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| 型號： | TDA7406T |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 消費(fèi)家電 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, PQFP44 |
| 封裝： | 10 X 10 MM, TQFP-44 |
| 文件頁數(shù)： | 26/49頁 |
| 文件大小： | 830K |
| 代理商： | TDA7406T |
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TDA7406T
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5.2 Noise Controlled Threshold Adjustment (NCT)
There are mainly two independent possibilities for programming the trigger threshold:
1. the low threshold in 8 steps (bits D1 to D3 of the noiseblanker-byte I)
2. and the noise adjusted threshold in 4 steps (bits D4 and D5 of the noiseblanker-byte I, see fig. 19).
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 increases 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 fig. 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 (fig. 20). In some cases the behavior of the noiseblank-
er 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 con-
trolled 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 noise-
blanker offers a deviation-dependent threshold adjustment. By rectifying the MPX signal a further signal repre-
senting 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 fig. 19).
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 TDA7406T offers a linear as well as a threshold
driven 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=infinite and VMPout=2.5V
5.3.4 AM mode of the Noiseblanker
The TDA7406T noiseblanker is also suitable for AM noise cancelling. The detector uses in AM mode the 450kHz
unfiltered 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 120kHz 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
(640s to 1.2ms). All threshold controls can be used like in FM mode.
6
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 block diagram, fig. 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 fig. 28. Another (internal) time constant is used to control the
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