TDA4504B Datasheet, PDF (6/26 Pages) NXP Semiconductors – Small signal combination for multistandard colour TV

Philips Semiconductors
Small signal combination for multistandard colour TV
Product specification
TDA4504B
FUNCTIONAL DESCRIPTION
Vision IF amplifier, demodulator
and video amplifier
Each of the three AC-coupled IF
stages permit the omission of DC
feedback and possess a control
range in excess of 20 dB.
The IF amplifier, which is completely
symmetrical, is followed by a passive
synchronous demodulator providing a
regenerated carrier signal. This is
limited by a logarithmic limiter circuit
prior to its application to the
demodulator.
A noise clamp circuit is provided at
the video input (pin 16) to limit
interference pulses below the sync tip
level and is more efficient than a
noise inverter in providing improved
picture stability during the presence of
interference.
The video amplifier has good linearity
and bandwidth figures.
AFC-circuit
Obtaining the AFC reference signal
from the demodulator tuned circuit
presents the advantage of utilizing a
single tuned circuit and one
adjustment. However, since the
frequency spectrum of the signal
applied to the demodulator is
determined by the characteristic of
the SAW filter, the resultant
asymmetrical spectrum with respect
to the vision carrier causes the AFC
output voltage to be dependent upon
the video signal. The TDA4504B thus
contains a sample-and-hold circuit.
With negative-going vision signals the
AFC is active only during the sync
pulse period. When positive-going
signals are applied to the device,
however, the AFC is continuously
active but filtered to ensure only a
small by-pass current is present in the
sample-and-hold circuit.
With weak input signals the drive
signal will contain considerable noise
which also possesses an
asymmetrical frequency spectrum
and could create an offset in the AFC
output voltage. The inclusion of a
notch in the demodulator tuned circuit
minimises this effect.
The sample-and-hold circuit is
followed by a high impedance output
amplifier. Thus the AFC control
gradient depends upon the load
impedance.
The AFC polarity switch is combined
with the start circuit (pin 12). It has a
negative slope when pin 12 is open or
connected to the main supply and a
positive slope when pin 12 is
grounded. The AFC is disabled when
the sample connection (pin 22) is
grounded.
February 1992
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