How do I calculate intermodulation product levels from two tone measurements?

Intermodulation product levels are calculated from a two-tone test, which is the standard method for characterizing the linearity of RF components: (1) Two-tone test setup: apply two equal-amplitude signals (tones) at frequencies f1 and f2 (closely spaced, e.g., f1 = 1000 MHz, f2 = 1001 MHz). The device nonlinearity generates intermodulation products at: 2×f1 - f2 = 999 MHz (IM3 low). 2×f2 - f1 = 1002 MHz (IM3 high). 3×f1 - 2×f2, 3×f2 - 2×f1 (IM5 products). And many others (IM2 at f1+f2 and f2-f1, higher orders). (2) Calculating IM3 levels from IP3: if you know the IIP3 of the device and the input power per tone (P_in): IM3 level (dBm) = 3 × P_in - 2 × IIP3. Example: IIP3 = +10 dBm, P_in = -20 dBm per tone. IM3 = 3 × (-20) - 2 × (+10) = -60 - 20 = -80 dBm. The IM3 products are at -80 dBm. The fundamental tones are at P_out = P_in + Gain. If gain = 15 dB: P_out = -20 + 15 = -5 dBm. IM3 relative to fundamental: -80 - (-5) = -75 dBc. (3) Conversely (calculating IP3 from measured IM3): measure the fundamental output power (P_f) and the IM3 output power (P_IM3). OIP3 = P_f + (P_f - P_IM3)/2 = P_f + delta/2. Where delta = P_f - P_IM3 (in dB). Example: P_f = +10 dBm, P_IM3 = -50 dBm. Delta = 60 dB. OIP3 = 10 + 60/2 = +40 dBm. IIP3 = OIP3 - Gain. (4) IM3 vs input power: IM3 rises 3 dB for every 1 dB increase in input power (as long as the device is in the small-signal regime). The delta between the fundamental and IM3 decreases by 2 dB for every 1 dB increase. At the IP3 point: delta = 0 (IM3 = fundamental, but this is extrapolated; the device compresses before reaching this point).