TDA7540N Datasheet, PDF (29/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system (ISS)

TDA7540N
Functional description
4.2
AM section
The up/down conversion is combined with gain control circuit sensing three input signals,
narrow band information at PIN 54, upconversion signal (IF2AGC) at PIN 71and wide band
information (RFAGC) at PIN 4.This gain control gives two output signals. The first one is a
current for pin diode attenuator and the second one is a voltage for preamplifier. Time
constant of RF- and IF-AGC is defined by internal 100k resistor and external capacitor at
PIN 67. The intervention points for AGC (DAGC,WAGC) are programmable by software.
In order to avoid a misbehaviour of AGC intervention point it is important to know that the
DAGC threshold has to be lower than WAGC threshold !
The oscillator frequency for upconversion-mixer1 is generated by dividing the VCO
frequency after VCO divider (VCOD) and AM predivider(AMD).
Two 10,7MHz ceramic filters before mixer2 input increases 900KHz attenuation.In mixer2
the IF1 is down converted into the IF2 450kHz. After filtering by ceramic filter a 450kHz
amplifier is included with an additional gain control of IF2 below DAGC threshold. Time
constant is defined by capacitance at PIN 78.
Mixer1 and mixer2 tanks are software controlled adjustable (IF1T, IF2T).
The demodulator is a peak detector to generate the audio output signal.
A separate output is available for AMIF stereo (AMST).
AM IF noise blanker
In order to remove in AM short spikes a noise cancellation conception is used in 450KHz IF
AM level. The advantage is to avoid long narrow AGC- and demodulator- time constants,
wich enlarge spike influences on audio signal and makes difficult to remove it in audio path.
The 10.7MHz AM IF signal generates before 10.7 MHz ceramic filter via limitation an
unweighted fieldstrenght signal including slope of noise spike. The comparison of these
detected slope between fast and slow rectifier ignores audio modulation whereby the
threshold of slow rectifier is programmable (AINBT). A comparator activates a pulse
generator.
The duration of this pulse is software programmable (AINT) and is smooth blanking out the
spikes in 450KHz AM mixer2. Additionally this funtionality is controlled by narrow AM
fieldstrenght (AINBD).
4.3
4.3.1
Stereodecoder
Decoder
The stereo decoder-part of the TDA7540N (see Figure 14) contains all functions necessary
to demodulate the MPX-signal like pilot tone-dependent MONO/STEREO-switching as well
as "stereoblend" and "highcut". Adaptations like programmable input gain, roll-off
compensation, selectable deemphasis time constant and a programmable field strength
input allow easy adaption to different applications.
The 4^th order input filter has a corner frequency of 80kHz and is used to attenuate spikes
and noise and acts as an anti-aliasing filter for the following switch capacitor filters.
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