Phase Locked Loop Explanation/Example

[tpowell1830]

Phase Locked Loop
I am a beginner in electronics hobby essentially because it has been 35 years since class and I was looking at the PLL circuit and trying to understand how it works. Can someone here in the forum please explain a PLL and give an example circuit? I went back to the text and I am clueless. I remember studying this, but also remember that I didn't really understand it back then, in the '80s.  I would like a laymans explanation, without too much theory, if possible, given that I understand the basics of electronics; i.e. ohms law.  Also, where would this type of circuit be used in today's devices?

Thanks in advance.

[DimitriP]

THe really short version:  compares the phase of two inputs and produces a error voltage based on the difference.
This voltage is used to "reign in" one of the two inputs so they match.

The long version is about 278   600 pages

Take a look at this:




and then this:

[Brumby]

THe really short version:  compares the phase of two inputs and produces a error voltage based on the difference.
This voltage is used to "reign in" one of the two inputs so they match.

The long version is about 278   600 pages

That's about right.

[DimitriP]

You MUST be kidding!

[Brumby]

I'd suggest starting off with the opening segment of this video - from 0:40 to 2:25

The things I have taken on board to help with my rudimentary understanding are these:
* We are more interested in the frequency of the incoming signal, rather than how 'pretty' it looks
* There is a local voltage controlled oscillator
* The output of the VCO is compared to the input signal and the difference in phase is used to vary the VCO until this phase difference is minimised.

The result is you not only get a signal from the VCO that is perfectly matched on frequency, but is also closely matched in phase, to the incoming signal - and once these are aligned, they 'lock' together.

This is really useful if you want a clean sine wave from a signal with noise, etc.

You can also 'multiply' the input frequency, but taking the output of the VCO and 'dividing' it before it is fed into the phase detector.

And I'm not even getting warmed up yet.



As said above, there is a lot more to this subject.

I'll give a brief explanation which is enough to get the generalities but you should go deeper into details.

Brief?

When it comes to PLLs.................

[danadak]

Some ref material -

https://archive.org/details/PhaseLockedLoop-AN-1410ConfiguringAndApplyingTheMC74HC4046APhaseLockedLoopOCR

ftp://bitsavers.informatik.uni-stuttgart.de/pdf/motorola/_dataBooks/Motorola_Phase-Locked_Loop_Systems_Data_Book_2ed_Aug73.pdf

http://www.analog.com/media/en/analog-dialogue/volume-33/number-1/articles/volume33-number1.pdf


Regards, Dana.

[tpowell1830]

Thanks to all who responded. I now have an understanding of how and why the system works (right before my head exploded)  .
Among other applications, the PLL can take a dirty signal and output a cleaner signal that is in phase and the same frequency as the control signal, as well as output a signal that is doubled or tripled etc. by a divide by N counter. This would be useful if you for instance need a frequency that is still in phase with the control, but is a multiple frequency of the control.

I think that I understood w2aew's explanation much better than the MIT professor's version. 

Again thanks!

[MrAl]

Hi,

The simplest explanation is that it is a linear voltage regulator that also includes stages that convert frequency to voltage and then voltage back to frequency.

A slightly closer view would reveal that it measures phase difference in order to determine the frequency difference, so it actually converts a phase difference to a voltage not the frequency directly, although there could be an additional circuit that detects frequency more directly in order to bring the circuit into the proper range for normal operation.

Simple example:
Start with a linear voltage regulator made with an op amp with negative feedback.  Break the feedback path.  On the output, connect a voltage to frequency circuit taking the new output as the frequency output from that circuit.  Make a new feedback path, connecting a frequency to voltage circuit from the new output back to the feedback input.  You now have a simplified PLL.

To make this more like a real PLL, instead of using a frequency to voltage converter, use a phase comparator that compares the phase of the output frequency to the phase of a local crystal oscillator and provides a linear output voltage that is proportional to the phase difference, and use the output of that as the feedback.

[steverino]

You MUST be kidding!

Are you indicating exasperation because of the length of the post or the content??

[DimitriP]

Mainly the length.       (although there may be some shorter ones I could pick on too....but I can't fix everything on the internet  )

"You do not really understand something unless you can explain it to your grandmother.”"
""Perfection is Achieved Not When There Is Nothing More to Add, But When There Is Nothing Left to Take Away""
Based on the two above criteria F for missing the point