ATR4255P datasheet(8/34 Pages) ATMEL

8 / 34 page

4883B–AUDR–01/06

ATR4255P

3.8

FM 1st IF Amplifier

A programmable gain amplifier is used in FM and WB mode between pin 38 and pin 30 to com-

pensate the loss in the external ceramic band filters. The gain of this amplifier is adjusted by

bits 89 to 91 (Table 3-6). The input and the output resistance is 330

Ω, and fits to external

ceramic filters.

Two different temperature coefficients (TC) of the FM IF amplifier can be selected by setting

bit 66 (Table 3-7).

3.9

AM 1st IF Amplifier

In AM mode, the gain of the 1st IF amplifier is controlled by the IF-AGC to extend the control

range of the IF-AGC.

3.10

2nd Mixer

The 2

nd mixer is used in AM, FM and WB mode. The mixer input has 330

Ω input resistance and

can be connected directly to an external ceramic filter.

In FM mode, the high output resistance of the second mixer is reduced to increase the band-

width of the tank at the mixer output. The output resistance can be selected by bits 60 and 61

(Table 3-8).

The LO frequency of the 2nd mixer (10.25 MHz) has to be applied at pin 22. This signal is usually

generated by the PLL circuit ATR4256.

Table 3-6.

Gain of the FM IF Amplifier

Gain FM IF

Bit 91

Bit 90

Bit 89

20 dB

22 dB

24 dB

26 dB

28 dB

29 dB

30 dB

31 dB

Table 3-7.

Temperature Coefficient Setting of FM IF Amplifier

Temperature Coefficient (TC) of the IF Amplifier

Bit 66

TKmin

TKmax

Table 3-8.

nd Mixer Output Resistance in FM Mode

Bit 61

Bit 60

Output Resistance (Bit 54 = 0)

Output Resistance (Bit 54 = 1)

3.3 k

Ω

~100 k

Ω

0.63 k

Ω

0.78 k

Ω

0.47 k

Ω

0.55 k

Ω

0.29 k

Ω

0.32 k

Ω

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