ATR4255P datasheet(10/34 Pages) ATMEL

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4883B–AUDR–01/06

ATR4255P

3.13

2nd IF Output

The 2

nd IF after the gain-controlled 2nd IF amplifier is available at pin 20 (bit 55 = 0). In AM

mode, this signal may be used for an external AM stereo decoder. Alternatively, a signal corre-

sponding to the logarithmic field strength after the integrated FM band filter, which is used for

multipath detection, can be switched to pin 20 by setting bit 55 to “1”.

3.14

Automatic IF Center Frequency Adjustment

Integrated active filters are used in the FM band filter, FM demodulator, and adjacent channel

sensor. The center frequency of these filters is automatically adjusted to the second IF fre-

quency of 450 kHz. The frequency of 10.25 MHz at pin 22 is used as a reference for this

alignment.

Figure 3-2.

Automatic IF Center Frequency Adjustment

For fine tuning, the center frequency of all these integrated active filters (band filter, demodula-

tor, and adjacent channel sensor) can be shifted in steps of 6.25 kHz using bits 56 to 59 (Table

3-13 on page 11). Additionally, the center frequency of the band filter can be adjusted separately

by means of bits 14 to 17, if bit 4 is set to “1”. If bit 4 is set to “0”, the default setting is used.

Table 3-12.

Pin 20 Output Setting

Pin 20

Bit 55

2nd IF output

Multipath field strength

Automatic

frequency

adjustment

FM demod.

band filter

Adj. channel

sensor

Bit 56 to 59

10.25 MHz

Bit 14 to 17

Bits 5 and 6

Center

freq.

Center

freq.

Center

freq.

Pin 37

Pin 22

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