TDA7407 datasheet(30/46 Pages) STMICROELECTRONICS

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Multipath detector

TDA7407

30/46

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.

5.2.11

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

5.2.12

Automatic noise controlled threshold adjustment (ATC)

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

the low threshold in 8 steps (bits D0 to D2 of the noiseblanker byte)

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.

RECTIFIER

LOWPASS

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

D98AU856

MPX

CONTROL

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