Hello,

I am trying to understand the concept of a PLL. I understand that it essentially locks a VCO to a reference signal. I don't understand what this reference signal is and where it comes from? Why does the reference signal maintain the needed frequency?

 

Hello,

The links on the following page of the EDUCYPEDIA has some links on the PLL subject:
http://educypedia.karadimov.info/electronics/pll.htm

The 4046 and the 567 are well known PLL chips.

Bertus

 

Short answer I found from there is:

"A high stability oscillator like a crystal oscillator etc. are used."

That is what I assumed, but then why use a PLL at all. Is it not better to use a crystal oscillator and a program counter?

 

Hello,

With a PLL you also have the posibility to multiply the frequency of the reference frequency.
With an oscilator and a counter, you can only go down in frequency.

Bertus

 

Hello,

So, that VCO, is esentially a frequency synthesizer? And it needs that reference crystal as the voltage may vary too much?

Thanks.

 

Hello,

The VCO is part of the synthesizer.
Take a look at the attached PDF's.

Bertus

 

My ARM mcu is clocked at a rock-stable 168MHz but it uses an 8MHz xtal.
That is an example of what a PLL can do.