TDA7513T Datasheet, PDF (24/59 Pages) STMicroelectronics – SINGLE-CHIP FM/AM TUNER WITH STEREO DECODER AND AUDIO PROCESSOR

TDA7513T
The RDS mute is activated from pin #RDSMUTE in AND with Bit 0 of APSD data byte 9.
5.5.1 Stereo Decoder Input stage, Ingain + Infilter
The stereo decoder is crossed by both the FM and the AM signal: the input impedance of the pin
#TUNER_IN is different between the two modes in order to allow the same external coupling components
between #TUNER_OUT and #TUNER_IN to realize different filtering functions. Whilst the input impe-
dence in FM is 100k , in AM the input impedance is decreased to 2kΩ: this allows the realization of typical
high-pass filters with a corner frequency of 70Hz for AM and less than 5Hz for FM. The low-pass section
of the typical AM transfer function is realized by use of the internal FM High-Cut filter.
The Ingain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally which is the
recommended value. The 4th order input filter has a corner frequency of 80kHz and is used to attenuate
spikes and nose and acts as an anti aliasing filter for the following switch capacitor filters.
5.5.2 Demodulator
In the demodulator block the left and the right channel are separated from the MPX signal. In this stage
the 19 kHz pilot tone is cancelled.
To reach a good channel separation the device offers an I2C bus programmable roll-off adjustment which
is able to finely compensate for the low-pass behaviour of the tuner section. An adjustment to better than
40dB channel separation is possible. The bits for this adjustment are located ogether with the fieldstrength
adjustment in one byte. This gives the possibility to perform an optimization step during the production of
the carradio where the channel separation in relation to the fieldstrength control are trimmed.
The setup of the Stereoblend characteristics, which is programmable in a wide range, is de-scribed in 2.8.
5.5.3 De-emphasis and Highcut.
One filter is provided to realize de-emphasis and High-Cut filtering.
The lowpass filter for the de-emphasis allows to choose between a time constant of 50µs and 75µs.
The filter time constant can further be controlled in both cases over the range = 2 DEEMPH. The control
is automatically performed as a function of the filtered field strength level: inside the highcut control range
(between VHCH and VHCL) the level is converted into a 5 bit word which drives the lowpass time constant.
The FM highcut function can be switched off by I²C bus (bit 0,of APSD data byte 11). The setup of the
highcut characteristics is described in 2.9.
In AM the high-cut filter can be programmed (bit 3 to 7 of APSD data byte 16) to a fixed value (inside the
above-mentioned programmable range) in order to provide the desired lowpass characteristic of the AM
signal.
5.5.4 PLL and Pilot Tone Detector
The PLL is tasked with locking on the 19kHz pilot tone during a stereo transmission to allow the correct
demodulation. The detector enables the stereo demodulation if the pilot tone reaches the selected pilot
tone threshold VPTHST. Two different thresholds are available. The detector output can be checked by
reading the status byte of the TDA7407 via I²C bus.
5.5.5 Fieldstrength Control
The filtered field strength signal is fed to the stereo decoder where it can be finely adjusted and normalized
so that it can be used to control the highcut and stereoblend functions. Furthermore the adjusted signal
can also be used to control the noise-blanker thresholds. The unfiltered field strength meter, on the other
hand, is used as input for the stereo decoder multipath detector. These additional functions are described
in sections 3.3 and 4.
5.5.6 LEVEL Input and Gain
To help suppress undesired high frequency modulation of the highcut and stereoblend functions the tuner
filtered field strength signal (LEVEL) is lowpassed by a combination of a 1st order RC low-pass at 53kHz
(working as anti-aliasing filter) and a 1st-order switched capacitor lowpass at 2.2kHz.
The second stage is a programmable gain stage to finely adapt the LEVEL signal internally against tuner
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