TDA6142-5X Datasheet, PDF (5/10 Pages) Siemens Semiconductor Group – FM-Demodulator for SAT TV with Switchable Input

TDA 6142-5X
Circuit Description
The FM modulated satellite IF signal is fed to both low-ohm symmetrical inputs of the limiting
amplifier via a SAW filter with two symmetrical outputs and different bandwidths. With an external
DC voltage at pin 16 of the IF input selector, an IF-input-signal can be switched through the IF-
input 1 or IF-input 2. By high frequency blocking of the limiting amplifier's symmetrical inputs,
asymmetrical operation is also possible, but with increased noise. An external selection circuit in the
penultimate amplifier stage produces good selectivity of the limiting amplifier and thus provides a
better suppression of harmonics.
The output signal of the amplifier is fed to a divider, which splits the frequency into four. The signal
is then led to a quadrature demodulator, once directly, and once with phase shift produced by an
external phase-shifting circuit.
The demodulated video signal is amplified and is available at the video output. The polarity of this
signal can be inverted by a DC voltage applied from outside to the polarity reversal input.
The demodulated video signal is likewise used for AFC regulation. It is fed via a preamplifier to an
inverting OP amplifier at which an internal reference voltage is present. Any frequency offset
appears as a varying DC voltage, which can in turn be led to an external A/D converter. The AFC
S-curve and the amplification of the OPs can be adjusted via an external resistor.
The information for the field strength of the FM modulated satellite IF signal is present as DC voltage
at both the AGC output and the AGC low pass filter. A pot can be used to change the AGC
threshold. The IF stage of the satellite tuner , or a prestage, can be regulated via the AGC output by
amplification.
Semiconductor Group
5