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Issue #209 December 2007
THE DARKER SIDE
Are You Locked?
A PLL Primer
by Robert Lacoste
Start | VCO Basics | PLL Basics | Integer or Fractional? | PLL Design | Silicon Trends | It's Your Turn! | Sources & PDF
IT’S YOUR TURN!
As I said in my introduction, PLLs are everywhere. Their immediate use is to generate a fixed-frequency clock or RF carrier based on a reference frequency. Take a look at your latest motherboard. There isn’t a high-frequency oscillator. The 3-GHz or so clock is generated inside the processor itself. However, a PLL’s application range is far wider. Digital transmissions (from Ethernet to ADSL or to wireless protocols) all transmit their bits of information serially and rely on PLLs to recover the clock from the signal itself. Because the transmitted signal has frequency content at the carrier frequency, a PLL with a locking range around this carrier frequency will quickly lock on it. By the way, to ensure that the lock will always occur, the datastream must have regular bit transitions even if the data is always 0 or 1. This is why bit-encoding methods like Manchester encoding or 8/10 bit encoding are used everywhere. PLL can also “clean” a noisy clock signal, act as a nearly perfect FM demodulator, and be used in a zillion of other applications.
Full books can be written on this subject. I can’t cover everything in a single column. However, my hope is that the subject is now demystified for you and that you will not be frightened to think “PLL” for your next design. PLLs are not back magic, even if they are sometimes on the darker side.
