Akai VS-2EGN - Page 96

11-3-2 VIDEO IF (VIF) IC AND PERIPHERAL CIR-
CUITS
IF signals having their characteristic determined by the
SAW filter, enter VIF ICI pins(1) and (19), pass through
the 3-stage VIF amp, are detected at the video detector,
then are amplified, and output to pin ({) via the emit-
ter follower. This IC has an AGC circuit for the gain
control, and an AFT voltage generator for the control of
the tuner local oscillator frequency.
1) Detector circuit
VIF INO OVIDEO OUT
TUNING CIRCUIT
Fig. 7-159 LLD circuit composition
An LLD (Low Level Detector) is employed as the detect-
or circuit (normally IC). Its feature is that a large video
detection output can be obtained from a low level video
IF input.
As shown in Fig. 7-159, the LLD consists of a limiter,
a double balanced detector, and a tuned circuit.
2) Fig. 7-160 shows the limiter circuit, the IC part
shows its equivalent circuit.
mores | TUNING CIRCUIT
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Amplitude variations of the VIF are sliced off by the dif-
ferential amplifier (TR1, TR2), and only the carrier is
fed to the LC tuned circuit. The square wave is generat-
ed by clipping this carrier by D1 and D2.
The limiter circuit is made up of C16, L4, and R20 con-
nected to ICI pins () and (9) (Fig. 7-113(D)). Adjust-
Service Manual VS-2EGN
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Fig. 7-160 Limiter circuit
ment to the VIF frequency is made by adjusting L4.
R20is a damping resistor, and itis for reducing the TR of
the tuned circuit. When the TR is too high, adjustment
becomes difficult, and a slight deviation of VIF results in
a large change in the detector output. A temperature
compensating capacitor is used for C16.
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