TDA7405 STMicroelectronics, TDA7405 Datasheet - Page 34
![IC PROCESSOR EQUALIZER 44-TQFP](/photos/6/84/68458/497-44-tqfp_44-lqfp_sml.jpg)
TDA7405
Manufacturer Part Number
TDA7405
Description
IC PROCESSOR EQUALIZER 44-TQFP
Manufacturer
STMicroelectronics
Type
Car Signal Processorr
Datasheet
1.STN1NF10.pdf
(57 pages)
Specifications of TDA7405
Mounting Type
Surface Mount
Package / Case
44-TQFP, 44-VQFP
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Applications
-
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Company:
Part Number:
TDA7405TR
Manufacturer:
STMicroelectronics
Quantity:
10 000
TDA7405
Figure 34. Highcut characteristics
11 FUNCTIONAL DESCRIPTION OF THE NOISEBLANKER
In the automotive environment the MPX-signal as well as the AM-signal is disturbed by spikes produced by the
ignition and other radiating sources like the wiper-motor. The aim of the noiseblanker part is to cancel the audi-
ble influence of the spikes. Therefore the output of the stereodecoder is held at the actual voltage for a time
between 22 and 38µs in FM (370 and 645µs in AM-mode). The blockdiagram of the noiseblanker is given in
Figure 34.
Figure 35. Block diagram of the noiseblanker
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 triggerstage a pulse former generates the "blanking"-pulse.
11.1 Trigger Path FM
The incoming MPX signal is highpass-filtered, amplified and rectified. This second order highpass-filter has a
corner-frequency of 140kHz. The rectified signal, RECT, is integrated (lowpass filtered) to generate a signal
called PEAK. The DC-charge/discharge behaviour can be adjusted as well as the transient behaviour(MP-dis-
charge control). Also noise with a frequency 140kHz increases the PEAK voltage. 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 acti-
vates the sample-and-hold circuits in the signalpath for the selected duration
11.2 Noise Controlled Threshold Adjustment (NCT)
There are mainly two independent possibilities for programming the trigger threshold:
34/57
1. the low threshold in 8 steps (bits D
2. and the noise adjusted threshold in 4 steps (bits D
1
to D
3
of the noiseblanker-byte I)
4
and D
5
of the noiseblanker-byte I, see Figure 22).