ATR4258 datasheet(14/38 Pages) ATMEL

14 / 38 page

4838B–AUDR–05/05

ATR4258

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.

4.17

Soft Mute

The soft mute functionality is implemented to reduce the output level of the FM demodulator 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 demodulator 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 capac-

itor between pin 34 and pin 29.

The field strength signal available at pin 9 is used for soft mute. Therefore, the soft mute thresh-

old 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 4-19.

Demodulator Gain in Weather Band Mode

Demodulator Gain in Weather Band Mode Relative to FM Mode

Bit 72

Bit 71

14 dB

17 dB

21 dB

23 dB

Table 4-20.

Demodulator Fine Adjustment

Bit 53

Fine tuning ON

Fine tuning OFF

Table 4-21.

Soft Mute Threshold

Relative Soft Mute Threshold

Bit 70

Bit 69

Bit 68

Soft mute OFF

–18 dB

–15 dB

–12 dB

–9 dB

–6 dB

–3 dB

0 dB

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