locked on frequency, the
Lock Detect output on U403, pin 2 is a high voltage
with narrow negative-going pulses. When the synthe
-
sizer is unlocked, the negative-going pulses are much
wider, the width may vary at a rate determined by the
frequency difference of f
V and fR.
The locked or unlocked condition of the synthe-
sizer is filtered by R440/C423 and applied to J401, pin
16, then sent to the RF Interface on J102, pin 16 for
detection.
6.2.9 BUFFER AMPLIFIER
A cascode amplifier formed by Q410/Q411 pro-
vides amplification and also isolation between the
VCO and exciter RF stages. A cascode amplifier is
used because it provides high gain, high isolation and
consumes only a small amount of power. The input
signal to this amplifier is tapped from VCO A007, pin
4 by C441. C441 also provides DC blocking. Bias for
the amplifier is provided by R464, R465, R466, R467
and R468. L406 is an RF choke and R483 lowers the
Q of the coil. RF bypass is provided by C434, C442,
C445, C443, C444 and C480. The output of Q410/
Q411 is matched to the Exciter RF stages by a section
of microstrip, C446, signal pad R459/R460/R461,
C498, C450 and L408.
6.2.10 RF AMPLIFIERS
RF amplifier Q412 is biased by CR402, R469,
R470, R471 and R472. C448 provides RF bypass
from the DC line. L409 is an RF choke to the supply
line. Q412 is matched to the 50 ohm signal pad by
low pass filter C449/L410/C451, C503 and signal pad
R473/R474/R475.
RF amplifier/buffer Q413 is similar in design to
Q412. The collector voltage of Q413 is switched by
Q405. When the Logic Push-To-Talk (LPTT) on
J401, pin 11 is low Q405 turns on and conducts the
15V supply to the collector of Q405 and to Q413. The
output of Q413 is matched to 50 ohms by C509, L412,
C510, and C465 provides DC blocking. A 3 dB atten
-
uator R490/R491/R492 follows amplifier Q413. The
RF output of the Exciter is on coaxial connector J402
to the Power Amplifier.
6.3 110W POWER AMPLIFIER
6.3.1 AMPLIFIER/PREDRIVER
RF input to the PA from the Exciter is through a
coaxial cable and connector to WO511. C501 couples
the RF to signal pad R501/R502/R503 that connects
the input to U501. U501 is a 6W amplifier/pre-driver
operating in the 380-520 MHz range.
Power control is connected to WO505 from the
RF Interface board (RFIB). RF is filtered from the
control voltage line by various capacitors to U501,
pin 2. This control voltage regulates the RF output of
the amplifier on U501, pin 5 to approximately 5W.
6.3.2 DRIVER
The output of U501 passes through several sec-
tions of 50 ohm microstrip and matching capacitors to
the gate of Q501. Driver Q501 is a MOSFET ampli
-
fier with a normal output of approximately 22W. Sup-
ply voltage is RF bypassed by various capacitors.
L501, C541, C542 and microstrip match the output of
the driver to 35 ohms. The RF is applied to the input
of the splitter and to the finals.
6.3.3 FINAL AMPLIFIERS
Q502 and Q503 are combined 60W amplifiers.
The 22W RF input from the driver Q501 is applied to
a 70.7 ohm Wilkinson splitter and then to the gate of
each MOSFET amplifier. The 60W outputs on the
drain of the amplifiers are combined using a Wilkin
-
son combiner. Q502 has a half-wave transmission