by reading the status byte of the TDA7403 via

I2C bus.

Fieldstrength Control

The fieldstrength input is used to control the high

cut and the stereoblend function. In addition the

signal can be also used to control the noise-

blanker thresholds.

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 lowpass 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.

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 to be 33%, 42%, 50% or

58% of REF5V (see fig. 16).

To adjust the external LEVEL voltage to the inter-

nal range two values must be defined: the LEVEL

the full channel separation is reached (VST) the

equation can be used to estimate the gain:

=

REF5V

Field strength voltage [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 33, 42, 50 or 58%

of REF5V.

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 TDA7403 offers this TC for

the reference voltages, too. The TC is selectable

with bit D7 of the "stereodecoder adjustment"

byte.

Figure 15. Internal stereoblend characteristics

INTERNAL

VOLTAGES

t

D97AU639

VSBL

REF 5V

SETUP OF VST

INTERNAL

VOLTAGES

t

33%

REF 5V

SETUP OF VMO

LEVEL

LEVEL INTERN

FIELDSTRENGHT VOLTAGE

VST

VMO

LEVEL INTERN

42%

50%

58%

VSBL

VST

VMO

FIELDSTRENGHT VOLTAGE

Figure 16. Relation between internal and external LEVEL voltage and setup of Stereoblend

TDA7403

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