ATR4258 datasheet(7/38 Pages) ATMEL

7 / 38 page

ATR4258

4838A–AUDR–10/04

Note:

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

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 out-

puts of these level detectors are used to control the current into the pin diode (see

Figure 3) 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 changing 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 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 3. FM RF-AGC Bit 92

Table 5. Local Oscillator Prescaler (VCO/Buffer Divider)

Prescaler VCO

Prescaler Buffer

Bit 12

Bit 11

Bit 10

1.5

001

1.5

101

Table 6. FM-AGC Threshold

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