Datenblatt-Suchmaschine für elektronische Bauteile |
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TDA7420 Datenblatt(PDF) 10 Page - STMicroelectronics |
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TDA7420 Datenblatt(HTML) 10 Page - STMicroelectronics |
10 / 29 page DESCRIPTION DESCRIPTION OF THE NOISEBLANKER In the normal automotive environment the MPX signal is disturbed by ignition spikes, motors and high frequency switches etc. The aim of the noiseblanker part is to cancel the influence of the spikes produced by these compo- nents. Therefore the output of the stereodecoder is switched off for a time of 40 µs (average spike du- ration). In a first stage the spikes must be detected but to avoid a wrong triggering on high frequency noise a complex trigger control is implemented. Behind the triggerstage a pulse former generates the 40 µs ”blanking” pulse. In the following section all of these circuits are de- scribed in their function and their programming, too (see fig.1). 1.1 Normal Trigger Path (RECT-PEAK, ACT, PEAK-COMP, BLANK-COMP, BIAS-MONO) The Incoming MPX signal is highpass-filtered, amplified and rectified (block RECT-PEAK). The second order highpass-filter has a corner-fre- quency of 140KHz. The gain of the rectifier can be controlled by the bit D2 of the noiseblanker byte2. If programming bit D2 to zero the gain is only half of the nominal value. All trigger thresholds must be roughly doubled in this case. The rectified signal, RECT, is used to generate by peak-rectification a signal called PEAK, which is available at the PEAK pin. Also noise with a frequency >100KHz increases the PEAK voltage. The value of the PEAK voltage influences the trigger threshold voltage Vth (block ATC). Both signals, RECT and PEAK+Vth are fed to a comparator (block PEAK-COMP) which outputs a sawtooth-sharped waveform at the TBLANK pin, it is triggered. A second comparator (block BLANK-COMP) forms the internal blanking duration of 40 µs. The noiseblanker is supplied by his own biasing circuit (block BIAS-MONO). 1.2 Automatic Threshold Control (ATC) There are two independent possibilities for pro- gramming the trigger threshold: a)the minimum threshold in 8 steps (bits D0-D2, NB-byte 1) b)the maximum threshold in 4 steps (bits D3- D4, NB-byte 1) (see fig.2) The low threshold is used in combination with a good MPX signal without any noise. The sensitivity in this operation is high, depending only on the programmed ”Low Trigger Threshold”, bits D0-D2 of the noiseblanker byte 1. It is independent of the PEAK voltage. The MPX signal is noisy (low fieldstrength) the PEAK signal increases due to the higher noise, which is also rectified (see part 1.1). With increasing of the PEAK voltage the trigger threshold voltage increases, too. This particular gain is programmable in 4 steps (see fig.2). 1.3 Automatic Threshold Control by the Stereoblend voltage (ATC-SB) Besides the noise controlled threshold adjustment there is an additional possibility for influencing the trigger. It is controlled by the difference between Vsb and Vr, similar to the Stereoblend. The reason for implementing such a second con- trol will be explained in the following: 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 (fig.3). But in some cases the behaviour of the noise- blanker can be improved by increasing the threshold even in a region of higher fieldstrength, for the MPX signal often shows distortion in this range. Because of the overlap of this range and the range of the stereo/mono transition it can be con- trolled by Vsb and Vr. This threshold increase is programmable in 3 steps or switched off (see fig.3). 1.4 Over Deviation Detector (MPX-RECT) Sometimes when listening to stations with a higher deviation than 75KHz the noiseblanker triggers on the high frequency modulation. To avoid this blanking, which causes noise in the output signal, the noiseblanker offers a deviation- dependent threshold adjustment. By rectifying the MPX signal a further signal rep- resenting the actual deviation is obtained. It is used to increase the PEAK voltage. Offset and gain of this circuit are programmable in 3 steps (the first step turns off the detector, see fig.4). TDA7420 10/29 |
Ähnliche Teilenummer - TDA7420 |
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Ähnliche Beschreibung - TDA7420 |
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