Service manual

CIRCUIT DESCRIPTION
6-9
Month 2000
Part No. 001-2004-601
With reference to the ground on the Exciter, the
3.5V reference stability is maintained by U126B/C/D
on the MAC. The 3.5V DC passes through summing
amplifier U129B and transmit modulation gate U118D
to P100, pin 29 (Tx MOD). P100, pin 29 is connected
to backplane connector J2, pin 8 and RFIB connector
J101, pin 22 to J102, pin 13. The transmit modula
-
tion and 3.5V reference enter the Exciter on J401, pin
13 and are routed to U402B, pin 6. R425 sets the
TCXO modulation level. The modulation signal and
the 3.5V DC are applied to U402A, pin 2.
6.2.4 BUFFER
A cascode amplifier formed by Q406/Q407 pro-
vides amplification and also isolation between the
VCO and synthesizer. A cascode amplifier is used
because it provides high reverse isolation. The input
signal to this amplifier is tapped from the VCO RF
output. DC blocking to the VCO is provided by C441
and to the buffer by C433. Bias for the amplifier is
provided by R451, R453, R454 and R455. Q407 is a
common-emitter amplifier and Q406 is a common-
base with C432 providing RF bypass. L403 decouples
the output from AC ground and R452 lowers the Q of
L403. The amplifier is coupled by C429 to U403,
pin 11.
6.2.5 SYNTHESIZER
The inputs/outputs of synthesizer U403 are
shown in Figure
6-4. The synthesizer output signal is
the transmit frequency. This signal is produced by a
VCO (voltage-controller oscillator) that is frequency
controlled by a DC voltage produced by synthesizer
chip U403. This DC voltage is filtered by a loop filter
made up of C805, C806 and R804 in the VCO cir
-
cuitry.
Frequencies are selected by programming
counters in U403 to divide by a certain number. This
programming is provided through J401, pins 12, 19
and 20. The frequency stability of the synthesizer is
established by the ±1.0 PPM stability of TCXO Y401.
This oscillator is stable from -30°C to +60°C (-22°F to
+140°F).
The VCO frequency of A007 is controlled by a
DC voltage produced by the phase detector in U403.
The phase detector senses the phase and frequency of
the two input signals and causes the VCO control volt
-
age to increase or decrease if they are not the same.
When the frequencies are the same, the VCO is then
"locked" on frequency.
Figure 6-5 EXCITER BLOCK DIAGRAM
BUFFER
SYNTHESIZER
DATA
CLK
BUFFER
VCO
BUFFER AMP
SYN CS EX
SYN LK EX
U403
Q406/Q407
U404B
BUFFER
Q410/Q411 Q412 Q413
AMP
Q405
Q403/Q404
BUFFER
Y401
TCXO
U404A
U402AU402B
EX MOD
LPTT
SWITCH
TO PAA007
AMP
AMP
U407B
V REF EX
PD OUT
REF IN
F IN