U4255BM-M Datasheet, PDF (5/32 Pages) ATMEL Corporation – AM / FM RECEIVER IC

AM RF-AGC
FM 1^st Mixer
AM 1^st Mixer
4519C–AUDR–07/02
U4255BM-M
Table 3. FM-AGC Threshold
FM-AGC Threshold
104 dBmV
101 dBmV
98 dBmV
92 dBmV
Bit 65
0
0
1
1
Bit 64
0
1
0
1
Figure 3. FM RF-AGC Bit 92
AGC
VS Pin 42
C111
Pin 5
R115
PIN Diode
B92
The AM RF-AGC controls the current into the AM pin diodes (Pin 7) and the source
drain voltage of the MOSFET in the AM preamplifier stage (Pin 6) to limit the level at the
AM mixer input (Pin 3, Pin 41). This threshold level can be set by Bit 62 and Bit 63. If the
level at the AM mixer input exceeds the selected threshold, the current into the AM pin
diodes is increased. If this step is not sufficient, the source drain voltage of the MOSFET
is decreased. The time constant of the AGC control loop can be adjusted by changing
the value of the capacitor at Pin 8.
Table 4. AM-AGC Threshold
AM-AGC Threshold
99 dBmV
102 dBmV
104 dBmV
105 dBmV
Bit 63
0
0
1
1
Bit 62
0
1
0
1
In the 1^st FM mixer stage, the FM reception frequency is down converted to the 1^st IF
frequency. The VCO frequency is used as LO frequency for the mixer.
The AM 1^st mixer is used for up-conversion of the AM reception frequency to the 1^st IF
frequency. Therefore, an AM prescaler is implemented to generate the necessary LO
frequency from the VCO frequency. The divide factor of the AM prescaler can be
selected according to Table 5.
5