Circuit Descriptions; Fm Tuner Section; Front End; If Amplifier And Detector - Pioneer SX-3900 Service Manual

4 . C I R C U I T D E S C R I P T I O N S
4 . 1 F M T U N E R S E C T I O N
Front End
The FM front end of SX-3900 includes
a 4
ganged tuning capacitor, a dual-gate MOS
FET-
Eo"Too"A l-slage RF amplifier' and
a modified
ai;;
circuit llcal oscillator' This oscillator is a
uoiiug" controled oscillator employing
a vari-cap
(varia'ble capacitance diode)' When the.quartz-lock
,yrt"..t (refer to "Quartz-lock system")
is not in
operution, a constant voltage is applied
to the
diode.
lF Amplifier and Detector
These employ 3 ICs and 3 dual-element ceramic
filters. fne lC (HA1201) of the first 2
stage con-
stitutes a single-stage differential amplifier
current-
ii-lti.rg limitet. The IC (PA300?-A)-in
the third
stage, an improvement on the former
iF system IC
liigool
ai,
includes an iF iimiter amplifier'
quadrature detector, meter drive' and other
cir-
cuits. Performance in terms of distortion'
S/N
ratio, delay characteristics, and other
parameters'
,ho*,
a marked improvement in comparison
to
the PA3001-A.
Ouartz-Lock SYstem
The quartz-lock system featured in the
SX-3900
stereo receiver is a frequency serl/o control
system
employing a crystal resonator' Any
displacement
i.r itt"-lntlrmediate frequency (IF) is
detected as a
DC voltage by the discriminator (equipped
with a
crystal resonator)' resulting in the local
oscillator
ir"qt"r-r"y
being corrected and subsequently
locked. This extremely stable frequency
servo con-
trol system thus ensures that tuned
frequencies
remain tuned securely for as long as required'
When the IF signal appears at pin no'17
of the
IF system IC (PA300?-A), it is amplified and
ap-
ptiei to a crystal detector (see Fig' 4-1) which
con-
sists of diodes connected in parallel in
a series
resonance circuit equipped with a crystal
resona-
i.r. irt" resonance frequency is the same
as the IF
fr"qr".r"y (10'?MHz), which means the impedance
at it ls time will be minimal, resulting
in the out-
fut being reduced to a minimum
level' If the input
iruq.r"rr"y increases- the reactance of the
capaci-
tanie stage (C) is reduced, and the reactance
of the
ir"rar"tun"u stage (L) increased, resulting
in AM
Jetection by D2 which leaves the
positive portion
of the IF signal. If the input frequency
decreases'
L stage reactance is decreased and C stage
reac-
tance increased, resulting in AM detection
by D1
which leaves the negative portion of the
IF signal'
it'r" t stage and C stage reactances inctease
as the
degree oidetuning in the respective directions
is
in&eased, resulting in a subsequent increase
in
the detector output. By thus attaining
S-curve
characteristics, FM detection becomes
possible'
Since the IF signal is an FM signal frequency
devia-
tiorl-a""
to iodulation,
it will be symmetrical
about a central axis. And if the central frequency
is
equal to the resonance frequency, the detector
out-
o,rt OC level will be zero. If, however, there is any
displacement in the central frequency, frequency
deviation in respect to the detector will
become
aryrrr-utri"a1, resulting in the generation
of a DC
"liiug".
This DC voitage is passed through LPF1
(IF filter) and LPF2 (AC filter) to form a correc-
iion voltage which is applied to the variable
capaci-
t a n c e d i o d e i n t h e l o c a l o s c i l l a t o r , t h e r e b y c o r r e c t -
ing the oscillator frequency to obtain a
constant
tn' (i.e. a constant tuned frequency)'
Since the central frequency of the crystal
detec-
tor is regulated by the crystal resonator'
tuned
frequencies of exiremely high stability
are ob-
tained.
l c ( P A 5 0 0 7 - A )
F M F R O N T E N D
CRYSTAL DETECTOR
| ----, I
: ( L ) 1 3
+ i
C I R C U I T
F i g . 4 - 1 O u a r t z L o c k S Y s t e m
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