SL1461MPAS datasheet(6/13 Pages) ETC2

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SL1461

AFC FACILITY

The SL1461 contains an analog frequency error detect

circuit, which generates DC voltage proportional to the

integral of frequency error. If the incident RF is high then the

AFC voltage increases, if low then the voltage decreases. The

AFC voltage can then be converted by an ADC to be read by

the micro controller for frequency fine tuning; if used in an I2C

system it is recommended the device is used with either the

SP5055 or SP5056 frequency synthesiser which contains an

internal ADC readable via the I2C bus.

The voltage corresponding to frequency alignment is

arbitrary and user defined; if used with the SP5055 it is

suggested the aligned voltage is 0.375 VCC, corresponding to

the centre code of the ADC on port 6.

The AFC detect circuit contains a deadband centred

around the aligned frequency. The deadband can be adjusted

from zero window to approximately 25MHz width assuming an

oscillator dF/dV of 15MHz/V. If the incident RF is within this

window the AFC voltage does not integrate, except by

component leakage.

With reference to Fig.5; in normal operation the

demodulated video is fed to a dual comparator where it is

compared with two reference voltages, corresponding to the

extremes of the deadband, or window. These voltages are

variable and set by the window adjust input.

The

comparators

produce

two

digital

outputs

corresponding to voltages above or below the voltage window,

or frequency above or below deadband. These digital control

signals are used to control a complimentary current source

pump. The current signals are then fed to the input of an

amplifier which is arranged as an integrator, so integrating the

pulses into a DC voltage.

If the frequency is correctly aligned both the current source

and sink are disabled, therefore the DC output voltage

remains constant. There will be a small drift due to component

leakage; the maximum drift can be calculated from;

dt +

2500.C where I +

VCC

REXT

, C + CEXT

WINDOW

ADJUST

VALIGN

FREQ

BASEBAND

VIDEO

VCC

VEE

CEXT

EXT

VAFC

Fig. 5 AFC system block diagram

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