TDA7513T datasheet(36/84 Pages) STMICROELECTRONICS

36 / 84 page

Functional description

TDA7513T

36/84

the highcut control range (between VHCH and VHCL) the level is converted into a 5 bit word

which drives the lowpass time constant. The FM highcut function can be switched off by I2C

bus (bit 0,of APSD data byte 11). The setup of the highcut characteristics is described in

2.9.

In AM the high-cut filter can be programmed (bit 3 to 7 of APSD data byte 16) to a fixed

value (inside the above-mentioned programmable range) in order to provide the desired

lowpass characteristic of the AM signal.

3.5.5

PLL and pilot tone detector

The PLL is tasked with locking on the 19kHz pilot tone during a stereo transmission to allow

the correct demodulation. The detector enables the stereo demodulation if the pilot tone

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

The detector output can be checked by reading the status byte of the TDA7407 via I2C bus.

3.5.6

Fieldstrength control

The filtered field strength signal is fed to the stereo decoder where it can be finely adjusted

and normalized so that it can be used to control the highcut and stereoblend functions.

Furthermore the adjusted signal can also be used to control the noise-blanker thresholds.

The unfiltered field strength meter, on the other hand, is used as input for the stereo decoder

multipath detector. These additional functions are described in sections 3.3 and 4.

3.5.7

LEVEL input and gain

To help suppress undesired high frequency modulation of the highcut and stereoblend

functions the tuner filtered field strength signal (LEVEL) is lowpassed by a combination of a

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

capacitor lowpass at 2.2kHz.

The second stage is a programmable gain stage to finely adapt the LEVEL signal internally

against tuner spread (see Testmode section 5 LEVELINTERN). The gain is widely

programmable in 16 steps from 0dB to 10dB (step = 0.67dB).

3.5.8

Stereoblend control

The stereoblend control block converts the internal LEVEL voltage (LEVELINTERN) into a

demodulator-compatible analog signal which is used to control the channel separation

between 0dB and the maximum separation. This control range has a fixed upper limit which

is the in-ternal reference voltage REF5V. The lower limit

can be programmed between 29.2% and 58% of REF5V in 4.167% steps (see

Figure 17).

To adjust the LEVEL voltage to the proper range two values must be defined: the LEVEL

gain LG and VSBL (see

Figure 17). To adjust the voltage where the full channel separation

is reached (VST) the LEVEL gain LG has to be defined. The following equation can be used

to estimate the gain:

The gain LG can be programmed with 4 bits. The MONO voltage VMO (0dB channel

separation) can be chosen selecting VSBL. All the necessary internal reference voltages

REF5V

Fieldstrengthvoltage STEREO

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