TDA7540N STMicroelectronics, TDA7540N Datasheet - Page 32
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TDA7540N
Manufacturer Part Number
TDA7540N
Description
IC TUNER AM/FM CAR RADIO 80-LQFP
Manufacturer
STMicroelectronics
Datasheet
1.TDA7540NTR.pdf
(76 pages)
Specifications of TDA7540N
Frequency
10.7MHz
Modulation Or Protocol
AM, FM
Applications
Automotive Audio
Current - Receiving
12mA
Data Interface
PCB, Surface Mount
Antenna Connector
PCB, Surface Mount
Voltage - Supply
7.7 V ~ 9 V
Operating Temperature
-40°C ~ 85°C
Package / Case
80-LQFP
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Features
-
Sensitivity
-
Memory Size
-
Data Rate - Maximum
-
Lead Free Status / Rohs Status
Compliant
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Company:
Part Number:
TDA7540N
Manufacturer:
MAXIM
Quantity:
3 032
Company:
Part Number:
TDA7540NCD
Manufacturer:
ST
Quantity:
1 160
Functional description
4.3.2
32/76
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 22
blanker is given
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
There are mainly two independent possibilities for programming the trigger threshold:
1.
2.
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
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
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
the low threshold in 8 steps (NBLTH)
and the noise adjusted threshold in 4 steps (NBCTH).
inFigure
15.
(Figure
μ
s and 38
4)
μ
s (programmable). The block diagram of the noise
(Figure
(Figure
3)
5)
TDA7540N