TDA7513T_07 Datasheet, PDF (37/84 Pages) STMicroelectronics – Single chip FM/AM tuner with stereo decoder and audio processor

TDA7513T
Functional description
3.5.9
3.5.10
3.5.11
3.5.12
like REF5V are derived from a bandgap circuit, therefore they have a temperature coefficient
which is practically zero.
Highcut control
The highcut control setup is similar to the stereoblend control setup, the starting point VHCH
can be set with 2 bits to be 42, 50, 58 or 66% of REF5V whereas the range can be set to be
17, 22, 28 or 33% of VHCH (see Figure 18).
Noise blanker
In the automotive environment the MPX signal is disturbed by spikes produced for example
by the ignition and by the wiper motor. The aim of the noiseblanker part is to cancel the
audible influence of these spikes. To perform this function the output of the stereo decoder is
held at the curent voltage for a time between 22 and 38μs (programmable). The block
diagram of the noiseblanker is shown in fig.20. In the first stage the spikes are detected but
to avoid a wrong triggering on high frequency (white) noise a complex trigger desensitization
control is implemented. Behind the trigger stage a pulse former generates the "blanking"
pulse
Trigger path
The incoming MPX signal is highpassed by a filter with a corner frequency of 140kHz,
amplified and rectified. The rectified signal (RECT) is lowpassed to generate the signal
PEAK. Also noise at a frequency higher than 140kHz increases PEAK. The lowpass output
voltage can be adjusted by changing the noise rectifier discharge current. The PEAK
voltage is fed to a threshold generator which adds to the PEAK voltage a constant voltage
VTH, thus producing the trigger threshold PEAK+VTH. Both RECT and PEAK+VTH are fed
to a comparator which trig-gers a re-triggerable monoflop. The monoflop output activates
the sample-and-hold circuits in the signalpath for a selectable duration.
Automatic noise controlled threshold adjustment (ATC)
There are mainly two independent possibilities to program the trigger threshold:
a) Programming the so-called "low threshold" in 8 steps;
b) Programming the so-called "noise adjusted threshold" in 4 steps
The "low threshold" is active in combination with a good MPX signal without any noise; the
PEAK voltage is less than 1V. The sensitivity in this operating mode is high.
If the MPX signal is noisy the PEAK voltage increases due to the higher noise, which is also
rectified. With increasing of the PEAK voltage the trigger threshold increases, too. This
particular mechanism ("noise adjusted threshold") is programmable in 4 steps.
3.6
3.6.1
Automatic threshold control mechanism
Automatic threshold control by the stereoblend voltage
Besides the noise controlled threshold adjustment there is an additional possibility to
influence the trigger threshold which depends on the stereoblend control.
The point where the MPX signal starts to become noisy is fixed by the RF part. Therefore
also the starting point of the normal noise controlled trigger adjustment is fixed. In some
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