TDA7407 datasheet(32/46 Pages) STMICROELECTRONICS

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Multipath detector

TDA7407

32/46

Figure 24.

Block diagram of the multipath detector

5.4.1

Programming

To obtain a good multipath performance an adaptation is necessary. Therefore the gain of

the 19kHz bandpass is programmable in four steps as well as the rectifier gain. The attack

and decay times can be set by the external capacitor value.

5.4.2

Quality detector

The TDA7407 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:

Quality = 1.6 (Vnoise -0.8V)+ a (REF5V - VMPOUT)

The noise signal is the PEAK signal without additional influences. The factor "a" can be

programmed from 0.7 to 1.15. the output is a low impedance output able to drive external

circuitry as well as simply fed to an A/D converter for RDS applications.

5.4.3

AF Search Control

The TDA7407 is supplied with several functionality to support AF checks using the stereo

decoder. As mentioned already before the high ohmic mute feature avoids any clicks during

the jump condition. It is possible at the same time, to evaluate the noise and multipath

content of the alternate frequency, by using the quality detector output. Therefore the

multipath detector is switched automatically to a small time constant. No additional pin

(AFS) is implemented in order to separate the audioprocessor mute and stereo decoder AF

functions. In Figure 25 the block diagram and control functions of the complete AFS

functionality is shown (please note that the pins AFS and SM are active low as well as all

control bits indicated by an overbar).

5.5

Test mode

During the test mode, which can be activated by setting bit D0 of the testing byte and bit D5

of the subaddress byte to "1", several internal signals are available at the CASSR pin.

During this mode the input resistor of 100kOhm is disconnected from the pin. The internal

signals available are shown in the software specification.

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