Extension of the concept of the third order intercept point

Mixer nonlinearity is often characterized by the third order intercept point that is the intersection of the straight lines that fit to the first and the third order intermodulation products vs. RF input power. The formula that expresses the third order intercept point as a function of the first and the third order intermodulation products, is valid when the slopes of the first and the third order intermodulation products vs. RF input power are related as 1:3. However, this condition is not always fulfilled. As a solution, we propose postulation of the formula for the case when the slope condition is not met. Advantages and disadvantages are discussed.