TDA7541B Datasheet, PDF (27/77 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

TDA7541B
Functional description
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 22 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.
Doc ID 16048 Rev 1
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