TDA7540B Datasheet, PDF (30/75 Pages) STMicroelectronics – AM/FM Car Radio Tuner IC with Stereo Decoder and Intelligent Selectivity System (ISS)

Functional description
TDA7540B
4.3.3
Automatic threshold control by the stereoblend voltage (see Figure 5)
Besides the noise controlled threshold adjustment there is an additional possibility for
influencing the noise blanker trigger threshold using the bits NBFS. This influence depends
on the stereoblend control.
The point where the MPX signal starts to become noisy is fixed by the RF part. This point is
also the starting point of the normal noise-controlled trigger adjustment. But in some cases
the noise blanker can create a wrong triggering, which create distortion, already in the
region of mono/stereo transition. Therefore a opportunity to control the PEAK voltage by the
stereo blend function it is implemented.
Over deviation detector (see Figure 4)
If the system is tuned to stations with a high deviation the noise blanker can trigger on the
higher frequencies of the modulation. To avoid this wrong behavior, which causes noise in
the output signal, the noise blanker offers a deviation-dependent threshold adjustment. By
rectifying the MPX signal a further signal representing the actual deviation is obtained. It is
used to increase the PEAK voltage. Offset and gain of this circuit are programmable in 3
steps (NBDTH) of the stereo decoder-byte (the first step turns off the detector).
Multipath-level
To react on high repetitive spikes caused by a Multipath-situation, the discharge-time of the
PEAK voltage can be decreased depending on the voltage-level at Pin MPout. There are
two ways to do this. One way is to switch on the linear influence of the Multipath-Level on the
PEAK-signal . In this case the discharge slew rate is 1V/ms^(a). The second possibility is to
activate a function, which switches to the 18k discharge if the Multipath-Level is below 2.5V.
AM mode of noise blanker
The TDA7540 offers an AM audio noise blanker too.
If the AM noise blanker is used the AM audio delay filter must be switched on. It is not
recommented to use the AM noise blanker without to use the AMIF noiseblanker inside the
tuner.
Together with the IF AM moise blanker, this audio noise blanking can work in two different
modes.
Mode 1 uses the same threshold controls like in FM mode. The detector uses in AM mode
the audio input for spike detection. A combination of programmable gain stage and low pass
filtering forms an envelope detector wich drives the noise blanker input via 10/20KHz,
1st/2nd order high pass filter.
In mode 2 only a fixed noise blanker threshold is used.
In order to blank the whole spike in AM mode the hold time of the S&H circuit is much longer
than in FM mode (640µs -1,2ms)
Functional description of the multipath-detector
Using the internal Multipath-Detector the audible effects of a multipath condition can be
minimized. A multipath-condition is detected by rectifying the 19kHz spectrum in the
fieldstrength signal. An external capacitor is used to define the attack- and decay-times (see
a. The slew rate is measured with RDischarge=infinite and VMPout=2.5V
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