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 external capacitor

and used to drive the AGC network, which allows

an input regulation of the video carrier from less

than 100

µV 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 characteristics as 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.

CIRCUIT OPERATION

TDA8120B

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