ATR4258 Datasheet, PDF (13/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

ATR4258
Integrated FM Band Filter
For FM reception a band filter with variable bandwidth is integrated in front of the
demodulator to provide channel selectivity on the 2^nd IF. The bandwidth of this filter can
be adjusted by bit 0 to 3 (see Table 18) to be suitable for the present receiving condition.
In WB mode, the bandwidth of the integrated filter is shifted to lower bandwidth values,
while the necessary channel selectivity is achieved by an external ceramic filter.
The center frequency of the integrated FM band filter can be adjusted by means of bit 14
to 17. The field strength after the integrated FM band filter that is available at pin 20
(bit 55 = 1) can be used for this purpose.
FM Demodulator
Table 18. Bandwidth of the Integrated Band Filter
IF Bandwidth FM
220
...
200
...
165
...
130
...
80
70
60
IF Bandwidth WB
195
...
160
...
120
...
80
...
35
–
–
Bit 3
0
...
0
...
0
...
1
...
1
1
1
Bit 2
0
...
0
...
1
...
0
...
1
1
1
Bit 1
0
...
1
...
1
...
0
...
0
1
1
Bit 0
0
...
1
...
0
...
1
...
1
0
1
For weather band reception, the gain of the FM demodulator is increased and can be
adjusted by means of bit 71 and bit 72 in order to increase the output voltage to com-
pensate the low frequency deviation in weather band.
An integrated demodulator fine adjustment allows automatic fine tuning of the demodu-
lator center frequency to the average frequency of the received signal. This feature is
implemented for use in weather band mode and can be activated by setting bit 53 to 0.
Figure 5. FM Demodulator Automatic Fine Tuning
FM Demod
Center freq.
+
Bit 53
Automatic
frequency
adjustment
MPX (Pin 11)
AM
B92
AGC
IFAGCH (Pin 35)
V3P
(Pin 27)
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