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SL1461KG 데이터시트(PDF) 5 Page - Zarlink Semiconductor Inc |
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SL1461KG 데이터시트(HTML) 5 Page - Zarlink Semiconductor Inc |
5 / 13 page 5 SL1461SA Advance Information DESIGN OF PLL LOOP PARAMETERS Fig.4 VCO R2 C1 BASEBAND OUTPUT R1 RF INPUT GAIN = K0 RAD SEC/VOLT GAIN = KD VOLT/RAD The SL1461SA is normally used as a type 1 second order loop and can be represented by the above diagram. For such a system the following parameters apply; 1 2 1 K 0KD 2 n 2 2 n and where: K0 is the VCO gain in radian seconds per volt KD is the phase detector gain in volts per radian n is the natural loop bandwidth is the loop damping factor R1 is loop amplifier input impedance Note: K0 is dependant on sensitivity of VCO used. KD = 0.25V/rad single ended, 0.5V/rad differential From these factors the loop 3dB bandwidth can be determined from the following expression; AFC FACILITY The SL1461SA 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 centre 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; |
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