February 1991

5

Philips Semiconductors

Product specification

Multistandard IF amplifier and demodulator

TDA8349A

bandgap also reduces the supply ripple on the video

output to values less than

−30 dB. The video amplifier has

a typical bandwidth of 10 MHz which allows application for

all new video standards with bandwidths of up to 10 to

12 MHz. The video output signal has an amplitude of

2 V (p-p). White spot protection comprises a white spot

clamp system combined with a delayed-action inverter

which is also highly resistant to high sound carriers. A

switchable DC shift for positively modulated IF signals

ensures correct signal handling. This switching is obtained

via pin 20, which is the same pin used for switching the

demodulation polarity in the demodulator.

The circuit also has a noise clamp which prevents the

video output becoming less than

±400 mV below the top

sync level at noise peaks. The output buffer of the video

amplifier consists of a class A/B circuit which can handle

large source as well as large sink currents. This makes the

circuit more flexible in several applications with one or

more ceramic filters connected to this output buffer.

AGC control circuit

This converts the AGC detector voltage (pin 5) into a

current signal which controls the gain of the IF amplifier. It

also provides a tuner AGC control output from pin 4,

current limiting is incorporated to prevent internal damage.

The AGC starting point is adjusted by a voltage between 3

and 5 V for pnp tuners and between 7 and 9 V for npn

tuners via pin 3.

AGC circuit

A new AGC system has been designed for the AGC. It will

be a top sync-detector for negatively modulated signals

and a top white level AGC for positively modulated signals.

For optimal flexibility reasons the load and unload currents

of the AGC are chosen such that both, a relatively fast set,

as well as a set with a low tilt are possible for positive (L)

and negative (B/G) modulated signals. For this reason a tilt

ratio between positive (L) and negative (B/G) of

approximately 3:1 has been chosen. This means that in a

fast set the choice of a typical tilt for negatively modulated

signals of 2% will obtain a typical tilt for positively

modulated signals (L) of 6%. For a digital set which

requires a small tilt the choice of tilt can be a factor of 5 or

10 smaller by increasing the AGC capacitor.

The chosen AGC currents:

Note

1. As long as no signal has been identified by the identification detector the unload current will be 50

µA.

MODE

UNLOAD CURRENT

LOAD CURRENT

TILT AT 2.2

µF

B/G

50

µA

1.5 mA

typ. 0.5% (line tilt)

L

500 nA (note 1)

1.5 mA

typ. 1.5% (field tilt)

MAC(positive)

200 nA

1.5 mA

typ. 1.2% (frame tilt)

MAC(negative)

500 nA

1.5 mA

typ. 1.5% (field tilt)

Switching between the first three modes can be achieved

by the system switch. This is a 3-level switch which when

grounded selects B/G; open or 5 V selects L, and with pin

The IC operates in a fourth mode if the identification

negatively modulated MAC.

During channel switching a situation can occur that

requires the AGC to increase the gain more than for

example 50 dB. If this increase of gain has to be done for

a positively modulated (L) signal, it will be achieved by the

500 nA load current and is therefore extremely slow.

Because the identification information can be used to

indicate that the signal is too small, in this event the

identification circuit will mute, it is possible to increase the

positive unload current to the same value as that used for

negatively modulated signals. This switching is fully

automatic and cannot be switched off.

AFC circuit

The AFC circuit consists of a demodulator stage which is

fed with signals 90

° out of phase. A very accurate internally

realized 90

° phase shift circuit makes it possible to use the

demodulator IF regenerator tuned circuit for tuning the

AFC circuit. To prevent video ripple on the AFC output

voltage a sample-and-hold circuit is used for negatively

modulated signals. The output signal of the demodulator is

sampled during sync level of the video signal and will be

stored with the aid of an external capacitor.