10
4883B–AUDR–01/06
ATR4255P
3.13 2
nd
IF Output
The 2
nd
IF after the gain-controlled 2
nd
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
2
nd
IF output 0
Multipath field strength 1
Automatic
frequency
adjustment
FM demod.
FM
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
