Frequency location of the two-tone testing

I do remember i've seen a topic discussing about how to locate the two frequency in the two-tone method for IIP3 testing. but i cannot find it now, anybody can help explain it again? e.g. choose f1=flo-100M, f2=flo-195M, and the IM3 tone 8M after the mixer. i really want to know why and how we choose the frequency apart of the two tone.

Well, you certainly can have diffierent results at different frequency spacings. You want to look at your modulation scheme and figure out what is appropriate. If you have a lot of envelope peaking with low frequency modulation, like voice, you obviously want the tones spaced close together (a few KHz) to see if your amplifier's bypass capacitors are adequate. If you have a lot of channels, line in a cable TV application, you want to guess which tones would give you the most trouble, in fact sometimes even to the point of doing a 5-tone test, spaced at your channel frequencies, to try to stress the amplifiers the most. In other words, the test should emulate how you are going to use the component.

i got ur point that "try to stress the amplifiers the most", but i am confusing about the "spaced at your channel frequencies", should i consider the multi tones spaced at different channels or, multi tones in a channel?

http://rfdesign.com/images/archive/0601Barkley36.pdf