Daewoo DSC-3220E/3220L service manual Standard Features, FPLL, VCO and AFC

Service manual SC-150

SAW filters

Ref.	Standard	Features
K3953M	B/G - D/K - I - L/L’	- IF filter for video application
		- TV IF filter with Nyquist slopes at 33.9 MHz and
		38.9 MHz
		- Constant group delay
K9650M	B/G - D/K - I - L/L’	- 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

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 can be adjusted at Pin 10 by a potentiometer or an external dc voltage (from interface circuit or microprocessor).

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 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”an d 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 (0o) and the quadrature signal (90o) of the generated picture carrier.

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