TDA7541 Datasheet, PDF (31/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

TDA7541
Functional description
4.1.5
4.1.6
Table 12. ISS control modes
Input signals
ac
ac+
sm
dev dev+
0
0
0
0
0
0
0
0
1
0
0
0
0
1
1
0
0
1
0
0
0
0
1
1
0
0
0
1
1
1
1
0
0
0
0
1
1
0
0
0
1
0
0
1
0
1
1
0
1
1
inton
0
0
0
1
0
0
1
1
1
1
Mode1
int80
0
0
0
1
0
0
1
1
0
0
Function
wide
wide
wide
narrow
wide
wide
narrow
narrow
mid
mid
inton
0
0
0
1
1
0
1
1
1
1
Mode2
int80
0
0
0
1
0
0
0
1
0
1
Function
wide
wide
wide
narrow
mid
wide
Mid
Narrow
Mid
Narrow
Weak signal mute
The filtered field strength signal (FSWO) is the reference for soft slope mute control in weak
signal condition to eliminate audible effects. The start point and mute depth are
programmable (WMTH, WMD) in a wide range. These settings together with FSWO bits are
influencing the weak signal mute behaviour. The time constant is defined by external
capacitance at PIN 42. Additional adjacent channel mute function is supported. A high pass
filter with -3dB threshold frequency of 100 kHz, amplifier and peak rectifier generates an
adjacent noise signal from Demodulator output. This value is compared with adjustable
threshold (ACMTH). For present strong adjacent channel the MPX signal is additional
attenuated (ACMD) and has the same time constant as weak signal mute.
Weather band input
If the weather band input is used the chip must be set in FM mode (FMON). In addition to
that the ISS filter must be switched ON (ISSENA), mixer2 quality must set to IF2Q=”00” and
the ISS filter must be set in manual control mode. The bit WBON is activating the WB input
and is switching the ISS filter in WB mode (24 kHz band width).
4.2
AM section
The up/down conversion in AM signal path is combined with gain control circuit sensing
three input signals, narrow band information (DAGC) referred to PIN 56, up conversion
signal (IFAGC) at PIN 61 and wide band information (RFAGC) at PIN 3.This gain control
gives two output signals. The first one is a current for pin diode attenuator and the second
one is a voltage for preamplifier. Time constant of RF- and IF-AGC is defined by internal
100k resistor and external capacitor at PIN 54. The intervention points for AGC (DAGC,
IFAGC and RFAGC) are programmable by software. The oscillator frequency for
upconversion mixer1 is generated by dividing the VCO frequency after VCO divider (VCOD)
and AM predivider (AMD).
Two 10,7MHz ceramic filters before mixer2 input increase 900KHz attenuation.
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