Which of the following is an odd-order intermodulation product of frequencies F1 and F2?
Why The order of an intermodulation product is the sum of the absolute values of the coefficients on the two frequencies. For 2F1 - F2 that is 2 + 1 = 3, an odd (third-order) product. Third-order products are the ones that matter most in practice because they land very close to the original signals and fall inside the receiver or transmitter passband.
Watch out The choice with 3F1 - F2 adds up to 3 + 1 = 4 and 5F1 - 3F2 adds up to 5 + 3 = 8, so both are even-order products even though the individual coefficients look odd. Even-order products usually fall far from the originals and are easier to filter out.
Add the coefficients, not look at them: 2+1=3 is odd. 2F1-F2 is the classic third-order troublemaker.