E.F. Johnson Company VIKING VX, Руководство пользователя

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CIRCUIT DESCRIPTION
6-11
August 2000
Part No. 001-2001-200
6.2.7 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
/f
R
.
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.
6.2.8 CHARGE PUMP, LOOP FILTER
The charge pump circuit charges and discharges
C519, C520 and C521 in the loop filter to provide the
12V VCO control voltage (see Section 6.1.12). Pulses
which control the charge pump are fed out of U403,
pins 3/4. When both phase detector inputs are in
phase, these output signals are high except for a very
short period when both pulse low in phase. If the fre-
quency of the f
R
input to the phase detector is higher
than that of the f
V
input (or if the phase of f
R
leads f
V
),
the VCO frequency is too low. The negative-going
pulses on the f
V
output (pin 4) then become much
wider and the f
R
output (pin 3) stays essentially high.
If the frequency of the f
V
input is greater than f
R
(VCO frequency too high), the opposite occurs.
Q414 and Q415 are drivers which make the 5V
levels and polarity of U403 phase detector outputs
compatible with the high voltage supply to Q416 and
Q417. Capacitors C523 and C517 momentarily
bypass R494 and R498 when negative-going pulses
occur. This speeds up the turn-off time of Q414 and
Q415 by minimizing the effect of the base charge.
When a negative-going pulse occurs on pin 4,
Q414 turns on which turns on Q416. Q416 sources
current to charge up the loop filter capacitors C519/
C520, thereby increasing the VCO control line volt-
age. When a negative-going pulse occurs on pin 3,
Q415 turns on which turns on Q417. Q417 sinks cur-
rent to discharge the loop filter capacitors C519/C520
thereby decreasing the VCO control line voltage. The
source current from Q416, when it is on, equals the
sink current from Q417, when it is on.
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 Q413 is biased by CR403, R477,
R478, R479 and R480. C508 provides RF bypass
from the DC line and R479/R480 provide supply volt-
age isolation. L 411 is an RF choke to the supply line.
Q413 is matched to 50 ohms by low pass filter C509/
L412/C510 and C465 provides DC blocking. 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 0.3W pre-driver Q501. R504, R505 and
R506 provide DC bias to the gate of Q501. C506,
C507 and C508 provide RF bypass from the DC sup-
ply line. L503 is an RF choke. C510 and C522 provide
RF bypass. C511/L504/C512/C513 match Q501 out-
put impedance to U501 input impedance. U502 pro-
vides Q501 with DC voltage regulated at 8V.
6.3.2 DRIVER
U501 is a 12W amplifier operating in the 132-
178 MHz range. The RF is applied to the input of the
splitter and to the finals.
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 10W.