1
\$\begingroup\$

What is the difference between an open-loop clock and a closed-loop clock?

Is a PLL with an oscillator lock a closed loop clock? Is a PLL without an oscillator lock an open-loop clock?

\$\endgroup\$
6
  • 2
    \$\begingroup\$ Your question does not make sense, please try to rephrase it more clearly. \$\endgroup\$ Nov 20, 2019 at 19:53
  • 1
    \$\begingroup\$ Hi, I think there might be a language barrier. Could you re-word your question in a few different ways and post all of them since your question is unclear. You can click edit to change your post. Maybe what you are trying to ask is: "Does an PLL without an oscillator to lock onto run open-loop to produce a clock?" \$\endgroup\$
    – DKNguyen
    Nov 20, 2019 at 19:53
  • \$\begingroup\$ Or maybe you are trying to ask "Is an unlocked PLL an open-loop clock? And is a locked PLL a closed-loop clock? And what is an open loop clock and what is a closed loop clock?" \$\endgroup\$
    – DKNguyen
    Nov 20, 2019 at 19:55
  • 2
    \$\begingroup\$ Sorry, English is not my main language. \$\endgroup\$ Nov 20, 2019 at 19:56
  • 2
    \$\begingroup\$ To get an answer to what you REALLY want to know you should explain the background - what is the REAL question? || Otherwise terminology can bury both question and answer. | An unlocked PLL clock is not constrained by any external signal so is "open loop". || For a FREQUENCY locked loop - which is often part of what people think of with a PLL, the PLL clock "locks" to the input signal so can be thought of as 'closed loop'. || You may have trouble with strict meanings when signal and clock are on the SAME frequency but do or don't have phase lock. \$\endgroup\$
    – Russell McMahon
    Nov 20, 2019 at 20:04

1 Answer 1

1
\$\begingroup\$

An "unlocked" PLL could be open or closed loop.

Consider if the capture range does not exceed the initial mixer error frequency then the feedback is too small to correct the VCO and lock it even though, the loop may be "closed"

Capture range is usually much less than the holding or tracking Locked range for a type I phase detector mixer.

This capture/ Locked range ratio depends on the loop phase margin, which depends on the loop filter phase lead /lag compensation which is a fancy description for a filter that looks like < R1+R2C1> in the feedback path .

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.