E.F. Johnson Company VIKING VX, Руководство пользователя

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CIRCUIT DESCRIPTION
6-7
August 2000
Part No. 001-2001-200
6.1.10 BUFFER AMPLIFIER
A cascode amplifier formed by Q401 and Q402
provides amplification and also isolation between the
TCXO and Synthesizer U401. A cascode amplifier is
used because it provides high reverse isolation. The
input signal to this amplifier is from TCXO Y401.
C405 provides DC blocking. Bias for the amplifier is
provided by R404, R406, R407, R408 and R409.
L401 is an RF choke. RF bypass is provided by C403,
C401 and C407. The output of Q401/Q402 is coupled
to U401 by C432.
6.1.11 LOCK DETECT
When the synthesizer is locked on frequency, the
Lock Detect output on U401, pin 2 is a logic high
voltage with very narrow negative-going pulses. Then
when the synthesizer is unlocked, these pulses become
much wider, the width may vary at a rate determined
by the frequency difference of f
V
and f
R
. The lock
detect pulses are applied to J401, pin 14 and sent to
the RF Interface on J103, pin 14 for detection and
sampling in the IAC.
6.1.12 CHARGE PUMP, LOOP FILTER
The charge pump circuit charges and discharges
C450, C451 and C452 in the loop filter to provide the
21V VCO control voltage (see Section 6.1.13). Pulses
which control the charge pump are fed out of U401,
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.
Q406 and Q407 are drivers which make the 5V
levels and polarity of U401 phase detector outputs
compatible with the high voltage supply to Q408 and
Q409. Capacitors C444 and C446 momentarily
bypass R432 and R437 when negative-going pulses
occur. This speeds up the turn-off time of Q406 and
Q407 by minimizing the effect of the base charge.
When a negative-going pulse occurs on pin 4,
Q406 turns on which turns on Q408. Q408 sources
current to charge up the loop filter capacitors C450/
C451, thereby increasing the VCO control line volt-
age. When a negative-going pulse occurs on pin 3,
Q407 turns on which turns on Q409. Q409 sinks cur-
rent to discharge the loop filter capacitors C450/C451
thereby decreasing the VCO control line voltage. The
source current from Q408, when it is on, equals the
sink current from Q409, when it is on.
6.1.13 VOLTAGE MULTIPLIER
The 17.5 MHz from Y401 is amplified by Q401/
Q402 and passed to the reference input of synthesizer
U401, pin 20. This signal is also coupled from the
output of Q401/Q402 through C408 to amplifier
Q403. Biasing for Q403 is provided by R410, R411
and R412. The output of Q403 is direct coupled to
switching transistors Q404/Q405.
When Q405 is turned on and Q404 if off, C409 is
grounded on the side connected to the emitter of
Q405. This allows the other side of C409 to charge
from the 12V supply through R414, CR402 to C409.
When Q404 turns on and Q405 is off. C414 charges
up to approximately 12V plus the voltage that was
stored across C809 from the last cycle. The output
voltage is 21V due to voltage loss in the transistor and
diodes. C413 is an RF bypass and C414 charges to
21V to stabilize the voltage. The 21V output is fil-
tered by C415/L403/C416 to remove the 17.5 MHz
ripple. The 21V output is applied to the charge pump
Q408/Q409 and the VCO control line.
6.1.14 BUFFER AMPLIFIER
A cascode amplifier formed by Q131 and Q132
provides amplification and also isolation between the
VCO and Receiver RF stages. A cascode amplifier is
used because it provides high reverse isolation. The
input signal to this amplifier is coupled from VCO
A401 by C131. C131 also provides DC blocking.
Bias for the amplifier is provided by R134, R133,
R138, R132, R131 and R136. L131 is an RF choke
and R135 sets the RF output impedance of the cas-
code. RF bypass is provided by C143, C142, C141,
C140, C139, C138, C133, C134, C135 and C136. The
output of Q131/Q132 is matched to the Receiver RF
stages by a section of microstrip, C144, signal pad
R139/R140/R141, C145, C146 and L133. C145 cou-
ples the signal to the input of the first injection
amplifier.