ATR4258 Datasheet, PDF (10/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

2^nd IF Amplifier
IF-AGC
Table 12. FM Bandwidth Mixer 2
Bit 61
Bit 60
FM Bandwidth Mixer 2
0
0
150 kHz
0
1
200 kHz
1
0
250 kHz
1
1
450 kHz
Note: The bandwidth is also dependant on the values of the application circuit.
In AM and WB mode, the input of the second IF amplifier is pin 28, is externally con-
nected to the 2^nd 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 2^nd 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
2^nd mixer tank and pin 28 in FM mode by setting bit 54 to 1.
Table 13. 2^nd IF Filter in FM Mode
2nd IF Filter
Internal filter
External and internal filter
Bit 54
0
1
The IF-AGC controls the level of the 2^nd IF signal that is passed to the AM demodulator
input or the integrated FM band filter and to the 2^nd 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.
Table 14. IF-AGC Time Constant
Mode
FM/WB
AM reception
AM search
Bit 92
1
0
0
Bit 88
X
0
1
IF AGC Time Constant
IFAGCL (fast)
IFAGCH (slow)
IFAGCL (fast)
In FM/WB mode, the output signal of the FM demodulator is applied to pin 35 via a
series resistor of about 95 kΩ. 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.
10 ATR4258
4838A–AUDR–10/04