FM-Demodulator for SAT TV with
Switchable Input
TDA 6142-5X
Preliminary Data
Bipolar IC
Features
q
High input sensitivity results in excellent carrier/noise
q
q
q
q
q
behaviour
Reduced noise bandwidth for the prescaler with 480-MHz
LC circuit
Limitation of the prescaler's internal bandwidth to 960 MHz
Due to the LC circuit improvement stability against
oscillations
Interferences reduced by damping higher frequencies with
LC circuit
Switch for double SAW filter input integrated.
P-DSO-20-1
Type
TDA 6142-5X
Ordering Code
Q67000-A5173
Package
P-DSO-20-1 (SMD)
Amplifier and IF demodulator for satellite applications, consisting of: four-stage limiter amplifier with
selection circuit and input selector for two IF-inputs, each with a base stage; divider by 4; video
amplifier; AGC regulator; AFC-output with adjustable steepness; polarity reversal of the video
signal.
Application
Used in indoor units.
Semiconductor Group
1
04.95
TDA 6142-5X
Pin Definitions and Functions
Pin No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Function
Connection for parallel resonance circuit
Connection for parallel resonance circuit
AGC-control voltage output
AGC-threshold adjust
Demodulator tank circuit
Demodulator tank circuit
Ground
AFC-preamplifier output
AFC-low pass filter and feedback point for AFC-sensitivity setting
AFC-output to A/D-converter and AFC-low pass filter
Video output
Video polarity switch input, positive/negative modulation
IF-reference input 1
IF-input 1, e.g.
∆
f
= 27 MHz
Reference voltage output
IF-input select for IF-input 1 or 2
IF-reference input 2
IF-input 2, e.g.
∆
f
= 18 MHz
Supply voltage 5 V
AGC-low pass filter
Semiconductor Group
3
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
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