TDA7541B Datasheet, PDF (35/77 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system

TDA7541B
Functional description
Trigger path
The incoming FM demodulator output signal is taken in front of the weak signal mute, high
pass filtered, amplified and rectified. This second order high pass filter has a corner-
frequency of 140 kHz. The rectified signal, VRECT, is low pass filtered to generate a signal
called V_PEAK. Noise with a frequency above 140 kHz increases the V_PEAK voltage. The
resulting voltage can be adjusted by use of the noise rectifier discharge current. The V_PEAK
voltage is fed to a threshold generator, which adds to the V_PEAK voltage a DC threshold
VTH. Both signals, V_RECT and V_PEAK+VTH are fed to a comparator, which triggers a re-
triggerable monoflop. The output of the monoflop activates the sample-and-hold circuit 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. Low threshold in 8 steps (NBLT)
2. Noise adjusted threshold in 4 steps (NBCT).
The low threshold is active in combination with a good MPX signal without any noise; the
V_PEAK voltage is less than 1V. The sensitivity in this operation is high.
If the MPX signal is noisy (low field strength) the V_PEAK voltage increases due to the higher
noise, which is also rectified. With increasing of the V_PEAK voltage the trigger threshold
increases, too. This particular gain is programmable in 4 steps (NBCT).
Figure 5. Adaptive threshold control of noise blanker
VTH
Min. Trig. Threshold
200 mV
8 STEPS
147 mV
0.8 V
1200 mV (00)
950 mV (01)
700 mV (10)
450 mV (11)
Noise Controlled Trig. Threshold
1.5 V
V _Peak
Automatic stereo blend controlled threshold adjustment
Besides the noise controlled threshold adjustment there is an additional possibility for
influencing the noise blanker trigger threshold using the bits NBFC. This influence depends
on the stereo blend 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 an opportunity to
control the V_PEAK voltage by the stereo blend function it is implemented.
Doc ID 16048 Rev 1
35/77