ATR4258 datasheet(14/38 Pages) ATMEL

14 / 38 page

ATR4258

4838A–AUDR–10/04

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.

Soft Mute

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 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 20. Demodulator Fine Adjustment

Demodulator Fine Adjustment

Bit 53

Fine tuning ON

Fine tuning OFF

Table 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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