TDA7541 datasheet(28/76 Pages) STMICROELECTRONICS

28 / 76 page

Functional description

TDA7541

28/76

Adjacent channel detector

The input of the adjacent channel detector is AC coupled from VFSW. A programmable

high-pass or band-pass (ACF) as well as rectifier generate a signal which is compared with

adjustable threshold (ACTH). The output signals of this comparator is controlling the charge

and discharge of the external capacitor at PIN30 or PIN31 (dependent on SEEK) with

programmable discharge current (TISS). The level at PIN30/31 is used to generate the two

digital signal ac and ac+ for ISS control. The adjacent channel information behind the

rectifier is available as analog output signal at the multiplexer output (PIN23).

Multipath detector

The input of the multi path detector is AC coupled from internal VFSW too. After filtering with

19 kHz band-pass and rectifying, this voltage is compared with an adjustable threshold

(MPTH). The output signal of this comparator can be used to switch off the adjacent channel

detection. This influence is selectable by I2C Bus (MPENA).

The multi path information behind the rectifier is available as analog output signal at

multiplexer output (PIN23).

450 kHz IF narrow band pass filter (ISS filter)

The device has an additional 450 kHz IF narrow band-pass filter for suppression of noise

and adjacent channel signal influences. This narrow filter has three switchable bandwidths,

narrow range of 80 kHz, mid range of 120 kHz and 24 kHz for weather band information.

Without ISS (Intelligent Selectivity System) filter the IF bandwidth (wide range) is defined

only by ceramic filter chain and mixer2 bandwidth. The filter is located between mixer2

output buffer and 450 kHz limiter stage. The centre frequency is matched to the

demodulator centre frequency.

Deviation detector

In order to avoid distortion in audio output signal the narrow ISS filter is switched OFF if over

deviation is present. Hence the demodulator output signal is detected. After AC coupling,

low-pass filtering and peak rectifying this signal is charging/discharging the external DEVTC

capacitor by an IIC programmable charge/discharge current (TDEV). The voltage at DEVTC

is compared with adjustable thresholds (DWTH, DTH) and generates two digital control

signals (dev, dev+). For weak signal condition the deviation threshold depends on FSWO.

ISS switch logic

All digital control signals coming from adjacent channel detector, deviation detector and

weak signal mute are acting via switching matrix on ISS filter switch. IF2 narrow band-pass

switch mode is controlled also by software (ISSENA, ISSON, WBON, ISSBW, and BWDEF).

The switching of the IF band-pass is also possible to influence by external manipulation of

DC voltage at PIN30.

The influence of the ISS software control on the functionality of the ISS filter is described in

Table 10. The value “X” for the control bit means the bit does not influence the filter control in

this condition.

TDA7541 数据表(PDF) 28 Page - STMicroelectronics

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