B.
ln
the case
of detuning
to
frequencies higher
than the
perfectly
tune{
point
ln this
cese,
if the circuit
is
set
to
input
the
positive
voltage, the
potential
acts to charge C405,
and
when
the
Vge
of
TR406
exceeds
0.6V,
TR406 is turned
"ON".
Sìnce
TR407
is biased
in
the reverse direction, it remains
in
rhÊ
'''OFF"
stare.
rvith
the
ground
potential
being applied
to the
base of TR409.
Upon
turninE
"ON"
of
TR406,
the base
potential
of T8408
is
also
raised
for
further
maintaining
"ON"
state. Accordingly,
the
emitter
potential
of TR408
is
raisecj, with
the
Vse
of TR409 reduced belcw
0.6V, thus
TR409
being
turned
"OFF",
and
positivevoltageappearsatthecollectorofTB40g. lnotherwords,theinputvoltagetocauseC40Stobecharged
up
to
the voltage
for turning
TR406
"ON"
determines
+Af
of Fig.
4.
Upon
exceeciing
+Àí, positive
voltage
ap-
pears
as the
output.
C.
ln
the case of
detuning to frequencies
lower than
the
perfectly
tuned
po¡nt
ln this
case,negative
potential
is input and
tiiis
voltage
gradually
chargesC405.
When the VBË
of
TR407
reaches
-0.6
V,
TR407 is turned
"ON",
with
negative
potentiai
appearing
at the emiliér, and
TR409 is brought
into
"OFF"
state,
with
positive potential
being developed at
its
collecior.
Meanwhile, TR406
biesed in the
reverse
direction
is kept
in
"OFF"
state, and
0V is applied to the
base of TF408'to
maintain
"ON"
state.
ln
other
words, the
input
voltage
to cause C405
to be charged up fo
the
voltage íor turning TF407
"ON"
determines
-Âf
cf
Fig.
4. Upon
exceeding
-Af,
positive
voltage
appears
as
input.
When
summarized, the above
description is that the outpút
a't
the
tuned
tin,e-(Il
and
éetarned
time
(III)
of Fig.
4
is of
0V,
and
only in the non-tuned
time
(lI)
,
positive
vo.ltage
is developed at
the ouiput, The
tuned
range
Ì'^f oí
-(I)
is deterrnined by the capaciry
of C405.
TRIO6
J1JL
I
I
I
i
I
!
:
I
o
I
c¡t
I
o
0
I
I
P?rtecily
tuñeC oo¡nt
IRI{}T
o
{¡NPUT
VOLTAGE
AND
OUTPUl
VOLTAGE)
Fig.
4
@
Sllv
level
switch circuit
Since
the
muting
band
switch circuit
functir¡ns in
the
similar
manner in
the
detuned staie
as
in
the
tunecl state,
noises
are
generated
in
the
signal
systern. As counte!'-measures against
subh
inconveniences, the
mutilìg
circu¡Î
functioning at
band
ranges wider
than
Aí is
required.
The
circuit
Íor
such
a
purpose
in
sho.¡¡n
in
Fig.
5.
The I'F
amplifierl3te¡:minal
voltageof
lC2A3variesinproport¡ontotheFMinputsignal
level.
Thisvoltageisapplie'Jto
T8413
through
R431. ln
the
absenæ of
signals. the
13
term¡nal
is
of
0V,\,vith the base
of
TR4i3
also
being of
0V,buttheVBE of theemitterof
TB41
3{-0.6V) bythepresenceof
D4O6,andthr.¡sTR4
13isin"ON"state.
Meanshile,
the
VBÉ
of
TR4i2 is
also set
to
be 0.6V, witn
TÞ,412
kept in
"ON"
state,v'/hile
f841 1 is in
"OFF"
state, thus
positive.potential
being developecj
es the output.
As
the FM input signal
increases,
the
poteniial
to
be
applied tq
TR413 is also
raised,
with increased
collec-
tor current,
in whìch case, the
collector
polent¡al
of
Tn413
is ciecreased to turn
"OFF"
TR412.
l.low-
e---
ever,
VB40 l
is
connected to the base
of TÌ412,and
o--|
|
OB
it is
so arranEed
thatJR412
is turned
"OFF"
only by
the input
voltâge of
TR413
when
S/N of
FM
input
signál exceecls
50
dB.
When
(muting
level
adjusting
volurne) TP412
is
turned
"OFF",
positive
potential
is
applied to
TR41 1
to turn
it
"ON",
with
the output
being
rendered
io be of
ground
potential.
ln
short,'
this
icircuit
ís one
v¡hich functions
only
when
the
S/N
of
the
FM
input signal is over
50 dB,
'
..:
'.
{f!IUTING
BAND
SWITCH
CIRCUIT}
Fis.3
Oulpuì
"
l¡prl
rc20¡
TRII¡
TRI I3
iz_
o
-s/N
lñplt
leY?l
and its
ourput
as
shown in Fig.
5.
(S/N
MUTING LEVEL
SYVITCH
CIRCUIT)
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