TDA7541B datasheet(35/77 Pages) STMICROELECTRONICS

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TDA7541B

Functional description

Doc ID 16048 Rev 1

35/77

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 VPEAK. Noise with a frequency above 140 kHz increases the VPEAK voltage. The

resulting voltage can be adjusted by use of the noise rectifier discharge current. The VPEAK

voltage is fed to a threshold generator, which adds to the VPEAK voltage a DC threshold

VTH. Both signals, VRECT and VPEAK+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:

Low threshold in 8 steps (NBLT)

Noise adjusted threshold in 4 steps (NBCT).

The low threshold is active in combination with a good MPX signal without any noise; the

VPEAK voltage is less than 1V. The sensitivity in this operation is high.

If the MPX signal is noisy (low field strength) the VPEAK voltage increases due to the higher

noise, which is also rectified. With increasing of the VPEAK voltage the trigger threshold

increases, too. This particular gain is programmable in 4 steps (NBCT).

Figure 5.

Adaptive threshold control of noise blanker

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 VPEAK voltage by the stereo blend function it is implemented.

VTH

Min. Trig. Threshold

Noise Controlled Trig. Threshold

0.8 V

1200 mV (00)

950 mV (01)

700 mV (10)

450 mV (11)

8 STEPS

200 mV

147 mV

Peak

1.5 V

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