TDA7541 datasheet(32/76 Pages) STMICROELECTRONICS

32 / 76 page

Functional description

TDA7541

32/76

The AMIF2 block contains mixer2, IF2 amplifier, demodulator and AGC2

The AM/FM mixer2 converts 10.7MHz IF1 into 450 kHz IF2. Mixer2 output passes a 450

kHz narrow band filter (LC plus ceramic filter). LC centre frequency is adjustable by IIC-bus

(IF2A). The following IF2 amplifier provides signal for fully integrated demodulator.

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

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

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 450KHz 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 10,7MHz 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

rectifier is programmable (AINBT). A comparator activates a pulse generator. The duration

of this pulse is software programmable (AINT) and is smooth blanking out the spikes in

mixer2.

4.3

Stereo decoder

4.3.1

Decoder

The stereo decoder-part of the TDA7541 contains all functions necessary to demodulate the

MPX-signal, like pilot tone-dependent MONO/STEREO switching as well as the stereo

blend and high cut.

Adaptations like programmable input gain, roll-off compensation, selectable deemphasis

time constant and a programmable field strength input allow easy adoption to different

applications.

The 4.th order input filter has a corner frequency of 80 kHz and is used to attenuate spikes

and noise and acts as an anti-aliasing filter for the following switch capacitor filters.

Demodulator

In the demodulator block the left and the right channel are separated from the MPX-signal.

In this stage also the 19-kHz pilot tone is canceled. For reaching a high channel separation

the TDA7541 offers an I2C bus programmable roll-off adjustment, which is able to

compensate the low pass behavior of the tuner section. Within the compensation range an

adjustment to obtain about 40dB channel separation is possible. The bits for this adjustment

are located together with the level gain adjustment in one byte. This gives the possibility to

perform an optimization step during the production of the car radio where the channel

separation and the field strength control are trimmed.

In addition to that the FM signal can be inverted.

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