What is a Frequency Doubler?

A Frequency Doubler is device that outputs the same signal that is fed into it but with double the frequency. So it gives out exactly twice (2x) the frequency as output.

So, for example, if a frequency doubler has a 500Hz signal as its input, it will output a 1KHz signal. Being that frequency doublers multiply the analog (AC) inputs by 2, they are referred to as analog multipliers.