TDA7407 datasheet(19/30 Pages) STMICROELECTRONICS

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gion of higher fieldstrength. Sometimes a wrong

triggering occures for the MPX signal often shows

distortion in this range which can be avoided

even if using a low threshold.

Because of the overlap of this range and the

range of the stereo/mono transition it can be con-

trolled by stereoblend. This threshold increase is

programmable in 3 steps or switched off with bits

D0 and D1 of the fieldstrength control byte.

Over Deviation Detector

If the system is tuned to stations with a high de-

viation the noiseblanker can trigger on the higher

frequencies of the modulation. To avoid this

wrong behaviour, which causes noise in the out-

put signal, the noiseblanker offers a deviation de-

pendent threshold adjustment.

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. 18).

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. 21). 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 requirement is given in fig.21.

To keep the old value of the Multipath Detector

during an AF-jump, the external capacitor can be

disconnected by the MP-Hold switch. This switch

can be controlled directly by the AFS-Pin.

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. 21.

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 TDA7407 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.

Figure 21. Block Diagram of the Multipath Detector

TDA7407

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