TDA7513T datasheet(35/84 Pages) STMICROELECTRONICS

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TDA7513T

Functional description

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3.5.1

Stereo decoder mute

The device has a fast and easy-to-control RDS mute function meant for "freezing" the stereo

decoder status during the RDS AF check time period. When this function is invoked three

effects take place:

The stereo decoder input impedance changes to infinity (condition known as high-

ohmic input); this prevents the decoupling capacitor between the pins #TUNER_OUT

(tuner output) and #TUNER_IN (stereo decoder input) to be discharged by a channel

with a potentially different DC output for the duration of the AF check;

The stereo decoder PLL pilot detector is held at the current value;

The external capacitor of the multipath detector used inside the stereo decoder for

quality control is disconnected from the dection circuit in order to make quality

checking the AF faster.

The RDS mute is activated from pin #RDSMUTE in AND with Bit 0 of APSD data byte 9.

3.5.2

Stereo decoder Input stage, Ingain + Infilter

The stereo decoder is crossed by both the FM and the AM signal: the input impedance of

the pin #TUNER_IN is different between the two modes in order to allow the same external

coupling components between #TUNER_OUT and #TUNER_IN to realize different filtering

functions. Whilst the input impedence in FM is 100k , in AM the input impedance is

decreased to 2k

Ω: this allows the realization of typical high-pass filters with a corner

frequency of 70Hz for AM and less than 5Hz for FM. The low-pass section of the typical AM

transfer function is realized by use of the internal FM High-Cut filter.

The Ingain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally

which is the recommended value. The 4th order input filter has a corner frequency of 80kHz

and is used to attenuate spikes and nose and acts as an anti aliasing filter for the following

switch capacitor filters.

3.5.3

Demodulator

In the demodulator block the left and the right channel are separated from the MPX signal.

In this stage the 19 kHz pilot tone is cancelled.

To reach a good channel separation the device offers an I2C bus programmable roll-off

adjustment which is able to finely compensate for the low-pass behaviour of the tuner

section. An adjustment to better than 40dB channel separation is possible. The bits for this

adjustment are located ogether with the fieldstrength adjustment in one byte. This gives the

possibility to perform an optimization step during the production of the carradio where the

channel separation in relation to the fieldstrength control are trimmed.

The setup of the Stereoblend characteristics, which is fully programmable, is de-scribed in

Chapter 3.5.8.

3.5.4

De-emphasis and highcut.

One filter is provided to realize de-emphasis and High-Cut filtering.

The lowpass filter for the de-emphasis allows to choose between a time constant of 50

μs

and 75

μs.

The filter time constant can further be controlled in both cases over the range = 2 DEEMPH.

The control is automatically performed as a function of the filtered field strength level: inside

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