TDA7541B datasheet(30/77 Pages) STMICROELECTRONICS

30 / 77 page

Functional description

TDA7541B

30/77

Doc ID 16048 Rev 1

4.1.5

Strong adjacent channel correction

Under strong adjacent channel it is possible to have disturbance of the audio signal created

by interference between ISS filter control (deviation detection) and adjacent channel mute.

The control bits SACCE and SACCT can be used to avoid this disturbance. If the function is

activated (SACCE = “1”) the IF counter together with the internal field strength is used to

suppress a malfunction of deviation detection. The bit SACCT is changing the sensitivity of

the SACC block (SACCT = “1” means higher sensitivity).

Figure 10 shows the ISS block diagram including the SACC function.

Note:

The IF counter must be switched ON if this function is used (Addr25D4 = “1”)

4.1.6

Weak signal mute

The filtered field strength signal (FSWO) is the reference for soft slope mute control in weak

signal condition to eliminate audible effects. The start point and mute depth are

programmable (WMTH, WMD) in a wide range. These settings together with FSWO bits are

influencing the weak signal mute behavior. The time constant is defined by external

capacitance at PIN 42. Additional adjacent channel mute function is supported. A high pass

filter with -3 dB threshold frequency of 100 kHz, amplifier and peak rectifier generates an

adjacent noise signal from Demodulator output. This value is compared with adjustable

threshold (ACMTH). For present strong adjacent channel the MPX signal is additional

attenuated (ACMD) and has the same time constant as weak signal mute.

Table 12.

ISS control modes

Input signals

Mode1

Mode2

ac+

dev

dev+

inton

int80

Function

inton

int80

Function

0000000

wide

0001000

wide

0001100

wide

0010011

narrow

0011000

wide

mid

0011100

wide

1000011

narrow

mid

1100011

narrow

1001010

mid

1101110

mid

narrow

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