—11—

CXA3176N

13) AFC

The AFC is of the current output type which outputs the frequency deviation in the form of the current and

converts it to the voltage. The output current range is approximately ±0.4 µA for the slow mode and ±70

µA for the fast mode.

CXA3176N AFC Principle Diagram

The Pin 7 voltage V continues to change till the Vin value reaches the Vref value. When these values are

equal, the Pin 7 output current becomes “0” and the voltage is determined by the charge and time.

The Pin 7 voltage V is output to Pin 6 through the amplifier.

Vout=Vref+4 (V–Vref)

Also, the voltage for Pin 7 is undefined with the IC itself.

The AFC voltage varies, for example, as shown below by the VCO characteristics. The AFC voltage

follows the VCO characteristics because this voltage is independent of the slope of the S curve. In other

words, the CXA3176N operates according to the VCO characteristics when the VCO characteristics have

the linearity with respect to the voltage and the VCO characteristics can be controlled within the range

shown in the graph below.

Vref

Vref

C

V=it/C

×4

LPF

Vref

BUF

7

6

VCO

Vin (S curve voltage)

Vout

(AFC voltage)

External parts

1st MIX for

FM receiver

1st OSC for

FM receiver

To CXA3176N

0.4V

fo

A

B

C

Input frequency