ATR4258 Datasheet, PDF (8/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

AM RF-AGC
FM 1^st Mixer
AM 1^st Mixer
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 64 and bit 65. 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 7. AM-AGC Threshold
AM-AGC Threshold
91 dBµV
94 dBµV
97 dBµV
100 dBµV
Bit 65
0
0
1
1
Bit 64
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 8. (The AM prescaler is only active in AM mode).
Table 8. Divide Factor of the AM Prescaler
Divider (AM Prescaler)
Divide by 2
Divide by 3
Divide by 4
Divide by 5
Divide by 6
Divide by 7
Divide by 8
Divide by 9
Divide by 10
Bit 93
1
1
1
1
1
1
1
1
1
Bit 92
0
0
0
0
0
0
0
0
0
Bit 84
0
0
0
0
0
0
0
0
1
Bit 83
0
0
0
0
1
1
1
1
X
Bit 82
0
0
1
1
0
0
1
1
X
Bit 81
0
1
0
1
0
1
0
1
X
8 ATR4258
4838A–AUDR–10/04