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T
t
r
t
I
t
I
r
t
I
t
t
rl
l
t
I
I
T
t
stage
(C)
is reduced,
and
the
reactance
of the
in-
ductance
stage
(L)
increased,
resulting
in
AM
detection
by D2
which
leaves
the
positive
portion
of tlie
IF
signal.
If
the input frequency
decreases,
L
stage reactance
is
decreased
and
C stage
reactance
increased,
resulting
in AiVl
detection
by
D1
which
leaves
the negative
portion
of the IF
signal. The
L
stage
and C
stage
reactances
increase
as the
deglee
of detuning
in
the respective
directions
is in-
creased,
resulting
in
a
subsequent
increase
in
the
detector
output. By
thus
attaining
S-curve
charac-
teristics, FM
detection
becomes
possible.
Since
the
IF
signal
is
an
FIvI
signal
frequency
deviation
due
to modulation
will
be symmetrical
about
a
central
axis, And
if
the
central frequency
is
equal to
the
resonance
frequency,
the detector
output
DC
level
will
be zero. If,
however,
there
is
any
displacement
in
the central
frequency,
frequency
deviation
in
respect
to the
detector
will
become
asymmetrical,
resulting
in
the
generation
of a DC
voltage.
This
DC voltage
is
passed
through LPF1
(IF
filter)
and
LPF2
(AC
filter)
to form
a correction
voltage
which
is
applied
to the variable
capacitance
diode
in
tlre iocal
oscillator,
thereby
correcting
the
oscillator frequency
to
obtain
a constant
tF
(i.e.
a
constant
tuned
frequency).
Since
the
central frequency
of the
crystal
detec-
tor is
regulated
by
the
crystal
resonator,
tuned
fre-
quencies
of
extremely
high
stability
are
obtained.
r
Limiting
the
Locking
Range
If
lhe
quartz-lock
range
is
too
wide,
it
will
overiap
with
strong
adjacent
broadcasting
frequen-
cies
and
result
in
considerable
tuning
difficulties.
A
DC
amplifier
is
therelore
used
as
a limiter
(limiter
action
by
NFB
circuit
zener
diodes)
which
restricts
the voltage
applied
to
the variable
capaci-
tance
diode,
thereby limiting
the
quartz-lock
range.
A
DC
voltage
appea$
at
pin
no.L3
of
the IF
system
IC
(PA3007-A)
when
the
antenna
input
level
drops
below
5pV,
or when
the
tuned
fre-
quency
has been
detuned
by
more
than
r100kHz.
This
DC voltage (FM
muting
signal)
is
applied
to
the
gate
of
Q2
(fET)
via
a Schmitt
circuit,
result-
ing
in
the FET
being
turned
on, and
the
quartz-
Iock
circuit
being
turned off.
sx-gaoo
4.2
AM
TUNER
The
AM
tuner
section
consists
of
a 2-ganged
tuning
capacitor
plus
an
IC
(HA119Z)
which
con-
tains
a l-stage
RF
amplifier,
converter,
2-stage
IF
amplifier,
detector,
and AGC
circuit.
The
AM
STEREO
OUT
terminal
on
the rear
panel
is
for
connecting
to an
AM
stereo
broadcast
decoder
adaptor.
The
signal
appearing
at this
ter-
minal
is
the converter
output
passed
via a
buffer
(
emitterf
ollower)
stage.
4,3
DISPLAY
CIRCUIT
Frequency
Display
Frequencies
received
by
the SX-9800
are
dis-
played
in
digital
form
by
fluorescent
indicator
tube
(FL
tube). Each
digit employs
up
to ?
segments
(a
-
e)
(see
Fig.
4-3)
to
display
all
numerals
from
0 to
9
(with
the exception
of the left
hand
digit
which
employs
only
2 segments
b and
c).
o
fi'
Fig.4-3
7-segment
digit
display
The
signal
source
during
both AM
and
FM
recep-
tion
is
the local
oscillator.
The
signal
is
passed
via
a buffer
amplifier (FET)
to
the
prescalar
IC
(M54451P)
where
it
is
subjected
to frequency
divi-
sion
(1/8
for
AM
and
L/80
for FM)
before
being
rFRlied
to
the
frequency
counter
IC
(pDb0Og).
This
IC
is
responsible
for
the
dynamic
drive
of
the
?-segment
S-digit
display
(each
digit
being
turned
on
according
to time-shared
sequential
scanning).
An
outline
of
the
composition
of
pDb009
is
given
in
block
diagram
form
in Fig.
4-b.
With
the
FL
tube
a
-
g
segments
(anode)
for
each
digit
con-
nected
in
parallel,
the
D1
-
Db
time
division pulse
signals
(see
Fig.
4-6)
apptied
to
each
grid
(inde-
pendent
grid
for
each
digit)
result
in
the
digik
being
lit
up in
succession
from
the left
hand
side.
Each
digit
is
lit
up for
lms
during
each
bms
inter-
AM
LOCAL
osc.sr6NAL
FM LOCAL
OSC.SIGNAL
FM"aM sELgcrroN
FL TUBE
a
S IGNAL
Fig.
4-4
Frequency
display
block
diagram
a
rc
(M5445rP)