TDA7540N datasheet(30/76 Pages) STMICROELECTRONICS

30 / 76 page

Functional description

TDA7540N

30/76

Demodulator

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

In this stage also the 19-kHz pilot tone is canceled. For reaching a high channel separation

the TDA7540 offers an I2C-bus programmable roll-off adjustment, which is able to

compensate the low pass behavior of the tuner section. Within the compensation range an

adjustment to obtain at least 40dB channel separation is possible. The bits for this

adjustment are located together with the level gain adjustment in one byte. This gives the

possibility to perform an optimization step during the production of the car radio where the

channel separation and the header are trimmed.

In addition to that the FM signal can be inverted.

Deemphasis and highcut

The deemphasis low pass allows to choose between a time constant of 50µs/ 75

μs

(DEEMP) and 25

μs/37.5μs (DESFT). The highcut control range will be in both cases τ

HC =

Deemp. Inside the highcut control range (between VHCH and VHCL) the LEVEL signal is

converted into a 5-bit word, which controls the low pass time constant between

Deemp...3xτDeemp. Thereby the resolution will remain always 5 bits independently of the

absolute voltage range between the VHCH- and VHCL-values.

The highcut function can be switched off by I2C-bus .

In AM mode (AMON = 1) the bits DEEMP and DESFT together with the AM corner

frequency bits (AMCF1...5) can be used as programmable AM frequency response. The

maximum corner frequency is defined by

Deemp , the minimum is defined by 3xτDeemp

19kHz PLL and pilot tone detector

The PLL has the task to lock on the 19kHz pilot tone during a stereo-transmission to allow a

correct demodulation. The included pilot tone-detector enables the demodulation if the pilot

tone reaches the selected pilot tone threshold VPTHST. Two different thresholds are

available. The status of the detector output can be checked by reading the status byte of the

TDA7540N via I2C-bus or by reading the STEREO pin (pin 25).

Field strength control

The field strength input is used to control the highcut- and the stereoblend-function. In

addition the signal can be also used to control the noise blanker thresholds and as input for

the multipath detector.

Level-input and -gain

As level input for the stereo decoder is used the FSU voltage (pin22). Appling a capacitor at

FSTC (pin33) a desired time constant can by reached together with the internal resistor of

10k between FSU pin and FSTC pin.

In addition to that the LEVEL signal is low pass filtered internally in order to suppress

undesired high frequency modulation on the highcut- and stereoblend-function . The filter is

a combination of a 1st-order RC-low pass at 53kHz (working as anti-aliasing filter) and a 1st-

order switched capacitor low pass at 2.2kHz. The second stage is a programmable gain

stage to adapt the LEVEL signal internally . The gain is widely programmable in 8 steps

from 0dB to 4,7dB (step=0.67dB). These 3bits are located together with the Roll-Off bits in

the "Stereo decoder 8"-byte to simplify a possible adaptation during the production of the

car radio.

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