TDA7541 datasheet(33/76 Pages) STMICROELECTRONICS

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TDA7541

Functional description

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Deemphasis and high cut

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

(DEEMP). The high cut control range will be in both cases THC = 2 x TDeemp. Inside the high

cut control range (between VHCHT and VHCLT) the LEVEL signal is converted into a 5-bit

word, which controls the low pass time constant between TDeemp...3 x TDeemp. Thereby the

resolution will remain 5 bits referred to the voltage range between the VHCHT- and minimum

VHCLT-values.

The high cut function can be switched off by I2C-bus.

Figure 3.

Programming of HC filter

In AM mode (FMON = 0) the DEEMP bit together with the AM corner frequency bits (AMCF)

can be used as programmable AM frequency response. The maximum corner frequency is

defined by TDeemp, the minimum is defined by 3 x TDeemp For the over all frequency

response it is need to take into account the frequency response of the AF output at

MPXOUT and the Stereo decoder demodulator too.

19 kHz PLL and pilot tone detector

The PLL has the task to lock on the 19 kHz 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

TDA7541 via I2C bus or by reading the STEREO status at MUX pin23.

Field strength control

The field strength input is used to control the high cut- and the stereo blend-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 (pin27). Applying a capacitor at

FSTC (pin28) a desired time constant can by reached together with the internal resistor of

10k between FSU pin and FSTC pin. The second stage is a programmable gain stage to

adapt the VFSTC signal internally. The gain (LG) is widely programmable in 16 steps from

0dB to 8.25dB (step=0.55dB). These 4 bits are located together with the Roll-Off

High Cut Filter

Time Constant

3*TDeemp

TDeemp

HCMax

VHCLT

VHCHT

VFSTC* LG

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