13

4838B–AUDR–05/05

ATR4258

4.15

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

(see Table 4-18) 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 selectiv-

ity 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.

4.16

FM Demodulator

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 compensate the low fre-

quency deviation in weather band.

An integrated demodulator fine adjustment allows automatic fine tuning of the demodulator cen-

ter 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 4-3.

FM Demodulator Automatic Fine Tuning

Table 4-18.

Bandwidth of the Integrated Band Filter

IF Bandwidth FM (kHz) IF Bandwidth WB (kHz)

Bit 3

Bit 2

Bit 1

Bit 0

220

195

0

0

0

0

...

...

...

...

...

...

200

160

0

0

1

1

...

...

...

...

...

...

165

120

0

1

1

0

...

...

...

...

...

...

130

80

1

0

0

1

...

...

...

...

...

...

90

45

1

1

0

0

80

Not usable

1

1

0

1

70

Not usable

1

1

1

0

60

Not usable

1

1

1

1

FM Demod

+

AM

AGC

B92

Center freq.

Bit 53

Automatic

frequency

adjustment

MPX (Pin 11)

IFAGCH (Pin 35)

V3P

(Pin 27)