How do I determine the required IP3 for each stage in a receiver to meet a system linearity spec?

Determining the required IP3 for each stage in a receiver chain requires working backward from the system SFDR or linearity specification using the cascade IP3 formula: (1) System specification: start with the required SFDR (or IM3 specification for a given input level). From SFDR: IIP3_system = N_floor + (3/2) × SFDR. Example: SFDR = 70 dB and N_floor = -100 dBm: IIP3_system = -100 + 105 = +5 dBm. (2) Cascade IP3 formula (in linear power, not dB): 1/IIP3_total = 1/IIP3_1 + G1/IIP3_2 + G1×G2/IIP3_3 + ... Where G1, G2 = stage gains (linear, not dB), and IIP3_n = stage IIP3 (in mW, not dBm). Each succeeding term has all previous gains multiplied in. (3) Allocation strategy: the stage with the most gain before it contributes the most to the cascade IP3 degradation. Typically: the last IF amplifier (before the ADC) sees the highest signal levels and dominates the cascade IIP3. So: assign the most demanding IIP3 to the last active stage. Assign moderate IIP3 to intermediate stages. The first stage (LNA) can have relatively low IIP3 (because its gain has not yet amplified the signal, but its NF is critical). (4) Example: 3-stage receiver: LNA (gain = 20 dB, IIP3 = 0 dBm) → filter (loss = 3 dB) → mixer (gain = -6 dB, IIP3 = +15 dBm). Cascade: 1/IIP3_total = 1/1mW + 100×0.5/(31.6mW) = 1 + 1.58 = 2.58. IIP3_total = 1/2.58 mW = 0.387 mW = -4.1 dBm. The cascade IIP3 (-4.1 dBm) is worse than any individual stage. The mixer term dominates because the LNA gain (20 dB = 100×) multiplies its contribution.