Third-Order Intercept Point

Formula

Description

The third-order intercept point (IP3) is a figure of merit for the linearity of an amplifier or mixer. It is the theoretical power level where the fundamental output and third-order intermodulation products would be equal (this point is never actually reached because the device compresses first). Higher IP3 means better linearity and less intermodulation distortion. OIP3 = IIP3 + Gain. The spurious-free dynamic range (SFDR) of a receiver is bounded by IP3 at the top and noise floor at the bottom.

Variables

• OIP3 — Output third-order intercept point (dBm)
• Pout — Fundamental output power per tone (dBm)
• IIP3 — Input third-order intercept point (dBm)
• Pin — Input power per tone (dBm)

Practical Notes

Third-order IMD products (at 2f1-f2 and 2f2-f1) are problematic because they fall close to the desired signals and cannot be filtered. IMD3 level = 3×Pin - 2×IIP3 for small signals. For cascaded stages, the system IIP3 degrades with each stage (noise and linearity trade off against each other). High-linearity LNAs and mixers are essential for receivers in crowded RF environments.