TDA7541B datasheet(31/77 Pages) STMICROELECTRONICS

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TDA7541B

Functional description

Doc ID 16048 Rev 1

31/77

4.1.7

Weather band input

If the weather band input is used the chip must be set in FM mode (FMON). In addition to

that the ISS filter must be switched ON (ISSENA=”1”), mixer2 quality factor must be set to

IF2Q=”00” and the ISS filter must be set in manual control mode (ISSON=”1”).

The bit WBON is:

●

activating the WB input

●

switching the ISS filter in WB mode (22 kHz band width)

●

switching the stereo decoder InGain to 26 dB.

In order to increase the selectivity in weather band mode the AM 450kHz ceramic filter can

be used in series to the ISS filter by activating ADSEL (please refer to Figure 10, ISS block

diagram and Section 4.2.1, usage of bit ADSEL).

4.2

AM section

signals, narrow band information (DAGC) referred to PIN 56, up conversion signal (IFAGC)

at PIN 61 and wide band information (RFAGC) at PIN 3.This gain control gives two output

signals. The first one is a current for pin diode attenuator and the second one is a voltage for

preamplifier. Time constant of RF and IF-AGC is defined by internal 100 k

Ω resistor and

external capacitor at PIN 54. The intervention points for AGC (DAGC, IFAGC and RFAGC)

are programmable by software. The oscillator frequency for upconversion-mixer1 is

generated by dividing the VCO frequency after VCO divider (VCOD) and AM pre-divider

(AMD).

Two 10.7 MHz ceramic filters before mixer2 input increase 900 kHz attenuation.

The AMIF2 block contains mixer2, IF2 amplifier, demodulator and AGC2 The AM/FM mixer2

converts 10.7 MHz IF1 into 450 kHz IF2. Mixer2 output passes a 450 kHz narrow band filter

(LC plus ceramic filter). LC centre frequency is adjustable by I2C bus (IF2A). The following

IF2 amplifier provides signal for fully integrated demodulator. If the bit ADSEL is activated,

the ISS filter is inserted between IF2 amplifier and demodulator in order to increase the

selectivity (please refer to Figure 11, AM path block diagram and Section 4.2.1, usage of bit

ADSEL).

Mixer2 and IF2 amplifier have a 2-stage AGC with careful take-over behavior to keep

distortion low. The IF2 AGC range is about 55 dB.

The input signal of IF2 amplifier is used in limiter circuit for in-band level detection. The

electrical characteristics are described in FM section.

The demodulator is a peak detector to generate the audio output signal.

At the MUX output the AMIF stereo is available.

AM IF noise blanker

In order to remove in AM short spikes a noise cancellation conception is used in 450 kHz IF

AM level. The advantage is to avoid long narrow AGC- and demodulator- time constants,

which enlarge spike influences on audio signal and makes difficult to remove it in audio path.

The 10.7 MHz AM IF signal behind IF1 amplifier generates via limitation an unweighted field

strength signal including slope of noise spike. The comparison of this detected slope

between fast and slow rectifier ignores audio modulation whereby the threshold of slow

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