Daewoo DDT-21H9ZDF(21"), Руководство по обслуживанию

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Service Manual CP-099/FS
5-2 IF section
The Picture IF signal (PIF) is fed through a SAW filter to the differential input (Pin 1-2) of the PIF amplifier.
5-2-1 Picture IF Amplifier
This amplifier consists of three AC-coupled amplifier stages. Each differential amplifier is gain controlled by the
automatic gain control circuit (PIF-AGC). The output signal of the PIF amplifier is applied to the FPLL carrier
generation and the video demodulator.
This block adapts the voltage, generated at the VIF/SIF-AGC detector, to the internal signal processing at VIF/SIF amplifier
and performs the tuner AGC control current generation. The onset of tuner AGC control current generation can be set either by
I
2
C-bus (see Table 23) or optional (in case that I
2
C-bus information cannot be stored) by a potentiometer at pin 9. The presence
of a potentiometer will automatically be detected and disables the I
2
C-bus setting.
Furthermore, derived from the AGC detector voltage, a comparator is used to test the corresponding VIF input level to be sma-
ller than 200 . This information can be read out ('VIF HIGH LEVEL') via I
2
C-bus.
5-2-2 Tuner and VIF-AGC
Gain control performed by sync-level detecion (negative modulation) or peak White detection (positive modulation).
For negative modulation the sync level voltage is stored at an interrated capacitor by means of a fast peak detector.
This voltage is compared with a reference voltage (nominal sync level) by a comparator which charges/discharges the (integra-
ated) AGC capacitor for generation of the required VIF gain. The time constants for decreasing/increasing gain are nearly equal
and the total AGC reaction time is fast to cope with 'aeroplane fluttering'.
For positive modulation the white peak level voltage is compared with a reference voltage (nominal white level)by a compara-
tor which charges (fast) or discharges (slow) the (external) AGC capacitor directly for generation of the required VIF gain.
This very large time constant for VIF gain increase in needed caused by the fact the peak whith level may appear only once in
a field. In order to reduce this time constant, an additional level detector increases the discharging current of the AGC capacitor
(fast mode) in the event of a decreasing VIF amplitude step controlled by the detected actual black level voltage. The threshold
level for fast mode AGC is typical-6 dB video amplitude.
The fast mode state also is transferred to the SIF AGC detector for speed-up. In case of missing peak white. pulses the VIF gain
increase is limited to t3 dB by comparing the detected actual black level voltage with a correspond-
ing reference voltage.
5-2-3 VIF-AGC detector
The VIF amplifier output signal is fed into a frequency detector and into a phase detector via a limiting amplifier for removing
the video AM. During acquisition the frequency detector produces a current proportional to the frequency difference between
the VIF and the VCO signal.
After frequency lock-in the phase detector produces a current proportional to the phase difference between the VIF and the VCO
signal. The currents from the frequency and phase detector are charged into the loop filter which control the VIF VCO and
locks it to the frequency and phase of the VIF carrier. For positive modulated VIF signal the charging currents are gated by co-
mposite sync in order to avoid signal distortion in case of overmodulation.
The gating depth is switchable via I
2
C-bus.
5-2-4 Frequency Phase-Locked Loop(FPLL) detector
The VCO of the VIF-FPLL operates as integrated low radiation oscillator at double the picture carrier frequency. The control
voltage required to tune the VCO to actually double the picture carrier frequency is generated at the loop filter by the frequency
Phase detector. The typical possible frequency range is about 50 to 140MHz.
The oscillator frequency is divided-by-two for providing two differential square wave signals with exactly 90 degrees phase di-
fference, independent of the frequency, for use in the FPLL detectors, the video demodulator in the intercarrier mixer.
5-2-5 VCO and divider