In the PLL it is said that Capture range is the frequency from which the PLL starts functioning and Locked range is the frequency where the output of VCO is equal to that of reference oscillator frequency. If that is the case why the Locked range is greater than Capture range? My doubt is it shouldn't get locked as it crosses capture range because it stops functioning as PLL.

  • \$\begingroup\$ Please add a link where this is discussed into your question. I don't know much about phase-locked loops but I imagine that the frequency needs to be fairly close to capture but then the device can stay locked on over a wider range. \$\endgroup\$
    – Transistor
    Nov 17, 2018 at 16:58
  • \$\begingroup\$ Yes. "It is said" should always be followed by something that answers the question "by whom?". And -- it is not just said. It is a real phenomenon, that you can test for yourself either by working through the math, or by building a PLL and trying it on the bench. \$\endgroup\$
    – TimWescott
    Nov 17, 2018 at 17:58
  • \$\begingroup\$ PLLs produce a beat-note as they try to lock. The average of that beat-note is what pushes the loop in the proper direction. Once in lock, all the waveform is available to support lock, and there is no beat-note. \$\endgroup\$ Nov 18, 2018 at 4:58

1 Answer 1


Words don’t necessarily mean what you think they mean. It is locked-in range. The range in which the frequencies remain equal and the phases in lock, not simply coincide by sheer chance.

That is the feedback loop remains in operation and the PLL is in lock.

Except for some non-linear phase-comparator designs, the loop filters will define how wide the capture range is, as the beat frequency must introduce variations on the VCO with the proper feedback polarity and gain.

But once the PLL is in lock, and the input frequency does not change too fast for the filters to follow or the VCO is not out of range, the PLL will remain in lock. That is the lock-in range.

  • 1
    \$\begingroup\$ A very loose analogy would be to hang a magnet with a rubber band and raise a piece of steel underneath it, then pull it down. You'll need to get higher to capture the magnet as you go up than the point at which the magnet will let loose going down. The physics are entirely different, but it illustrates that such behavior is a thing in the real world. \$\endgroup\$
    – TimWescott
    Nov 17, 2018 at 17:57

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