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

voltage where the full channel separation is

fined. The following equation can be used to esti-

mate the gain:

=

REF5V

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

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

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

LOWPASS

TIME CONSTANT

D97AU640

FIELDSTRENGHT

VHCH

VHCL

3

•

Figure 19. Highcut Characteristics

TDA7407

17/30