TDA7407_07 Datasheet, PDF (30/46 Pages) STMicroelectronics – Advanced car signal processor

Multipath detector
TDA7407
5.2.11
The block diagram of the noiseblanker is given in Figure 23.
In the first stage the spikes must be detected but to avoid wrong triggering on high
frequency (white) noise, a complex trigger control is implemented. Behind the trigger stage
a pulse former generates the "blanking" pulse. To avoid any crosstalk to the signal path the
noiseblanker is supplied by his own biasing circuit.
Trigger path
The incoming MPX signal is highpass filtered, amplified and rectified. This second order
highpass filter has a corner frequency of 140kHz.
The rectified signal, RECT, is lowpass filtered to generate a signal called PEAK. Also noise
with a frequency 140kHz increases the PEAK voltage. The resulting voltage can be adjusted
by use of the noise rectifier discharge current.
The PEAK voltage is fed to a threshold generator, which adds to the PEAK voltage a DC
dependent threshold VTH. Both signals, RECT and PEAK+VTH are fed to a comparator
which triggers a re-triggerable monoflop. The monoflop's output activates the sample and
hold circuits in the signal path for selected duration.
Figure 23. Block Diagram of the Noiseblanker
MPX
MPX
RECTIFIER
RECT
+
-
VTH
+
LOWPASS
CONTROL
PEAK
+
D98AU856
MONOFLOP
THRESHOLD
GENERATOR
ADDITIONAL
THRESHOLD
CONTROL
HOLDN
5.2.12
Automatic noise controlled threshold adjustment (ATC)
There are mainly two independent possibilities for programming the trigger threshold:
a) the low threshold in 8 steps (bits D0 to D2 of the noiseblanker byte)
b) the noise adjusted threshold in 4 steps (bits D3 and D4 of the noiseblanker byte,
see Figure 23).
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 operation 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 gain is programmable in 4 steps.
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