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Issue #209 December 2007
THE DARKER SIDE
Are You Locked?
A PLL Primer
by Robert Lacoste
Do you cringe when someone mentions “phase-locked loops”? Robert demystifies the subject with this PLL primer. Get ready to use a PLL in your next design.
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Welcome back to The Darker Side. This month, I will dig into one of the most useful electronics building blocks, the phase-locked loop, nicknamed PLL. I don’t know why, but a significant number of electrical engineers are frightened when someone pronounces this acronym. This may be because they are sometimes presented in textbooks through their mathematical properties rather than through their silicon side. That is a shame because PLLs are actually quite simple and they are definitely useful! Let’s try to prove it.
A PLL enables you to generate an output frequency based on a reference input clock. The output frequency can be either higher or lower than the input, but the fundamental point is that the PLL will make sure that they will both stay locked to each other. That means the precision of the output will be the same as the input. If you have a high-stability, 10-MHz, 2-ppm clock, you will be able to generate virtually any frequency with 2 ppm precision. Moreover, a PLL could enable you to “clean” a noisy reference clock, filtering any short-term noise or jitter while keeping the long-term stability of the reference thanks to the locked loop (see Figure 1).
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| Figure 1—The first classic application of a phase-locked loop is to generate a precise frequency (122.4 MHz) based on another precise reference frequency (10 MHz). Another important use of PLLs is to clean a noisy reference frequency, removing any short-term phase variations, while keeping the same long-term precision. |
With these properties, it is not a surprise to find PLLs in virtually all radio-frequency devices. Transmitters need a PLL to generate and modulate the carrier frequency and receivers need them to generate the local oscillators and to recover the bit rate from a noisy signal. PLLs are also present in a large number of digital systems. You probably meet a PLL every time you open a device that needs more than one clock source, with variable clocks, or in devices that need to work with noisy digital signals. Think about it twice. That means everywhere. Some examples include Ethernet interfaces, multirate MP3 players, DVD readers, pixel clock recovery for TV or cellular receivers, and of course your PC. Without PLLs, multigigahertz processors would not exist.
