TDA7541 datasheet(37/76 Pages) STMICROELECTRONICS

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TDA7541

Functional description

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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 (640µs -1,2ms). It is not recommended to use the AM noise

blanker without to use the AMIF noise blanker inside the tuner.

4.3.3

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.

4.3.4

Quality detector

The TDA7541 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:

The VPEAK 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 6dB to 15dB (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.

4.3.5

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.

Qual

0.8 b

PEAK

0.8

–

() aV

REF1

MPTC

–

()

⋅

⋅⋅

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