FFT gives downsampled sinewave

Hi all!

I am working with the FFT core and am not getting an appropriate response. I am using a 128 point streaming FFT with input from a function generator. I simulated the core and got the results I expected, but when I went over to the FPGA, I get a down sampled sine wave (or variation of). i.e. I can cause aliasing, and get a 1 Hz output from the FFT core.

I understand a lot about the actual FFT function and whatnot, but I've been playing with the sample rates and things just don't seem to add up.

So my question is: Has anyone seen this before?

Thanks!