Daewoo DTL- 25G6F, Руководство по обслуживанию

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Vision IF Amplifier
The video IF signal(VIF) is fed through a SAW filter to the differential input(pin 6-7) of the VIF amplifier. This amplifier
consists of three AC-coupled amplifier stages. Each differential amplifier is gain controlled by the automatic gain
control(VIF-AGC). The output signal of the VIF amplifier is applied to the FPLL carrier generation and the video
demodulator.
Tuner-and VIF-AGC
At Pin 8, the VIF-AGC charges/discharges the AGC capacitor to generate a control voltage for setting the gain of the VIF
amplifier and tuner in order to keep the video output signal at a constant level. Therefore, in the case of all negative
modulated signals(e.g., B/G standard) the sync. level of the demodulated video signal is the cirterion for a fast
charge/discharge of the AGC capacitor.
For positive modulation(e.g., L standard) the peak white level of video signal controls the charge current. In order to
reduce reaction time for fositive modulation, where a large time constant is needed, an additional black level detector
controls the discharge current in the event of decreasing VIF input signal. The control voltage(AGC voltage at Pin 8) is
transferred to an internal control signal, and is fed to the tuner AGC to generate the tuner AGC can be adjusted at Pin 10
by a potentiometer or an external dc voltage(form interface circuit of microprocessor).
FPLL, VCO and AFC
The FPLL circuit(frequency phase locked loop) consists of a frequency and phase detector to generate the control
voltage for the VCO tuning. In the locked mode, the VCO is controlled by the phase detector and in unlocked mode, the
frequency detector is superimposed. The VCO operates with an external resonance circuit(L and Cparalled) and is
controlled by internal varicaps. The VCO control voltage is also converted to a current and represents the AFC output
signal at Pin 22. At the AFC are possible: AFCcurve “rising” or “falling” and AFC “off”.
A practicable VCO alignment of the external coil is the adjustment to zero AFC output current at Pin 22. At center
frequency the AFC output current is equal to zero. Furthemore, at Pin 14, the VCO center frequency can be switched for
setting to the required L’ value(L’ standard).
The optional potentiometer at Pin 26 allows an offset compensation of the VCO phase for improved sound quality(fine
adjustment). Without a potentiometer(open circuit at Pin 26), this offset compensation is not active.
The oscillator signal passes a phase shifter and supplies the in-phase signal(0¡£) and the quadrature signal(90¡£) of the
generated picture carrier.
Video Demodulation and Amplifier
The video IF signal, which is applied from the gain controlled IF amplifier, is multiplied with the inphase component of the
VCO signal. The video demodulator is designed for low distortion and large bandwidth. The demodulator output signal
passes an integrated low pass filter for attenuation of the residual vision carrier and is fed to the video amplifier. The
video amplifier is realized by an operational amplifier wth internal feedback and 8MHz bandwidth(-3dB). Astandard
dependent dc level shift in this stage delivers the same sync. level for positive and negative modulation. An additional
noise clipping is provided. The video signal is fed to VIF-AGC and to the video output buffer. This amplifier with a 6dB
gain offers easy adaption of the sound trap. For nominal video IF modulation the video output signal at Pin 12 is 2V
pp
.
Sound IF Amplifier and SIF-AGC
The SIF amplifier is nearly identical with the 3-stage VIF amplifier. Only the first amplifier stage exists twice and is
switchable by a control voltage at Pin 3. Therefore with a minimal external expense it is possible to switch between two
differnt SAW filters. Both SIF inputs features excellent corss-talk attenuation and an input impedance which is
independent from the switching condition.
signal to the AM demodulator and QPS mixer.
Circuit Description