U4255BM-M Datasheet, PDF (6/32 Pages) ATMEL Corporation – AM / FM RECEIVER IC

FM 1^st IF Amplifier
AM 1^st IF Amplifier
2^nd Mixer
Table 5. Divide Factor of the AM Prescaler
Divider AM Prescaler
Bit 93 Bit 92
Divide by 10
1
0
Divide by 6
1
0
Divide by 7
1
0
Divide by 8
1
0
Divide by 4
1
0
Bit 84
x
x
x
x
x
Bit 83
0
0
0
1
1
Bit 82
0
0
1
0
0
Bit 81
0
1
0
0
1
A programmable gain amplifier is used in FM and WB mode between Pin 38 and Pin 30
to compensate the loss in the external ceramic band filters. The gain of this amplifier is
adjusted by Bit 89 to Bit 91. The input and the output resistance is 330 W and fits to
external ceramic filters.
Two different temperature coefficients of the FM IF amplifier can be selected by Bit 66.
Table 6. Gain of the FM IF Amplifier
Gain FM IF
20 dB
22 dB
24 dB
26 dB
28 dB
29 dB
30 dB
31 dB
Bit 91
0
0
0
0
1
1
1
1
Bit 90
0
0
1
1
0
0
1
1
Bit 89
0
1
0
1
0
1
0
1
Table 7. Temperature Coefficient Setting of FM IF Amplifier
Temperature Coefficient (TC) of the IF Amplifier
TK_min
TK_max
Bit 66
0
1
In AM mode, the gain of the 1^st IF amplifier is controlled by the IF-AGC to extend the
control range of the IF-AGC.
The 2^nd mixer is used in AM, FM and WB mode. The mixer input has 330 W input resis-
tance and can be connected directly to an external ceramic filter.
In FM mode, the high output resistance of the second mixer is reduced to increase the
bandwidth of the tank at the mixer output. The output resistance can be selected by
Bit 60 and Bit 61.
Table 8. 2^nd Mixer Output Resistance in FM Mode
Bit 61
Bit 60
Output Resistance (Bit 54 = 0)
0
0
3.3 kW
0
1
0.63 kW
1
0
0.47 kW
1
1
0.29 kW
Output Resistance (Bit 54 = 1)
~ 100 kW
0.78 kW
0.55 kW
0.32 kW
The LO frequency of the 2^nd mixer (10.25 MHz) has to be applied at Pin 22. This signal
is usually generated by the PLL circuit U4256BM.
6 U4255BM-M
4519C–AUDR–07/02