ATR4255 datasheet(6/32 Pages) ATMEL

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ATR4255

4837A–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 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.

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 1st 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 5.

FM 1st IF Amplifier

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

Ω and fits to

external ceramic filters.

Two different temperature coefficients of the FM IF amplifier can be selected by bit 66.

Table 4. AM-AGC Threshold

AM-AGC Threshold

Bit 63

Bit 62

99 dBµV

102 dBµV

104 dBµV

105 dBµV

Table 5. Divide Factor of the AM Prescaler

Divider AM Prescaler

Bit 93

Bit 92

Bit 84

Bit 83

Bit 82

Bit 81

Divide by 10

Divide by 6

Divide by 7

Divide by 8

Divide by 4

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