tda7513t STMicroelectronics, tda7513t Datasheet - Page 36

• Manufacturer Part Number: tda7513t
• Description: Single-chip Fm/am Tuner With Stereo Decoder And Audio Processor
• Manufacturer: STMicroelectronics

Functional description
3.5.5
3.5.6
3.5.7
3.5.8
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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 I
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.
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 I
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.
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 spread (see Testmode section 5 LEVELINTERN). The gain is widely
programmable in 16 steps from 0dB to 10dB (step = 0.67dB).
Stereoblend control
The stereoblend control block converts the internal LEVEL voltage (LEVELINTERN) into a
demodulator-compatible analog signal which is used to control the channel separation
between 0dB and the maximum separation. This control range has a fixed upper limit which
is the in-ternal reference voltage REF5V. The lower limit
can be programmed between 29.2% and 58% of REF5V in 4.167% steps (see
To adjust the LEVEL voltage to the proper range two values must be defined: the LEVEL
gain LG and VSBL (see
is reached (VST) the LEVEL gain L
to estimate the gain:
The gain L
separation) can be chosen selecting VSBL. All the necessary internal reference voltages
G
can be programmed with 4 bits. The MONO voltage VMO (0dB channel
Figure
L
G
=
17). To adjust the voltage where the full channel separation
------------------------------------------------------------------------------------------- -
Fieldstrengthvoltage STEREO
G
has to be defined. The following equation can be used
REF5V
[
]
Figure
TDA7513T
2
C bus.
17).
2
C