ATR4258 Datasheet, PDF (14/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

Soft Mute
The center frequency of the FM demodulator can be adjusted by means of bit 56 to 59.
At the center frequency, the DC voltage at the MPX output pin 11 is equal to the MPX
offset voltage that can be measured at pin 11 while MPX mute is active (bit 7 = 1). This
adjustment will affect the center frequency of all integrated filters as mentioned before.
Table 19. Demodulator Gain in Weather Band Mode
Demodulator Gain in Weather Band Mode Relative to FM Mode
14 dB
17 dB
21 dB
23 dB
Bit 72
0
0
1
1
Bit 71
0
1
0
1
Table 20. Demodulator Fine Adjustment
Demodulator Fine Adjustment
Fine tuning ON
Fine tuning OFF
Bit 53
0
1
The soft mute functionality is implemented to reduce the output level of the FM demodu-
lator at low input signal levels to limit the noise at the MPX output in this case. If the
input level falls below an adjustable threshold continuously, the output of the FM demod-
ulator is continuously muted with decreasing input level until a maximum mute value is
reached. The threshold for the start of soft mute and the maximum mute can be
adjusted. The signal level for 3 dB mute can be set by means of bit 68 to bit 70 and the
maximum value for soft mute can be selected by bit 67. The steepness and the time
constant of the soft mute can be adjusted by the resistor and capacitor between pin 34
and pin 29.
The field strength signal available at pin 9 is used for soft mute. Therefore, the soft mute
threshold that referred to the input of the FM mixer depends on the gain from the FM
mixer input to the field strength sensor and on the setting of field strength offset (bit 15
to bit 21).
Table 21. Soft Mute Threshold
Relative Soft Mute Threshold
Soft mute OFF
-18 dB
-15 dB
-12 dB
-9 dB
-6 dB
-3 dB
0 dB
Bit 70
0
0
0
0
1
1
1
1
Bit 69
0
0
1
1
0
0
1
1
Bit 68
0
1
0
1
0
1
0
1
14 ATR4258
4838A–AUDR–10/04