TDA7541B Datasheet, PDF (37/77 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

TDA7541B
Functional description
4.3.3
4.3.4
4.3.5
Noise blanker in AM mode
In AM mode the noise blanker is activated if a spike on the audio signal is bigger than a fixed
threshold. 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 (64 0µs -1,2 ms). It is not recommended to use the AM noise
blanker without to use the AMIF noise blanker inside the tuner.
Functional description of the multipath detector
Using the internal multi path detector the audible effects of a multi path condition can be
minimized. A multi path condition is detected by rectifying the 19 kHz spectrum in the field
strength signal. An external capacitor is used to define the attack- and decay-times. The
MPTC pin is used as detector output connected to a capacitor. Using this configuration an
external adaptation to the user's requirement is possible without affecting the "normal" field
strength input (FSTC) for the stereo decoder.
To keep the old value of the multi path detector during an alternative frequency jump, the
MPFAST bit can disconnect the external capacitor.
Selecting MPINT the channel separation is automatically reduced during a multipath
condition according to the voltage appearing at the MPTC pin.
To obtain a good multipath performance an adaptation is necessary. Therefore the gain of
the 19 kHz band pass is programmable in four steps (MPG) and the rectifier gain is
programmable in four steps (MPRG). The attack- and decay-times can be set by the
external capacitor value and the multipath detector charge current MPCC.
Quality detector
The TDA7541B offers a quality detector output, which gives a voltage representing the FM-
reception conditions. To calculate this voltage the MPX-noise and the multipath-detector
output are summed according to the following formula:
V_Qual = 0.8 ⋅ b ⋅ (V_PEAK – 0.8) + a ⋅ (V_REF1 – V_MPTC)
The V_PEAK signal is described in noise blanker session. The factor 'a' can be programmed
from 0.6 to 1.05 (QDC) and the factor b can be programmed from 6 dB to 15 dB (QNG). The
quality output voltage can be read at the MUX pin. The MUX pin is a low impedance output
and is able to drive external circuitry as well as simply fed to an AD-converter for RDS
applications.
AFS control and stereo decoder mute
In case of AFS (alternative frequency search) jump it is recommended to set the stereo
decoder in mute condition (SDM) and in addition to set the SEEK mode. Since these two
bits are placed in the first written byte, this can be done in the same write cycle as the PLL
jump. The stereo decoder mute is high impedance mute, which means the charge on the
coupling capacitor will be kept. Simultaneously the pilot detector circuit of the stereo
decoder is switched into hold mode. The SEEK mode is switching the multi path detector
into fast mode. The external capacitor at MPTC is disconnected from multipath detector
which keeps the stereo blend condition, but makes the quality information in fast mode
available at the MUX pin.
Doc ID 16048 Rev 1
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