TDA7541 Datasheet, PDF (34/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

Functional description
TDA7541
compensation bits in byte14 to simplify a possible adaptation during the production of the
car radio.
Stereo blend control
The stereo blend control block converts the internal LEVEL-voltage into a demodulator
compatible analog signal, which is used to control the channel separation between 0dB and
the maximum separation. Internally this control range has a fixed upper limit, which is the
internal reference voltage VREF1. The lower limit can be programmed between 29 and 58%
of VREF1 in 4% steps. In order to adjust the external voltage VFSTC to the internal control
range two values must be defined: the Level gain LG and VSBL. Full channel separation is
reached when the internal level voltage (VST) becomes bigger than VREF1. Therefore the
following equation can be used to estimate the gain:
LG= ------------------^V----^R----^E----^F---¹-------------------
V_FSTC @ fullstereo
The MONO-voltage VMO (0dB channel separation) can be chosen selecting SBC.
Figure 4. Programming of stereo blend
4.3.2
Highcut control
The high cut control set-up is similar to the stereo blend control set-up: the starting point
VHCH can be set with 2 bits to be 42, 50, 58 or 66% of VREF1 whereas the range can be
set to be 11, 18.3, 25.7 or 33% of VHCH.
Noise blanker
In the automotive environment spikes produced for example by the ignition or the wiper-
motor disturb the MPX-signal. The aim of the noise blanker part is to cancel the audible
influence of the spikes. Therefore the output of the stereo decoder is held at the actual
voltage for a time between 22ms and 38ms (programmable).
In a first stage the spikes must be detected but to avoid a wrong triggering on high
frequency (white) noise a complex trigger control is implemented. Behind the trigger stage a
pulse former generates the "blanking"-pulse. An own biasing circuit supplies the noise
blanker in order to avoid any cross talk to the signal path.
Trigger path
The incoming FM demodulator output signal is taken in front of the weak signal mute, high
pass filtered, amplified and rectified. This second order high pass filter has a corner-
34/76