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參數(shù)資料
| 型號： | TDA7540N |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 調(diào)諧器 |
| 英文描述： | 2-BAND, AUDIO TUNER, PQFP80 |
| 封裝： | 14 X 14 MM, 1.40 MM HEIGHT, ROHS COMPLIANT, LQFP-80 |
| 文件頁數(shù)： | 26/76頁 |
| 文件大?。?/td> | 1225K |
| 代理商： | TDA7540N |
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Functional description
TDA7540N
4.3.2
Functional description of the noise blanker
In the automotive environment spikes produced by the ignition or for example the wiper-
motor disturb the MPX-signal. The aim of the noise blanker part is to cancel the audible
influence of the spikes. Therefore the output of the stereo decoder is held at the actual
voltage for a time between 22μs and 38μs (programmable). The block diagram of the noise
blanker is given inFigure 15.
In a first stage the spikes must be detected but to avoid a wrong triggering on high
frequency (white) noise a complex trigger control is implemented. Behind the trigger stage a
pulse former generates the "blanking"-pulse. An own biasing circuit supplies the noise
blanker in order to avoid any cross talk to the signal path.
Trigger path
The incoming MPX signal is high pass filtered, amplified and rectified. This second order
high pass filter has a corner-frequency of 140kHz. The rectified signal, RECT, is low pass
filtered to generate a signal called PEAK. Also noise with a frequency 140kHz increases the
PEAK voltage. The resulting voltage can be adjusted by use of the noise rectifier discharge
current. The PEAK voltage is fed to a threshold generator, which adds to the PEAK-voltage
a DC-dependent threshold VTH. Both signals, RECT and PEAK+VTH are fed to a
comparator, which triggers a re-triggerable monoflop. The monoflop's output activates the
sample-and-hold circuits in the signal path for the selected duration.
Automatic noise controlled threshold adjustment (Figure 3)
There are mainly two independent possibilities for programming the trigger threshold:
1.
the low threshold in 8 steps (NBLTH)
2.
and the noise adjusted threshold in 4 steps (NBCTH).
The low threshold is active in combination with a good MPX signal without any 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 particular gain is programmable in 4 steps (NBCTH).
Automatic threshold control by the stereoblend voltage (Figure 5)
Besides the noise controlled threshold adjustment there is an additional possibility for
influencing the noise blanker trigger threshold using the bits NBFS. This influence depends
on the stereoblend control.
The point where the MPX signal starts to become noisy is fixed by the RF part. This point is
also the starting point of the normal noise-controlled trigger adjustment. But in some cases
the noise blanker can create a wrong triggering, which create distortion, already in the
region of mono/stereo transition. Therefore a opportunity to control the PEAK voltage by the
stereo blend function it is implemented.
Over deviation detector (Figure 4)
If the system is tuned to stations with a high deviation the noise blanker can trigger on the
higher frequencies of the modulation. To avoid this wrong behavior, which causes noise in
the output signal, the noise blanker offers a deviation-dependent threshold adjustment. By
rectifying the MPX signal a further signal representing the actual deviation is obtained. It is
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