Samsung CX528ZSE Service Manual page 25

(4) Operations of the SIF section.
CERAMIC
FILTER
SIF IN
SIF FILTER
DC ATT VR SIF DET
VIDEO AMP
B/W NOISE
CANCELLER
FM IF AGC FM DET DC ATT AF AMP AF AMP VIDEO
DISCRIMINATOR FILTER OUT IN NFB OUT OUT
Fig 9. SIF block diagram of LA7520/KA2919.
The SIF section provides all functions for the SIF sub-system, which consist of 3—stage limiter amplifier,
FM quadrature detector, DC volume control and audio pre-amplifier with NFB terminal.
A. SIF limiter :
2601 is 6.5MHz ceramic filter that passes the 2nd SIF signal only eliminates the video signal and
other signals.
This inputted signal is amplified to a level of several volts for the FM detector and limited (clipped) to
eliminate AM interferences, limiting the amplitude variations does not distort the frequency variations
of the signal. A limiter is the SIF amplifier tuned to center frequency, but with the DC operating vol—
tages reduced so that the limiter operates as a saturated amplifier.
B. FM detector :
The output of the limiter amplifier is applied to the FM detector internally, and is tuned to 6.5MHz.
The FM detector is a quadrature detection type using the limiter output and 90° phase shifted signal
by an SIF detection circuit (R607, Z602, R606, C604) connected between Pin 1 and Pin 30. Refer to
Fig. 10.
The characteristics of distortion and bandwidth of
the FM detector are determined by R606 and R607 ai SISERIM MTOR
C604 controls the DC level of AC output from Pin 2. Pin 30
Detected AF signal from Pin 2 is connected to the
DC volume control circuit (Pin 3) through a de— Fig 10. FM detection
emphasis circuit of R608 and C605
De—emphasis circuit attenuates the higher frequencies to compensate for the boost of high frequen—
cies to by the broadcast transmitter.
Boosting of the high frequencies is referred to as pre-emphasis. The purpose is to improve the signal—
to~-noise ratio for FM reception.
(2602)
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