TDA4472-A Datasheet, PDF (3/14 Pages) TEMIC Semiconductors – Video-IF (VIF) with FPLL Demodulation and Quasi Parallel Sound (QPS) Processing

TELEFUNKEN Semiconductors
TDA4472-A
Circuit Description
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). 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 gain of
VIF amplifier and tuner in order to keep the video output
signal at a constant level. Therefore in case of all negative
modulated signals the sync level of the demodulated
video signal is the criterion for a fast charge/discharge of
the AGC capacitor. 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). 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).
FPLL, VCO and AFC
The FPLL circuit (frequency phase locked loop) consists
of a frequency and phase detector to generate control volt-
age 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. A practi-
cable VCO alignment of the external coil is the
adjustment to zero AFC output current at Pin 22. At centre
frequency the AFC output current is equal to zero. 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°C) and the quadrature signal
(90°C)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 with internal feedback and
8 MHz bandwidth (–3 dB). An additional noise clipping
is provided. The video signal is fed to VIF-AGC and to the
video output buffer. This amplifier with 6 dB gain offers
easy adaption of the sound trap. For nominal video
IF modulation the video output signal at Pin 12 is 2 V_pp.
Sound IF Amplifier and SIF-AGC
The SIF amplifier is nearly identical with the 3-stage
VIF amplifier. Merely 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 different SAW filters. Both
SIF inputs features excellent cross-talk attenuation and
an input impedance which is independent from the
switching condition.
The SIF-AGC is related to the average level of the FM-
carrier and controls the SIF amplifier to provide a
constant SIF signal to the QPS mixer.
Quasi-Parallel-Sound (QPS) Mixer
The QPS mixer is realized by a multiplier. The SIF signal
(FM or NICAM carrier) is converted to the intercarrier
frequency by the regenerated picture carrier (quadrature
signal) which is provided from the VCO. The intercarrier
signal is fed via an output amplifier to Pin 24.
AFC Switch
The AFC output signal at Pin 22 can be controlled by a
switching voltage at Pin 19. It is possible to switch off the
AFC.
VCR Mode
For the VCR mode in a TV set (external video source
selected) it is recommendable to switch off the IF circuit.
With an external switching voltage at Pin 6 or 7 the
IF amplifiers are switched off and all signal output levels
at Pin 12, 24 are according to the internal DC voltage.
Internal Voltage Stabilizer
The internal bandgap reference ensures constant perfor-
mance independent of supply voltage and temperature.
Rev. A1: 26.07.1995
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