L2572

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

13 of 14

www.unisonic.com.tw

QW-R102-032.A

AFC FACILITY

An analog frequency error detect circuit, which generates DC voltage proportional to the integral of frequency error

is contained in UTC L2572.As the incident RF is high then the AFC voltage increases, as low then the voltage

decreases. ADC converts the AFC voltage can then be an to be read by the micro controller for frequency fine

tuning.

Around the aligned frequency the AFC detect circuit contains a deadband centre. From zero window to

approximately 25MHz width assuming an oscillator dF/dV of 15MHz/V the deadband can be adjusted. The AFC

voltage does not integrate if the incident RF is within this window, except by component leakage.

With reference to Fig. 6; the demodulated video is fed to a dual comparator which can be compared with two

reference voltages in normal operation, corresponding to the extremes of the deadband, or window. These voltages

are variable and the window adjust input can set it.

Two digital outputs corresponding to voltages above or below the voltage window, or frequency above or below

deadband can be produced by the comparators. These digital control signals can control a complimentary current

source pump. The current signals are then fed to an amplifier’s input. it is arranged as an integrator, so integrating

the pulses into a DC voltage.

Both the current source and sink are disabled if the frequency is correctly aligned, therefore the DC output voltage

can be constant. Due to component leakage there will be a small drift; the maximum drift can be calculated from

here:

C

I

dt

dV

.

2500

=

where

EXT

R

Vcc

I

=