TDA7400D datasheet(19/28 Pages) STMICROELECTRONICS

19 / 28 page

By rectifying the MPX signal a further signal rep-

resenting the actual deviation is obtained. It is

used to increase the PEAK voltage. Offset and

gain of this circuit are programmable in 3 steps

with the bits D6 and D7 of the stereodecoder byte

(the first step turns off the detector, see fig. 15).

FUNCTIONAL DESCRIPTION OF THE MULTI-

PATH 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 block diagram fig. 23). the

MPOUT pin is used as detector output connected

to a capacitor of about 47nF and additionally the

MPIN pin is selected to be the fieldstrength input.

Using the configuration an external adaptation to

the user’s requiremet is given in fig.25.

Selecting the "internal influence" in the configura-

tion byte, the channel separation is automatically

reduced during a multipath condition according to

the voltage appearing at the MP_OUT pin.

possible application is shown in fig. 26.

Programming

To obtain a good multipath performance an adap-

tation is necessary. Therefore tha 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.

QUALITY DETECTOR

The TDA7400D offers a quality detector output

which gives a voltage representing the FM recep-

tion 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 addi-

tional influences. The factor "a" can be pro-

grammed from 0.7 to 1.15. the output is a low im-

pedance output able to drive external circuitry as

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

applications.

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.

RECTIFIER

BANDPASS

19KHz

to SB

D98AU857

MP_IN

GAIN

2 BITS

GAIN

2 BITS

CHARGE

1 bit

VDD

MPOUT

LEVEL

int. INFLUENCE

47nF

Figure 18. Block Diagram of the Multipath Detector

TDA7400D

19/28

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