TDA8120B Datasheet, PDF (5/6 Pages) STMicroelectronics – MULTISTANDARD VIDEO AND SOUND IF SYSTEM

TDA8120B
CIRCUIT OPERATION
The TDA8120B (see block diagram) consists of a
video section and a sound section. The integration
of both sections on the same chip requires a high
isolation at IF frequencies. This is achieved by
physically separating the two sections, with sepa-
rate power supplies and ground pins. In addition,
special care has been taken in the choice of pad
positions for the IF inputs and sound/video outputs.
The video section consists of three AC-coupled IF
stages with more than 60dB AGC range, flat ampli-
tude/frequency response from 10 to 85MHz and
linearized phase slope from 30 to 50MHz. Video
carrier regeneration is performed by a tuned limiter.
The carrier is then applied to the video demodulator
through a special circuit which switches the carrier
phase from 0 to 180°so that the video polarity can
be maintained constant when the standard
switches from B/G to L. A noise inverter and a white
spot inverter are included to eliminate ultra-black
and white pulses.
A top sync or a top white clamping circuit and a
minimum DC video component detector are imple-
mented by two double comparators the charac-
teristics of which may be controlled by an external
control input to adapt to the modulation type for
each standard. The voltage at the output of the two
comparatorsis memorized by an externalcapacitor
and used to drive the AGC network, which allows
an input regulation of the video carrier from less
than 100µV to 100mV. A delayed control storage
with current output for the turner AGC completes
the video section.
The sound section consists of three IF stages with
the same characteristicsas the video IF stages and
an identical network to control and set the gains of
the three IF amplifiers. The output of the third IF
stage feeds the AM/AGC detector and the QSS
section.
The AM/AGC detector consists of a wideband
limiter for AM sound regeneration or video carrier
regeneration used to feed the synchronous multi-
plier and consequently to obtain the AM demodu-
lated audio signal. In addition, a DC voltage
proportional to the peak-to-peak value of the video
carrier is produced. Two comparators complete the
sound AGC loop.
The subsequent QSS section consists of a refer-
ence amplifier tuned to the video IF which buffers
a wideband limiter to reject completely the video
AM information withou t introducing incidental
phase modulation (IPM).
Following the limiter there are a 90° phase shifter
and a linear-to-logarithmic converter which drives
a linear multiplier as a demodulator for the intercar-
rier 2nd sound IF. This quadrature multiplier rejects
all video components transmitted in DSB that is low
frequency components of the video signal.
In addition to the sound and video sections, the
TDA8120B includes a block for standard switching
(B/G or L) controlled by a TTL-compatible input.
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