DAEWOO DTE29U8
Manual de Servicio

CP830F Service Manual

5.2.2 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. SAW filters Ref. K3953M Standard B/G - D/K - I - L/L� Features IF filter for video application TV IF filter with Nyquist slopes at 33.9 MHz and 38.9 MHz Constant group delay IF filter for audio application TV IF audio filter with two channels Channel 1 (L�) with one pass band for sound carrier at 40.40 MHz Channel 2 ( L, D/K, I, B/G) with one pass band for sound carriers between 32.40 MHz and 33.40 MHz

K9650M

B/G - D/K - I - L/L�

5.2.3 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 criterion 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 positive 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 current at Pin 11 (open collector output). The take over point of the tuner AGC is adjusted at Pin 10 by an external dc voltage from microprocessor. A PWM output from microcontroller is low pass filtered for this AGC control. See also �AGC� adjustment for details on how to align TOP in SERVICE mode. 5.2.4 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 C parallel) 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 switch (Pin 19) three operating conditions of the AFC are possible: AFC curve �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. Furthermore, 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~ ) and the quadrature signal ~ )of the generated picture carrier.

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