TDA7540N datasheet(32/76 Pages) STMICROELECTRONICS

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Functional description

TDA7540N

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4.3.2

Functional description of the noise blanker

In the automotive environment spikes produced by the ignition or for example 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 22μs and 38μs (programmable). The block diagram of the noise

blanker is given inFigure 15.

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 MPX signal is high pass filtered, amplified and rectified. This second order

high pass filter has a corner-frequency of 140kHz. The rectified signal, RECT, is low pass

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 the selected duration.

Automatic noise controlled threshold adjustment (Figure 3)

There are mainly two independent possibilities for programming the trigger threshold:

the low threshold in 8 steps (NBLTH)

and the noise adjusted threshold in 4 steps (NBCTH).

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 (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 particular gain is programmable in 4 steps (NBCTH).

Automatic threshold control by the stereoblend voltage (Figure 5)

Besides the noise controlled threshold adjustment there is an additional possibility for

influencing the noise blanker trigger threshold using the bits NBFS. This influence depends

on the stereoblend control.

The point where the MPX signal starts to become noisy is fixed by the RF part. This point is

also the starting point of the normal noise-controlled trigger adjustment. But in some cases

the noise blanker can create a wrong triggering, which create distortion, already in the

region of mono/stereo transition. Therefore a opportunity to control the PEAK voltage by the

stereo blend function it is implemented.

Over deviation detector (Figure 4)

If the system is tuned to stations with a high deviation the noise blanker can trigger on the

higher frequencies of the modulation. To avoid this wrong behavior, which causes noise in

the output signal, the noise blanker offers a deviation-dependent threshold adjustment. By

rectifying the MPX signal a further signal representing the actual deviation is obtained. It is

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