TDA7541B Datasheet, PDF (36/77 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

Functional description
TDA7541B
Figure 6. Stereo blend controlled threshold adjust
VPEAK
MONO
STEREO
≈ 3V
N OIS E
2.0V (00)
1.6V (01)
1.4V (10)
ATC_SB OFF (11)
0.8V
noisy signal
good signal
E'
Automatic deviation controlled threshold adjustment
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
used to increase the PEAK voltage. The gain of this circuit is programmable in 3 steps
(NBDC) of the stereo decoder-byte (the first step turns off the detector).
Figure 7.
Deviation controlled threshold adjust
VPEAK
[V_OP]
00
2.0
01
1.5
10
1.0
0.8
11 Detector off
20
32.5
45
75
DEVIATION [KHz]
Multipath influence on noise blanker
To react on high repetitive spikes caused by a Multipath-situation, the discharge-time of the
V_PEAK voltage can be decreased depending on the voltage-level at Pin MPTC. There are
two ways to do this.
a) Switch on the linear influence of the Multipath-Level on the PEAK-signal. In this
case the discharge slew rate is 1 V/ms. The slew rate is measured with R_Discharge
= infinite and VMPTC = 2.5 V
b) Activate a function, which switches to the 18k discharge resistor if the Multipath-
Level is below 2.5 V. If multipath influence on noise blanker is switched ON than
MPF bit has to be set to 0.
36/77
Doc ID 16048 Rev 1