RFDesign.wikiRadio-frequency design, explained

What is third-order intercept point (IP3)?

From RF Design Wiki · By the · Published · Last reviewed · How we research

The third-order intercept point is the extrapolated power level at which the third-order intermodulation products of a two-tone test would equal the fundamental output. It is quoted at the input (IIP3) or output (OIP3 = IIP3 + gain). From one measurement: IIP3 = Pin + ΔP/2, where ΔP is the difference in dB between each output fundamental and the IM3 product.[1][2]

How is a two-tone test performed?

Two equal-amplitude tones at f1 and f2 are combined with good isolation (to avoid generating IM in the sources) and applied to the device. IM3 products appear at 2f1 − f2 and 2f2 − f1. The measurement should be made in the region where IM3 rises 3 dB per dB, well below compression, and the analyzer’s own distortion must be checked by changing its input attenuation.[3]

How do you cascade IIP3?

With linear powers: 1/IIP3tot ≈ 1/IIP31 + G1/IIP32 + G1G2/IIP33 + …. Unlike noise, later stages dominate linearity because preceding gain drives them harder. This tension between noise (wants early gain) and linearity (wants little early gain) is the central trade-off of receiver line-ups.[1]

For a memoryless third-order polynomial, the input 1 dB compression point is about 9.6 dB below IIP3. Real devices often deviate by several dB, so both should be measured.[1]

What about IP2?

The second-order intercept IIP2 is defined analogously with products at f1 ± f2, slope 2 dB/dB: IIP2 = Pin + ΔP. It is critical in direct-conversion receivers where f1 − f2 falls at baseband.[1]

Frequently asked questions

Is a higher IP3 better?

Yes; higher IP3 means lower intermodulation for a given input.

Can IP3 actually be reached?

No; it is an extrapolation. Devices compress long before.

What is a typical LNA IIP3?

It varies widely; discrete LNAs commonly range from about −10 to +10 dBm IIP3.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.
  2. “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
  3. “Spectrum Analysis Basics (Application Note 150)”. Keysight Technologies. application note.
Categories: Fundamentals