ATR4258 datasheet(7/38 Pages) ATMEL

7 / 38 page

4838B–AUDR–05/05

ATR4258

Note:

The U4256 FMOSCIN (pin 19) input frequency is limited to 160 MHz.

4.4

FM RF-AGC

The FM RF-AGC circuit includes a wide-band level detector at the input pin 1 of the FM mixer

and an in-band level detector at the output of the FM IF amplifier (pin 30). The outputs of these

level detectors are used to control the current into the pin diode (see Figure 4-1) in order to limit

the signal level at the FM mixer input and the following stages. The maximum pin diode current

is determined by R115 and the time constant of the AGC control loop can be adjusted by chang-

ing the value of C111.

The AGC threshold level at the input of the FM mixer can be adjusted by bit 64 and bit 65

according to Table 4-6. The in-band AGC threshold refers to the FM mixer input (pin 1, pin 2)

depends on the gain of the FM IF amplifier and can be adjusted by bit 89 to bit 91.

Figure 4-1.

FM RF-AGC Bit 92

Table 4-5.

Local Oscillator Prescaler (VCO/Buffer Divider)

Prescaler VCO

Prescaler Buffer

Bit 12

Bit 11

Bit 10

1.5

Table 4-6.

FM-AGC Threshold

Bit 65

Bit 64

100 dBµV

97 dBµV

94 dBµV

91 dBµV

AGC

B92

PIN Diode

Pin 42

Pin 5

C111

R115

ATR4258 Datasheet(PDF) 7 Page - ATMEL Corporation