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CHAPTER
4
1
1
C
H_N
LC
AL
_D_E_S_C
R
l_PJ_l
ON
INTRODUCTION
4.1
This
chapter describes
the significant features of the
circuit
design
and
functions.
The
reader should
have
a clear
understanding
of the functional principles
described
in
Chapter 3 and a
basic
knowledge
of solid state
theory
and
logic.
4.2
The
circuits
are
mounted
on
printed circuit
board (p.c.b.) assemblies and
these
will
be described
individually.
The
circuit
diagrams
at
the
back
of
the
book
are
arranged
in
order
of
Assembly
reference
number
(19-0841
to
19-0849) applicable
to
the
model
9081
.
The
circuits for
assemblies
19-0842,
19-0843 and 19-0846
carry the
necessary
notes
to
enable them
to
be used
for
assemblies
19-0948, 19-0949 and 19-0950 which
are
applicable
to
model 9082.
Other
assemblies are
common
to
both models.
It
will
be
useful
also
to refer to
the
block diagram
Fig.
3.2
at
the
end
of
Chapter
3
to
observe
the theoretical
relationship
between
the different
circuits.
The
practical (interconnection) relationships are
shown
in
the overall
block diagram
(Fig.
22
at
the
back
of the
book).
LOGIC
CIRCUIT
SYMBOLS
4.3
Extensive use
is
made
of integrated circuits
(IC's)
and
these are identified
by a
number and
suffix letter.
In
the circuit description a particular 1C pin
will
be
identified
by
a reference such
as
1
IC2b/4'
which
indicates pin
4 on
that
particular
gate
or
amplifier.
The
logic
symbols used
in
the
circuits
are those
found
in
most manufacturers
1C
data
sheets
to
which
reference should
be
made
if
detailed information
is
required.
MAIN
OSCILLATOR
(VCOl)
ASSEMBLY
19-0841
(Fig.
3)
4.4
This
is
the
r.f.
source
shown
as
VCOl
on the block diagram
Fig.
3.2.
Referring
to
the circuit
diagram.
Fig.
3,
the
resonant
element
of the oscillator
is
the
strip-
line,
with voltage variable
capacitance
(varactor)
diodes D1
and D2.
This line
is
r.f.
decoupled
from the
modulation
input
by
Ferrite
bead
FX1
and
inductor
LI
.
Oscillator
d.c.
tuning voltage
is
applied via
pin
5
and
R6.
Components
R4, R6,
C3
and
C4
provide
filtering
and
shaping on the d.c. tuning
and
the
f.m. modulation
inputs.
4.5
The
oscillator transistor
is
Q4
which
is
coupled
to
the resonant
line
by
strip-lines
connected
to
emitter
and
collector.
The
oscillatory
output
is
tapped from
a
point
close
to
the
end
of the resonant
element
via
R16
and
is
fed
to
pin
6
of the
board.
Transistors
Q1
and
Q2
stabilize the
d.c.
supply
rails.
4.6
The
oscillator
output
level
is
controlled
by
Q3,
the
base
of
which
is
held
at the
required potential
by
the divider R14,
R1
5
and
the
forward
volt
drop
in
D3, 4
and
5.
D6
prevents the
D3
,
R1
5 junction going
positive.
4-1
9081
&
9082 Vol
.
2
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