TDA7407 datasheet(17/30 Pages) STMICROELECTRONICS

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LEVEL Input and Gain

To suppress undesired high frequency modula-

tion on the highcut and stereoblend function the

LEVEL signal is lowpass filtered firstly.

The filter is 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 adapt the LEVEL signal internally to different IF

device (see Testmode section 5 LEVELINTERN).

The gain is widely programmable in 16 steps

from 0dB to 10dB (step = 0.67dB). These 4 bits

are located together with the Roll-Off bits in the

"Stereodecoder Adjustment" byte to simplify a

possible adaptation during the production of the

carradio.

Stereoblend Control

The stereoblend control block converts the inter-

nal LEVEL voltage (LEVEL INTERN) into an de-

modulator compatible analog signal which is used

to control the channel separation between 0dB

and the maximum separation. Internally this con-

trol 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 external LEVEL voltage to the inter-

nal 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 LG has to be de-

fined. The following equation can be used to esti-

mate the gain:

REF5V

Fieldstrengthvoltage[STEREO]

The gain can be programmed through 4 bits in

the "Stereodecoder-Adjustment" byte.

The MONO voltage VMO (0dB channel separa-

tion) can be choosen selecting VSBL

All necessary internal reference voltages like

REF5V are derived from a bandgap circuit.

Therefore they have a temperature coefficient

near zero. This is useful if the fieldstrength signal

is also temperature compensated.

But most IF devices apply a LEVEL voltage with a

TC of 3300ppm. The TDA7407 offers this TC for

the reference voltages, too. The TC is selectable

with bit D7 of the "stereodecoder adjustment"

byte.

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).

INTERNAL

VOLTAGES

D97AU639

VSBL

REF 5V

SETUP OF VST

INTERNAL

VOLTAGES

33%

REF 5V

SETUP OF VMO

LEVEL

LEVEL INTERN

FIELDSTRENGHT VOLTAGE

VST

VMO

LEVEL INTERN

42%

50%

58%

VSBL

VST

VMO

FIELDSTRENGHT VOLTAGE

Figure 18. Relation Between Internal and External LEVEL Voltage and Setup of Stereoblend

LOWPASS

TIME CONSTANT

D97AU640

τDeemp

FIELDSTRENGHT

VHCH

VHCL

•

τDeemp

Figure 19. Highcut Characteristics

TDA7407

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