TDA7513T Datasheet, PDF (25/59 Pages) STMicroelectronics – SINGLE-CHIP FM/AM TUNER WITH STEREO DECODER AND AUDIO PROCESSOR

TDA7513T
spread (see Testmode section 5 LEVELINTERN). The gain is widely programmable in 16 steps from 0dB
to 10dB (step = 0.67dB).
5.5.7 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 figs. 14, 15).
To adjust the LEVEL voltage to the proper range two values must be defined: the LEVEL gain LG and
VSBL (see fig. 15). To adjust the voltage where the full channel separation is reached (VST) the LEVEL
gain L_G has to be defined. The following equation can be used to estimate the gain:
L_G
=
-------------------------------------^R-----^E-----^F----⁵----^V---------------------------------------
Fieldstrengthvoltage[STEREO]
The gain L_G can be programmed with 4 bits. The MONO voltage VMO (0dB channel separation) can be
chosen selecting VSBL. All the necessary internal reference voltages like REF5V are derived from a band-
gap circuit, therefore they have a temperature coefficient which is practically zero.
5.5.8 Highcut Control
The highcut control setup is similar to the stereoblend control setup : the starting point VHCH can be set
with 2 bits to be 42, 50, 58 or 66% of REF5V whereas the range can be set to be 17, 22, 28 or 33% of
VHCH (see fig. 19).
5.6 NOISE-BLANKER
In the automotive environment the MPX signal is disturbed by spikes produced for example by the ignition
and by the wiper motor. The aim of the noiseblanker part is to cancel the audible influence of these spikes.
To perform this function the output of the stereodecoder is held at the curent voltage for a time between
22 and 38µs (programmable). The block diagram of the noiseblanker is shown in fig.20. In the first stage
the spikes are detected but to avoid a wrong triggering on high frequency (white) noise a complex trigger
desensitization control is implemented. Behind the trigger stage a pulse former generates the "blanking"
pulse
5.6.1 Trigger Path
The incoming MPX signal is highpassed by a filter with a corner frequency of 140kHz, amplified and rec-
tified. The rectified signal (RECT) is lowpassed to generate the signal PEAK. Also noise at a frequency
higher than 140kHz increases PEAK. The lowpass output voltage can be adjusted by changing the noise
rectifier discharge current. The PEAK voltage is fed to a threshold generator which adds to the PEAK volt-
age a constant voltage VTH, thus producing the trigger threshold PEAK+VTH. Both RECT and
PEAK+VTH are fed to a comparator which trig-gers a re-triggerable monoflop. The monoflop output acti-
vates the sample-and-hold circuits in the signalpath for a selectable duration.
5.6.2 Automatic Noise Controlled Threshold Adjustment (ATC)
There are mainly two independent possibilities to program the trigger threshold:
a programming the so-called "low threshold" in 8 steps;
b programming the so-called "noise-adjusted threshold" in 4 steps
The "low threshold" is active in combination with a good MPX signal without any noise; the PEAK voltage
is less than 1V. The sensitivity in this operating mode is high.
If the MPX signal is noisy the PEAK voltage increases due to the higher noise, which is also rectified. With
increasing of the PEAK voltage the trigger threshold increases, too. This particular mechanism ("noise-
adjusted threshold") is programmable in 4 steps.
25/59