TDA8349A Datasheet, PDF (5/17 Pages) NXP Semiconductors – Multistandard IF amplifier and demodulator

Philips Semiconductors
Multistandard IF amplifier and demodulator
Product specification
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:
MODE
B/G
L
MAC(positive)
MAC(negative)
UNLOAD CURRENT
50 µA
500 nA (note 1)
200 nA
500 nA
LOAD CURRENT
1.5 mA
1.5 mA
1.5 mA
1.5 mA
TILT AT 2.2 µF
typ. 0.5% (line tilt)
typ. 1.5% (field tilt)
typ. 1.2% (frame tilt)
typ. 1.5% (field tilt)
Note
1. As long as no signal has been identified by the identification detector the unload current will be 50 µA.
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
20 connected to V_CC selects positively modulated MAC.
The IC operates in a fourth mode if the identification
capacitor at pin 19 is connected to V_CC, it can be used for
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.
February 1991
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