Which of the following is an odd-order intermodulation product of frequencies F1 and F2?

Intermodulation products are spurious signals created when two or more frequencies mix together in a non-linear circuit, such as a receiver or amplifier. The "order" of an intermodulation product is calculated by adding together the absolute values of the frequency coefficients (the multipliers). Looking at the correct answer (2F1 - F2), the multipliers are 2 and 1. Adding 2 + 1 gives us 3, making it a third-order product. Because 3 is an odd number, this is an odd-order intermodulation product. By contrast, the other choices produce even-order products: 3F1 - F2 gives 3 + 1 = 4 (even), and 5F1 - 3F2 gives 5 + 3 = 8 (even). Odd-order products, particularly third-order ones, are especially problematic in radio communications because they often fall very close to the original frequencies and can easily slip past filters.