This is part 3 in series of two tone intermodulation distortion. When we usually talk about two tone intermods, we mean it for two RF tones for which 2nd harmonics or even order terms fall far away, and we say we only care about IM3 and IM5 because they are close to our signal. That is clearly not the case for two BB tones. Here everything falls either in-band or very close to the signal band leaking into adjacent channels. That is why baseband emissions are one of the top contributors in ACLR. Let’s have a visual look at baseband tones intermodulation.
Image below says it all. You can see how 2nd order terms (BBHD2 & IMD2s), 3rd order products (BBHD3 & IMD3s), 4th order products (BBHD4 & IMD4s) and 5th order products (BBHD5 & IMD5s), they all fall in ACLR zone. On the other hand, IM3 and IM5s (the ones you hear most about), envelope, and some IMD4s fall in-band degrading signal quality, and thus EVM.
For this case, you see distortions are more spread out. You get mix-and-match of odd and even order non-linearities in-band and out-of-band.
Date Published: 09 Jan 2023
Last Edit: 02 Feb 2023