TDA7540N datasheet(33/76 Pages) STMICROELECTRONICS

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TDA7540N

Functional description

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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/ms1). The second possibility is to

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

If multipath influence on noise blanker is switched ON than MPF bit has to be set to 0.

1) The slew rate is measured with R

Discharge=infinite and VMPout=2.5V

AM Mode of Noise Blanker

The TDA7540N offers an AM audio noise blanker too.

If the AM noise blanker is used the AM audio delay filter and the AM audio filter must be

switched on. It is not recommented to use the AM noise blanker without to use the AMIF

noiseblanker inside the tuner.

The noise blanker is activated if the spike is bigger than a fixed threshold

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 (640µs -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

block diagram, Figure 16). The MP_OUT-pin is used as detector-output connected to a

capacitor of about 47nF. Using this configuration an external adaptation to the user's

requirement is possible without affecting the "normal" fieldstrength input (LEVEL) for the

stereo decoder.

To keep the old value of the Multipath Detector during an AF-jump, the MP-Hold switch can

disconnect the external capacitor. This switch is controlled directly by the AFS-Pin.

Selecting MPION the channel separation is automatically reduced during a multipath

condition according to the voltage appearing at the MP_OUT-pin.

Programming

To obtain a good multipath performance an adaptation is necessary. Therefore the gain of

the first 19kHz-bandpass is programmable in two steps (MPG), the gain of the second

19kHz-bandpass is programmable in four steps (MPBPG) and the rectifier gain is

programmable in four steps(MPRG). Please note that the frequency of the first multipath

bandpass (MPF) must be set to 19kHz! The attack- and decay-times can be set by the

external capacitor value and the multipath detector charge current MPCC.

4.3.4

Quality detector

The TDA7540N offers a quality detector output, which gives a voltage representing the FM-

reception conditions. To calculate this voltage the MPX-noise and the multipath-detector

output are summed according to the following formula:

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