Kenwood KR-4070 Service Manual page 8

KR-4070
CIRCUIT DESCRIPTION
The IF circuit employs IC HA1137W in which quadrature
detector circuit, muting circuit, and meter circuit are incor-
porated. (See block diagram.)
Quadrature detection is a sort of phase detection. A signal
from the third !F AMP is directly applied to one side of input
circuits of the multiplier of quadrature detector. and another
signal is applied to the other side through the phase shifter of
Lg3. The variable in phase difference obtained from these
two signals is utilized for detection.
AUXILIARY CIRCUITS
S-Meter (Signal meter) circuit
The S-meter output 1s obtained directly from No. 13 pin of
{Cb2.
T-meter (Tuning meter) circuit
When a tuned frequency is drift from its center, a voltage is
'produced between the No. 7 pin and No. 10 pin of ICg1.
This voltage is used to deflect the meter pointer.
A voltage is applied to the No. 5 pin of ICg1 to operate the
audio muting amplifier |Cg1. Since the voltage at the No. 12
pin of ICg1 is applied to the No. 10 pin of !Cg3. the signal is
forcedly changed into a monaural signal to prevent the stereo
lamp being mis-operated.
FM MUTING CIRCUIT
Detection procedure
Intensity of input level is detected from a position where a
signal equivalent to the quadrature detection input is ob-
tainable (NOISE DETECTOR). The obtained signal is
supplied to the MUTE DRIVER CIRCUIT. On the other hand,
DC output (discrimination curve) of the quadrature detector
is fed to the OV SWITCH and output is generated as shown in
Fig. 2-(a). This output and above-mentioned input level
detecting signal are put in the OR CIRCUIT, thus generating
an OR output as shown in Fig. 2-(c).
Namely. when the input signal is weak or deviates from the
discrimination curve of the FM detector, operating voltage for
muting is generated exceeding the muting level. (The
muting start leve! corresponds to the antenna input below
BuV.)
(a) OV Switch Output (b) Mute Driver Circuit Output
IF sar he characteristic
Discri
has curve
oc
stating component f
a ei ee MHz
10.7 MHz Ly
\ circuit
$c) OR Circuit Output
!
|
——+++4—— Muting level
10.7 MHz
Fig. 2 HA1137W MUTING DETECTION
MPX CIRCUIT
Operation of LA3350S
The MPX circuit uses PLL IC LA-3350S (See the block
diagram).
The PLL is composed of a phase detector (PD), a low pass
filter (LPF) and a voltage controlled oscillator (VCO). forming
a electronic closed loop servo system.
19 kHz
Pilot
PD
vco
Phase
signal
erase A
Phase
OC voltage
proportional to
the phase
difference
38 kHz
Sub-carrie:
19 kHz
Fig. 3 PLL 1C FUNDAMENTAL BLOCK DIAGRAM
As shown in the figure, a 76 kHz square wave Signal is
produced in VCO, which is divided into 19 kHz through the 2-
step divider and its phase is compared with that of the 19
kHz pilot signal by PD.
lf there is a difference between these phases, a DC voltage
corresponding to the phase difference is developed and is fed
to VCO through LPF and DC amplifier. as a DC contro!
voltage. With this DC voltage, VCO is controlled so that the
output is in phase with the pilot signal.
Thus, a 38 kHz sub-carrier phase locked in the 19 kHz pilot
signal is obtained to prevent the deterioration of separation
due to phase deviation.