phase lock loop

does anybody know if you would be able to use phase lock loop to take a sample of a standing wave (produced from a guitar) in order to compare its intial frequency level with a reference frequency (produced by an oscilator)
just an idea that somebody gave me and i was wondering if it would work....

I am not sure what you want to do, but a guitar puts out several harmonics in addition to the fundamental component when you play a string. You wont be able to compare the raw sound from the guitar with the output of a pll/vco. You would have to do some filtering of the initial wave first. now, the guitar is going to be audio frequency, so you can probably do this without a pll just synthesizing the wave in software.