So the other day in a lab my group was working with a signal and some noise and the signal we got was a bit interesting... Essentially we were using a white noise with 0.5 Vpp and a 1kHz square wave with the 10 times the amplitude (5V) running them through a 741 op-amp summing amplifier. We were supposed to experiment with different ratios of approximate voltage and looking at FFTs of the data we collected. The first ratio was ~1/1 so we were dividing the square wave signal by a factor of 10. However just running the signal (no noise) through the amplifier gave us an interesting display and I'm still not particularly sure why it is happening: Here's are some simulations that show pretty much the same thing we observed in the lab http://filer.case.edu/pal25/square%20wave.JPG [Broken] -- Output on oscilloscope http://filer.case.edu/pal25/circuit.JPG [Broken] -- Schematic of circuit (essentially it's two function generators in a summing amplifier) There are spikes on the square wave if we run it through the amplifier and I'm not particularly sure why we've tried using a better op-amp (OP42G) and we still get effectively the same display which leads me to believe it's something else.