frequency comparison

hello... me again....
rite.... does nebody have ne ideas on how to compare frequencys????
im trying to set a reference frequency that can be compared with a frequecny obtained from an external source (this case a guitar)
ive considered using beat frequencies, but i desperatly need some other options as i dont really seem to be gettin anywhere.....
help please.....

You might give these circuits a try:

**broken link removed**

http://www.geocities.com/IECMaster/circuits_msr/cir_msr003.html

**broken link removed**

With the last circuit, you could set the comparison frequency (adjusting Rs or C) to match your reference, then apply the test signal.