TDA7402 Datasheet, PDF (36/59 Pages) STMicroelectronics – CARRADIO SIGNAL PROCESSOR

TDA7402
4.0 FUNCTIONAL DESCRIPTION OF THE NOISEBLANKER
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 audi-
ble influence of the spikes. Therefore the output of the stereodecoder is held at the actual voltage for a time
between 22 and 38µs in FM (370 and 645µs in AM-mode). The blockdiagram of the noiseblanker is given in
fig.31.
Figure 31. Block diagram of the noiseblanker
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 triggerstage a pulse former generates the ”blanking”-pulse.
4.1 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 behaviour can be adjusted as well as the transient behaviour(MP-dis-
charge 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 acti-
vates the sample-and-hold circuits in the signalpath for the selected duration.
4.2 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 fig. 19).
The low threshold is activ 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 fig. 22).
4.3 Additional Threshold Control Mechanism
4.3.1 Automatic Threshold Control by the Stereoblend voltage
Besides the noise controlled threshold adjustment there is an additional possibility for influencing 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
36/59