ATR4258 datasheet(8/38 Pages) ATMEL

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ATR4258

4838A–AUDR–10/04

AM RF-AGC

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.

FM 1st Mixer

In the 1st FM mixer stage, the FM reception frequency is down converted to the 1st IF

frequency. The VCO frequency is used as LO frequency for the mixer.

AM 1st Mixer

The AM 1

st mixer is used for up-conversion of the AM reception frequency to the 1st 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 7. AM-AGC Threshold

AM-AGC Threshold

Bit 65

Bit 64

91 dBµV

94 dBµV

97 dBµV

100 dBµV

Table 8. Divide Factor of the AM Prescaler

Divider (AM Prescaler)

Bit 93

Bit 92

Bit 84

Bit 83

Bit 82

Bit 81

Divide by 2

100000

Divide by 3

100001

Divide by 4

100010

Divide by 5

100011

Divide by 6

100100

Divide by 7

100101

Divide by 8

100110

Divide by 9

100111

Divide by 10

101

ATR4258 데이터시트(PDF) 8 Page - ATMEL Corporation

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