I was inspired a while back by the simplicity of the FFT application written by Shane Ormond and featured on the 43oh blog. It was easy to duplicate and I've made a few changes, additions, and such that seemed worth documenting.
I didn't have a signal generator other than the 1kHz square wave on my oscilloscope and some clunky code that I wrote for a microcontroller so I ordered an inexpensive AD9850 and hooked it up to a FR6989 LP so I could use the LCD to display frequency. I've been pleased with the AD9850 and it is hard to beat it for the price. The sine wave is more than sufficient