So, since my last big question thread turned into some wonderfully patient explanations of IP3 and why I might care, I thought I'd play around with measuring it. I took some of the amplifiers I've made, and measured them to the best of my ability. A question arose today that I can't really find an answer for... I know that I don't have the best equipment, so I suppose any measurement I make is a "worst case" situation . But anyway, I noticed that if I measure the signal from my generator, and then put the amplifier in, I see the following results on my oscilloscope's FFT (left is siggen -> scope -> dummy load; right is siggen -> amp -> scope -> dummy load): My question is, since I see intermodulation products without the amplifier, does that somehow spoil my measurement when I put the amplifier inline? Is there any problem with these measurements that means I can't draw useful conclusions about the amplifier? Thanks for any help!