TDA7540B datasheet(30/75 Pages) STMICROELECTRONICS

30 / 75 page

Functional description

TDA7540B

30/75

Automatic threshold control by the stereoblend voltage (see 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 (see 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

used to increase the PEAK voltage. Offset and gain of this circuit are programmable in 3

steps (NBDTH) of the stereo decoder-byte (the first step turns off the detector).

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. There are

two ways to do this. One way is to switch on the linear influence of the Multipath-Level on the

PEAK-signal . In this case the discharge slew rate is 1V/ms(a). The second possibility is to

activate a function, which switches to the 18k discharge if the Multipath-Level is below 2.5V.

AM mode of noise blanker

The TDA7540 offers an AM audio noise blanker too.

If the AM noise blanker is used the AM audio delay filter must be switched on. It is not

recommented to use the AM noise blanker without to use the AMIF noiseblanker inside the

tuner.

Together with the IF AM moise blanker, this audio noise blanking can work in two different

modes.

Mode 1 uses the same threshold controls like in FM mode. The detector uses in AM mode

the audio input for spike detection. A combination of programmable gain stage and low pass

filtering forms an envelope detector wich drives the noise blanker input via 10/20KHz,

1st/2nd order high pass filter.

In mode 2 only a fixed noise blanker threshold is used.

In order to blank the whole spike in AM mode the hold time of the S&H circuit is much longer

than in FM mode (640

µs -1,2ms)

4.3.3

Functional description of the multipath-detector

Using the internal Multipath-Detector the audible effects of a multipath condition can be

minimized. A multipath-condition is detected by rectifying the 19kHz spectrum in the

fieldstrength signal. An external capacitor is used to define the attack- and decay-times (see

The slew rate is measured with RDischarge=infinite and VMPout=2.5V

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