TDA7540B datasheet(31/75 Pages) STMICROELECTRONICS

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TDA7540B

Functional description

31/75

block diagram, fig. 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 TDA7540 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:

VQual = 0.8b (VNoise-0.8 V)+ a (VREF1-VMpout).

The noise-signal is the PEAK-signal without additional influences (see noise blanker

description). The factor 'a' can be programmed from 0.6 to 1.05(QDC) and the factor b can

be programmed from 6dB to 15dB ( QNG). The output is a low impedance output able to

drive external circuitry as well as simply fed to an AD-converter for RDS applications.

4.3.5

AFS control and stereo decoder mute

The TDA7540 is supplied with several functionality to support AF-checks using the stereo

decoder. The additional pin (AFS) is implemented in order to speed up the stereo decoder

AF-functions compared to IIC controlling.

The block diagramm of AFS function is shown in Figure 17.

In order to separate the different functions of the AFS pin, two different logic thresholds are

implemented. Below the higher threshold voltage (2.4V) only the multipath-detector is

switched into small time constant (internal logical signal MPfast).

Below the lower threshold voltage (0.8V) the full AFS function is activated. The MPXIN pin is

switched into high impedance mode (internal signal AFSMute), which avoids any clicks

during the jump condition. If the stereo decoder is not muted, it is possible at the same time

to evaluate the noise- and multipath-content of the alternate frequency using the Quality

detector output.

Furthermore the AFS pin does also freeze the condition of pilot locking and magnitude

(internal signal PDhold). The Pdhold signal is defined by Vth1 or Vth2, dependent on the

PDH signal.

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