TDA7406T Datasheet, PDF (29/49 Pages) STMicroelectronics – CAR RADIO SIGNAL PROCESSOR

TDA7406T
4.2 2.2. InGain + InFilter
The InGain stage allows to adjust the MPX-signal to an internal magnitude of about 1V_rms which is the recom-
mended value. The 4.th order input filter has a corner frequency of 80kHz and is used to attenuate spikes and
noise and acts as an anti-aliasing filter for the following switch capacitor filters.
4.3 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 cancelled. For reaching a high channel separation the TDA7406T offers an I²C-bus pro-
grammable roll-off adjustment which is able to compensate the lowpass behavior of the tuner section. If the tun-
er's attenuation at 38kHz is in a range from 7.2% to 31.0% the TDA7406T needs no external network in front of
the MPX-pin. Within this range an adjustment to obtain at least 40dB channel separation is possible. The bits
for this adjustment are located together with the fieldstrength 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. The setup of the Stereoblend characteristics which is programmable in a wide
range is described in 4.8.
4.4 De-emphasis and Highcut
The de-emphasis-lowpass allows to choose a time constant between 37.5 and 100µs. The highcut control range
will be 2 x τ_Deemp or 2.7 x τ_Deemp dependent on the selected time constant (see programming section). The bit
D7 of the hightcut-byte will shift time constant and range.
Inside the highcut control range (between VHCH and VHCL) the LEVEL signal is converted into a 5 bit word
which controls the lowpass time constant between τ_Deemp...3 (3.7) x τ_Deemp. Thereby the resolution will remain
always 5 bits independently of the absolute voltage range between the VHCH- and VHCL-values. In addition
the maximum attenuation can be fixed between 2 and 10dB.
The highcut function can be switched off by I²C-bus (bit D7, Highcut byte set to “0”).
The setup of the highcut characteristics is described in 4.9.
4.5 PLL and Pilot tone-Detector
The PLL has the task to lock on the 19kHz pilot tone during a stereo-transmission to allow a correct demodula-
tion. The included pilot tone-detector enables the demodulation if the pilot tone reaches the selected pilot tone
threshold V_PTHST. Two different thresholds are available. The detector output (signal STEREO, see Block dia-
gram) can be checked by reading the status byte of the TDA7406T via I²C-bus. During a Softmute the PLL can
be set into “Hold”-mode which freezes the PLL's state (bit D₄, Softmute byte). After releasing the Softmute the
PLL will again follow the input signal only by correcting the phase error.
4.6 Fieldstrength Control
The fieldstrength input is used to control the highcut- and the stereoblend-function. In addition the signal can be
also used to control the noiseblanker thresholds and as input for the multipath detector. These additional func-
tions are described in sections 5.3 and 6.
4.7 LEVEL-Input and -Gain
To suppress undesired high frequency modulation on the highcut- and stereoblend-control signal the LEVEL
signal is lowpass filtered firstly. The filter is a combination of a 1.st-order RC-lowpass at 53kHz (working as anti-
aliasing filter) and a 1.st-order switched-capacitor-lowpass at 2.2kHz. The second stage is a programmable gain
stage to adapt the LEVEL signal internally to different IF-devices (see Testmode section 5: LEVELHCC). The
gain is widely programmable in 16 steps from 0dB to 6dB (step=0.4dB). These 4 bits are located together with
the Roll-Off bits in the “Stereodecoder-Adjustment”-byte to simplify a possible adjustment during the production
of the car radio. This signal controls directly the Highcut stage whereas the signal is filtered again (fc=100Hz)
before the stereoblend stage (see fig. 24).
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