ATR4258 Datasheet, PDF (7/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

FM RF-AGC
ATR4258
Table 5. Local Oscillator Prescaler (VCO/Buffer Divider)
Prescaler VCO Prescaler Buffer
Bit 12
Bit 11
1
1
X
0
1.5
3
0
0
1.5
1.5
1
0
2
2
X
1
3
3
X
1
Note: The U4256 FMOSCIN (pin 19) input frequency is limited to 160 MHz.
Bit 10
0
1
1
0
1
The FM RF-AGC circuit includes a wide-band level detector at the input pin 1 of the FM
mixer and an in-band level detector at the output of the FM IF amplifier (pin 30). The out-
puts of these level detectors are used to control the current into the pin diode (see
Figure 3) in order to limit the signal level at the FM mixer input and the following stages.
The maximum pin diode current is determined by R115 and the time constant of the
AGC control loop can be adjusted by changing the value of C111.
The AGC threshold level at the input of the FM mixer can be adjusted by bit 64 and
bit 65 according to Table 6. The in-band AGC threshold refers to the FM mixer input
(pin 1, pin 2) depends on the gain of the FM IF amplifier and can be adjusted by bit 89
to bit 91.
Table 6. FM-AGC Threshold
FM-AGC Threshold
100 dBµV
97 dBµV
94 dBµV
91 dBµV
Figure 3. FM RF-AGC Bit 92
Bit 65
0
0
1
1
Bit 64
0
1
0
1
VS Pin 42
C111
Pin 5
R115
PIN Diode
AGC
B92
7
4838A–AUDR–10/04