Fourier Spectrum of two Signals

Please help me understand what is wrong with my thinking. I have two high frequency sources into an oscilloscope with one source a few hertz difference. I have a spectrum analyzer and expected a low frequency signal due to the beat frequency of the sources. What is going wrong? I'm confused.
- duncan (age 18)
Boise, ID, ADA

The spectrum analyzer gives the square,  or maybe the absolute value, of the Fourier transform of your signal. Here's the key: the Fourier transform is a linear operator. That means that the Fourier transform of the sum of two signals is just the sum of their separate transforms. If neither one had a component at some low frequency, neither will their sum.

If you want to see those beat frequencies show up in the Fourier spectrum, you need to add a non-linear element to the circuit. A rectifying diode would be about the simplest. 

Mike W.

(published on 09/28/2013)