Differences between reference signal and VCO signal (PLL)

**broken link removed**

PLL Documentations say that if pll is used as a frequency synthesizer, VCO is used with frequency divider.

Why?

Why do we need a pll for this purpose? (I mean a VCO)

A reference oscillator signal is there already. We can add a divider so it becomes another synthesizer. Isn't it?

What is wrong with reference signal + divider? So we need to use VCO + divider?

Thanks..

It's the VCO that is generating the higher frequency, and the output. This is divided down to match phase with the lower reference frequency. The phase difference is used to control the frequency of the VCO to achieve the phase lock.

If you want a lower output frequency than your reference, then you are correct that you could just divide it directly, if you can get the exact frequency you want with an integer divisor.

Thank you FoxyRick

Are these 2 oscillators (VCO and reference signal oscillator) different?

In fact I was trying to ask why is there a such complex structure(2 oscillator+divider+phase comp...) is using to generate a signal?

If you want to increase the signal frequency, you already have a oscillator, just increase it. Is using pll most efficient way to generate a high frequency signal?

There must be a logic behind using a such complex diagram. I am trying to understand it.

Thank you...

Yes, the two oscillators are different. The reference should be accurate and stable, so that the output from the PLL's VCO is also.

A simple use of a PLL is to provide a higher, accurate (as accurate as the reference) digital clock. They are often inside a chip like a microprocessor or FGPA. It allows a simpler, cheaper and, importantly, a lower EMI-producing external clock source.

That's a 'niche' use of them really though. They are widely used in radio communications. For example, a CB radio might have 80 channels, which need to be accurately generated. By using a PLL with a lower, accurate crystal clock source (reference), it can be multiplied up into each frequency/channel step required by changing the division ratio. Each frequency output step will be as accurate as the single reference. The alternative, without a PLL, is the old-fashioned way of using a continuously tuned oscillator (inaccurate, drifting, always needs adjusting) or a separate crystal for each required channel/frequency - that's not practical for lots of channels.