13

4883B–AUDR–01/06

ATR4255P

Figure 3-3.

FM Demodulator Automatic Fine Tuning

The center frequency of the FM demodulator can be adjusted by means of bits 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.

3.17

Soft Mute

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

low input signal levels, and to limit the noise at MPX output at low input signal levels. If the input

level falls below an adjustable threshold, continuously the output of the FM demodulator is con-

tinuously 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 bits 68 to 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 pins 34 and 29.

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

old referred to the input of the FM mixer depends on the gain from FM mixer input to the field

strength sensor.

Table 3-16.

Demodulator Gain in Weather Band Mode

Demodulator Gain in Weather Band Mode Relative to FM Mode

Bit 72

Bit 71

12 dB

0

0

15 dB

0

1

18 dB

1

0

21 dB

1

1

Table 3-17.

Demodulator Fine Adjustment

Demodulator Fine Adjustment

Bit 53

Fine tuning ON

0

Fine tuning OFF

1

FM demod.

+

(10 nF)

AM

AGC

B92

Center freq.

Bit 53

Automatic

frequency

adjustment

MPX

IFAGCH

V3