TDA7405 datasheet(32/56 Pages) STMICROELECTRONICS

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TDA7405

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4.2 InGain + Infilter

The InGain stage allows to adjust the MPX-signal to a magnitude of about 1Vrms internally 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 pilottone is cancelled. For reaching a high channel separation the A631 offers an I

2C-bus programmable

roll-off adjustment which is able to compensate the lowpass behavior of the tuner section. If the tuner's attenu-

ation at 38kHz is in a range from 7.2% to 31.0% the A631 needs no external network in front of the MPX-pin.

Within this range an adjustment to obtain at least 40 dB channel separation is possible. The bits for this adjust-

ment are located together 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 and the fieldstrength con-

trol are trimmed. The setup of the Stereoblend characteristics which is programmable in a wide range is de-

scribed in 2.8.

4.4 Deemphasis and Highcut

.The deemphasis-lowpass allows to choose a time constant between 37.5 and 100µs. The highcut control range

will be 2 x tDeemp or 2.7 x tDeemp dependent on the selected time constant (see programming section). The bit

D7 of the hightcut-byte will shift timeconstant 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 tDeemp...3 (3.7) x tDeemp. 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 I

2C-bus (bit D7, Highcut byte set to "0").

The setup of the highcut characteristics is described in 4.9.

4.5 PLL and Pilottone-Detector

The PLL has the task to lock on the 19kHz pilottone during a stereo-transmission to allow a correct demodula-

tion. The included pilottone-detector enables the demodulation if the pilottone reaches the selected pilottone

threshold VPTHST. Two different thresholds are available. The detector output (signal STEREO, see Blockdi-

agram) can be checked by reading the status byte of the A631 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 3.3 and 4.

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 carradio. This signal controls directly the Highcut stage whereas the signal is filtered again (fc=100Hz)

before the stereoblend stage (see Figure 25).

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