Which intermodulation products are closest to the original signal frequencies?

Intermodulation products are created when two or more signals mix together in a non-linear circuit, such as a receiver's front end or an amplifier. The mathematical combinations of these frequencies (mF1 ± nF2) determine the 'order' of the product, which is the sum of m and n. Even-order products (like the 2nd order, F1 + F2) usually fall far away from the original frequencies and are easily blocked by tuned circuits. However, odd-order products, particularly the 3rd order (such as 2F1 - F2 or 2F2 - F1), fall extremely close to the original signals. Because these odd-order products land practically on top of the desired frequencies, they are very difficult to filter out and frequently cause adjacent-channel interference. This is why receivers boast about their 'third-order intercept point' as a measure of handling strong nearby signals.