Re: Why use VCO?
Well I can give one example for you ... you have to understand mixing and intermediate frequencies to get it:
I've just worked on a project where we used a magnetron as a source. The frequency drifts with temperature changes. The source is mixed a few times and filtered (A SAW filter, very high Q). If the source drifts too much then when we mix, the resulting signal does not make it through the passband. The only thing we could do was design a PLL with a VCO so we could change intermediate frequencies, which would bring the resultant back into the passband.
Hope this isn't too complicated.