Daewoo DV-F24S, Service Manual

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3-3. IF CIRCUIT OPERATION
1. VIDEO SIGNAL FLOW
The signal from the ANT is amplified for selectivity, to decrease image interference, and to increase S/N using the RF AMP.
The RF signal at the MIXER is subtracted from the LOCAL OSC frequency using the upperside band method, to change it
into the IF signal; 38.9MHz. The IF signal converted from the RF signal in the tuner block is amplified by about 30dB to
increase S/N in the pre-amp block. The reason for this is that the SAW filter has its own insertion loss of about –18 to
–22dB. The SAW filter is a kind of BPF, used to remove the near channel harmonics and make the desired frequency
response. An IF AMP of about 60 to 70dB gain is desired for receiver sensitivity and selectivity. The vision IF AMP consists
of three AC-coupled differential amplifier stages; each stage uses a controlled feedback network called AGC. To maintain
the video output signal at a constant level the automatic control voltage is generated according to the transmission
standard. For negative modulation in the PAL standard the peak-sync level is detected, and for positive modulation in the
SECAM standard the peak-white level is detected. The AGC detector charges and discharges the AGC capacitor to set
the IF gain and the tuner gain. The standard is switched by the µ-COM. We can also adjust the tuner AGC voltage take
over point. This allows the tuner and the IF SAW filter to be matched to achieve the optimum IF input signal. The IF
amplifier output signal is fed to a frequency detector and to a phase detector. The frequency detector is operational before
lock-in. A DC current is generated which is proportional to the frequency difference between the input signal and the VCO
frequency. The control voltage for the VCO is provided by the phase detector.
The video modulator is a type of multiplier. The vision IF input signal is multiplied by the in-phase component of the VCO
output. The demodulated output signal is fed via an integrated LPF (about 12MHz) to the video amplifier for suppression
of the carrier harmonics.
The polarity of the video signal is switched in the demodulated stage according to the standard switch. The VCO operates
with a symetrically-connected reference L-C circuit, running at the double vision carrier frequency (77.8MHz) to decrease
the frequency error. Frequency control is performed by an internal varicap diode. The voltage used to set the VCO
frequency to the actual double vision carrier frequency, is also amplified and converted to give the AFC output current.
The AFC output is fed to the tuner to change the LOCAL OSC frequency and to the µ-COM for channel searching. The
VCO signal is divided by two in a travelling wave divider, which generates two differential output signals with exactly 90
degrees phase difference, independant of frequency. The video signal passing through the 5.5MHz sound trap is fed to
the buffer.
RF
AMP
MIXER
PRE-
AMP
SAW
FILTER
IF
AMP
VIDEO
DET &
DE MOD
SOUND
TRAP
BUFFER
VIDEO
OUT
VCO AFC
AGC
DET
LOCAL
OSC
µ-COM
STANDARD S/W
TUNER
ANT