LMX2433 datasheet(23/53 Pages) TI1

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LMX2430, LMX2433, LMX2434

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SNAS187D – FEBRUARY 2003 – REVISED JANUARY 2016

9 Detailed Description

9.1 Overview

The basic phase-lock-loop (PLL) configuration consists of a high-stability crystal reference oscillator, a frequency

synthesizer such as the LMX243x, a voltage controlled oscillator (VCO), and a passive loop filter. The frequency

synthesizer includes a phase detector, current-mode charge pump, programmable reference R and feedback N

frequency dividers. The VCO frequency is established by dividing the crystal reference signal down through the

reference divider to obtain a comparison reference frequency. This reference signal, fr, is then presented to the

input of a phase / frequency detector and compared with the feedback signal, fp, which was obtained by dividing

the VCO frequency down by way of the feedback divider. The phase and frequency detector (PFD) measures the

phase error between the fr and fp signals and outputs control signals that are directly proportional to the phase

error. The charge pump then pumps charge into or out of the loop filter based on the magnitude and direction of

the phase error. The loop filter converts the charge into a stable control voltage for the VCO. The function of the

PFD is to adjust the voltage presented to the VCO until the frequency of the feedback signal and phase match

that of the reference signal. When this phase-locked condition exists, the VCO frequency is N times that of the

comparison frequency, where N is the feedback divider ratio.

9.2 Functional Block Diagram

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LMX2433 Datasheet(PDF) 23 Page - Texas Instruments