TDA7411 datasheet(26/43 Pages) STMICROELECTRONICS

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TDA7411

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Figure 22. Block diagram of the noise blanker

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 "blank-

ing"-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 gener-

ator, 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 ac-

tivates the sample-and-hold circuits in the signal path for the selected duration.

Automatic Noise Controlled Threshold Adjustment (ATC)

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

1. the low threshold in 8 steps (bits D0 to D2 of the noise blanker byte)

2. and the noise adjusted threshold in 4 steps (bits D3 and D4 of the noise blanker-byte, see fig. 15).

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 (see fig. 17).

RECTIFIER

LOWPASS

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

modAU856

MPOUT

CONTROL

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