TDA7402 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'TDA7402' PDF : 69 Pages View PDF
TDA7402
Functional description of the noise blanker
6
Functional description of the noise blanker
In the automotive environment the MPX-signal as well as the AM signal is disturbed by
spikes produced by the ignition and other radiating sources like the wiper motor. The aim of
the noiseblanker 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 22 and 38µs in FM (370
and 645µs in AM mode). The block diagram of the noise blanker is given in Figure 34.
Figure 34. Block diagram of the noise blanker
AM/FM
) - Obsolete Product(s) 6.1
Obsolete Product(s 6.2
AMIF
RECTIFIER +
14-56KHz LPF
(1st. order)
MPX
MPOUT
140KHz HPF
(1st. order)
140KHz HPF
(2nd. order)
RECTIFIER
RECT
+
-
VTH
+
INTEGRATOR
DISCHARGE
CONTROL
PEAK
+
D00AU1132
MONOFLOP
FM: 22 to 40μs
AM: 370 to 6400μs
THRESHOLD
GENERATOR
ADDITIONAL
THRESHOLD
CONTROL
HOLDN
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.
Trigger path FM
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 integrated
(lowpass filtered) to generate a signal called PEAK. The DC-charge/discharge behavior can
be adjusted as well as the transient behavior (MP discharge control). Also noise with a
frequency 140kHz increases the PEAK voltage. 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 signalpath for the
selected duration.
Noise controlled threshold adjustment (NCT)
There are mainly two independent possibilities for programming the trigger threshold:
1. the low threshold in 8 steps (bits D1 to D3 of the noiseblanker byte I)
2. and the noise adjusted threshold in 4 steps (bits D4 and D5 of the noiseblanker byte I,
see Figure 21).
The low threshold is active in combination with a good MPX signal without noise; the PEAK
voltage is less than 1V. The sensitivity in this operation is high.
If the MPX signal is noisy (low fieldstrength) 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 gain is programmable in 4 steps (see Figure 25).
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