U4255BM-M Datasheet, PDF(7/32 Page) ATMEL Corporation

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U4255BM-M Datasheet, PDF (7/32 Pages) ATMEL Corporation – AM / FM RECEIVER IC

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2nd IF Amplifier

IF-AGC

2nd IF Output

U4255BM-M

Table 9. FM Bandwidth Mixer 2

Bit 61

Bit 60

FM BW Mixer 2

150 kHz

200 kHz

250 kHz

450 kHz

Remark: The bandwidth is also depending from the values of the application circuit.

In AM and WB mode, the input of the second IF amplifier is Pin 28, which is externally

connected to the 2nd mixer tank through the AM ceramic filter to achieve channel selec-

tivity. During normal FM operation (Bit 54 = 0), the input of the second IF amplifier is

connected 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.

Table 10. 2nd IF Filter in FM Mode

2nd IF Filter

Internal filter

External and internal filter

Bit 54

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

input or the integrated FM bandfilter 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 mode

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

Table 11. IF-AGC Time Constant

Mode

Bit 92

FM/WB

AM reception

AM search

Bit 88

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 kW. 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.

The 2nd IF after the gain-controlled 2nd IF amplifier is available at Pin20 (Bit 55 = 0). In

AM mode, this signal may be used for an external AM stereo decoder. Alternatively, a

signal corresponding to the logarithmic field strength after the integrated FM band filter,

which is used for multipath detection, can be switched to Pin 20 by setting Bit 55 = 1.

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