TDA7402 datasheet(37/59 Pages) STMICROELECTRONICS

37 / 59 page

37/59

TDA7402

of the normal noise-controlled trigger adjustment is fixed (fig. 24). In some cases the behavior of the noiseblank-

er can be improved by increasing the threshold even in a region of higher fieldstrength. Sometimes a wrong

triggering occurs for the MPX signal often shows distortion in this range which can be avoided even if using a

low threshold. Because of the overlap of this range and the range of the stereo/mono transition it can be con-

trolled by stereoblend. This increase of the threshold is programmable in 3 steps or switched off.

4.3.2 Over Deviation Detector

If the system is tuned to stations with a high deviation the noiseblanker can trigger on the higher frequencies

of the modulation or distortion. To avoid this behavior, which causes audible noise in the output signal, the

noiseblanker offers a deviation-dependent threshold adjustment. By rectifying the MPX signal a further signal

representing the actual deviation is obtained. It is used to increase the PEAK voltage. Offset and gain of this

circuit are programmable in 3 steps with the bits D6 and D7 of the noiseblanker-byte I (bit combination ’00’ turns

off the detector, see fig. 23).

4.3.3 Multipath-Level

To react on high repetitive spikes caused by a Multipath-situation, the discharge-time of the PEAK voltage can

be decreased depending on the voltage-level at pin MPout. The A619 offers a linear as well as a threshold driv-

en control. The linear influence of the Multipath-Level on the PEAK-signal (D7 of Multipath-Control-Byte) gives

a discharge slewrate of 1V/ms1 . The second possibility is to activate the threshold driven discharge which

switches on the 18kOhm discharge if the Multipath-Level is below 2.5V (D7 of noiseblanker-byte II-byte).

1 The slewrate is measured with RDischarge=infinite and VMPout=2.5V

AM mode of the Noiseblanker

The A619 noiseblanker is also suitable for AM noise cancelling. The detector uses in AM mode the 450kHz

unfiltered IF-output of the tuner for spike detection. A combination of programmable gain-stage and lowpass-

filter forms an envelope detector which drives the noiseblanker’s input via a 120kHz 1.st order highpass. In order

to blank the whole spike in AM mode the hold-times of the sample and hold circuit are much longer then in FM

(640

µs to 1.2ms). All threshold controls can be used like in FM mode.

TDA7402 数据表(PDF) 37 Page - STMicroelectronics