This paper proposes an empirical equation for input carrier power level dependence of 3rd order reflected passive intermodulation (PIM) product. Three kinds of microwave passive devices, namely, directional coupler, N type coaxial adapter and microstrip line, are measured by the PIM test setup in the 900MHz band. To obtain systematical experimental results, the power levels of the two carriers are tuned in the following three ways: 1. Keep the two carriers with equal amplitude and simultaneously increase both of them; 2. Keep one of the carriers constant and increase the other carrier; 3. Keep the sum of the two carrier power constant and change their ratio.
Enlightened by the classic polynomial theory of intermodulation, an empirical equation based on polynomial is proposed to describe the relationship between input carrier power and PIM product. It is found that the empirical equation describes the measurement data very well with the fitting parameters in the equation vary for different devices. The empirical equation can be used as a simple tool of PIM evaluation when engineers tackle problems on site.