Mixing f1 with f2 produces f1+f2 as well as f1-f2 afaik? You could wolfram alpha plot sin(2*t), sin(3*t), sin(2*t)*sin(3*t), t=0..3*pi to see the effect. I meant Interference in sense of multiple frequencies put "on top" of each other resulting an audible waveform. Not in the sense of EMI. (See above) I think the second "frequency" could be anything from another switching stage to frequency response of any component though.. Since I had only noisy chokes and transformers so far, I can't tell from experience though. Assuming the IC runs stable and is correctly compensated, It does nothing but applying a rect with defined frequency and rise time with a calculated pulse width to a choke that feeds a capacitor. Not sure what can be out of specs there... The actual wave form comes from load, inductor and capacitor.