ATR4258 datasheet(10/38 Pages) ATMEL

10 / 38 page

ATR4258

4838A–AUDR–10/04

Note:

The bandwidth is also dependant on the values of the application circuit.

2nd IF Amplifier

In AM and WB mode, the input of the second IF amplifier is pin 28, is externally con-

nected to the 2nd mixer tank through the AM ceramic filter to achieve channel selectivity.

During normal FM operation (bit 54 = 0), the input of the second IF amplifier is con-

nected to the 2nd mixer output (pin 23, pin 24) and the integrated FM band filter is used

for channel selectivity only. It is possible to use an additional external filter between the

2nd mixer tank and pin 28 in FM mode by setting bit 54 to 1.

IF-AGC

The IF-AGC controls the level of the 2nd IF signal that is passed to the AM demodulator

input or the integrated FM band filter and to the 2nd IF output, pin 20.

Two different time constants of the IF-AGC can be selected by the capacitors at pin 35

(IFAGCH) and pin 36 (IFAGCL). The short time constant (IFAGCL) is used in FM/WB

mode and in AM search mode. The long time constant (IFAGCH) is used for AM

reception.

In FM/WB mode, the output signal of the FM demodulator is applied to pin 35 via a

series resistor of about 95 k

Ω. This low-pass filtered output signal of the FM demodula-

tor is used for the FM demodulator fine adjustment, for muting and as a reference for the

deviation sensor.

Table 12. FM Bandwidth Mixer 2

Bit 61

Bit 60

FM Bandwidth Mixer 2

150 kHz

200 kHz

250 kHz

450 kHz

Table 13. 2nd IF Filter in FM Mode

2nd IF Filter

Bit 54

Internal filter

External and internal filter

Table 14. IF-AGC Time Constant

Mode

Bit 92

Bit 88

IF AGC Time Constant

FM/WB

IFAGCL (fast)

AM reception

IFAGCH (slow)

AM search

IFAGCL (fast)

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