TDA7405 datasheet(35/56 Pages) STMICROELECTRONICS

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TDA7405

The low threshold is active in combination with a good MPX signal without 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 in-

creases due to the higher noise, which is also rectified. With increasing of the PEAK voltage the trigger threshold

increases, too. This gain is programmable in 4 steps (see Figure 22).

5.3 Additional Threshold Control Mechanism

5.3.1 Automatic Threshold Control by the Stereoblend voltage

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

threshold which depends on the stereoblend control.

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

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

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

5.3.2 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 Figure 23).

5.3.3 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 A631 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/ms

1 . 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 = nfinite and VMPout = 2.5V

5.3.4 AM mode of the Noiseblanker

The A631 noiseblanker is also suitable for AM noise canceling. The detector uses in AM mode the 450kHz un-

filtered 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 120 kHz 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

(640µs to 1.2ms). All threshold controls can be used like in FM mode.

5.4 Functional Description of the Multipath-Detector

Using the Multipath-Detector the audible effects of a multipath condition can be minimized. A multipath-condi-

tion is detected by rectifying the spectrum around 19kHz in the fieldstrength signal. An external capacitor is used

to define the attack- and decay-times for the Stereoblend (see blockdiagram, Figure 32). Due to the very small

charge currents this capacitor should be a low leakage current type (e.g. ceramic). Using this configuration an

adaptation to the user's requirement is possible without effecting the "normal" fieldstrength input (LEVEL) for

the stereodecoder. This application is given in Figure 32. Another (internal) time constant is used to control the

Highcut through the multipath detector

Selecting the "internal influence" in the configuration byte the Stereo-Blend and/or the Highcut is automatically

invoked during a multipath condition according to the voltage appearing at the MP_OUT-pin.

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