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Figure 6-4 exciter block diagram, 7 buffer amplifier, 8 exciter synthesizer – Viking 242-2009-632 User Manual

Page 82

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CIRCUIT DESCRIPTION

6-10

March 1999
Part No. 001-2009-600

Figure 6-4 EXCITER BLOCK DIAGRAM

BUFFER

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

U402A

U402B

EX MOD

LPTT

SWITCH

TO PA

A007

AMP

AMP

U407B

V REF EX

HS LK EX

HS CS EX

U401

BUFFER

Q401/Q402

SYNTHESIZER

MAIN

PD

OUT

IN

REF

IN

F

SYNTHESIZER

PD

OUT

IN

REF

IN

F

HIGH STABILITY

TX OCXO

FROM Q204

6.2.7 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 gain, high isolation and
consumes only a small amount of power. The input
signal to this amplifier is coupled from TCXO Y401,
pin 5 by C415. C415 also provides DC blocking.
Bias for the amplifier is provided by R420, R421,
R422, R423 and R418. L401 is an RF choke. RF
bypass is provided by C411, C413 and C414. The
output of Q401/Q402 is coupled to U401, pin 11 by
C412.

6.2.8 EXCITER SYNTHESIZER

The synthesizer inputs/outputs are shown in Fig-

ures 6-3 and 6-4. The synthesizer output signal is the
transmit frequency. This signal is produced by a VCO

(voltage-controller oscillator) that is frequency con-
trolled 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
circuitry.

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

±

0.1 PPM stability of the high sta-

bility loop that is stable from -40

°

C to +70

°

C

(-40

°

F to +158

°

F).

The VCO frequency of A007 is controlled by a

DC voltage produced by integrating the phase detec-
tor output pulses of U403. The phase detector senses
the phase and frequency of two input signals and
causes the VCO control voltage to increase or decrease
if they are not the same. When the frequencies are the
same, the VCO is then "locked" on frequency.