U4255BM-M Datasheet, PDF (10/32 Pages) ATMEL Corporation – AM / FM RECEIVER IC

FM Demodulator
For weather band reception, the gain of the FM demodulator is increased and can be
adjusted by means of Bit 71, Bit 72 in order to increase the output voltage to compen-
sate 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
MPX
center freq.
+
Bit 53
Automatic
Frequency
adjustment
AM
B92
AGC
IFAGCH
V3
(10nF)
The center frequency of the FM demodulator can be adjusted by means of Bit 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.
Table 16. Demodulator Gain in Weather Band Mode
Demodulator Gain in Weather Band Mode Relative to FM Mode
12 dB
15 dB
18 dB
21 dB
Bit 72
0
0
1
1
Bit 71
0
1
0
1
Table 17. Demodulator Fine Adjustment
Demodulator Fine Adjustment
Fine tuning ON
Fine tuning OFF
Bit 53
0
1
10 U4255BM-M
4519C–AUDR–07/02