Understanding IP3: What It Is and What It Isn't
What IP3 Is
IP3 stands for Third-Order Intercept Point. It’s a theoretical figure of merit used to describe the linearity of RF systems — especially amplifiers, mixers, and receivers. It indicates how well a system can handle strong signals without generating undesired intermodulation products.
Third-order intermodulation products arise when two strong signals (at frequencies f1 and f2) are input into a nonlinear device. The system produces unwanted signals at 2f1 - f2 and 2f2 - f1. These fall close to the original signals and are hard to filter. Unlike second-order products (which can fall far away in frequency), third-order products tend to land inside your band of interest.
IP3 is defined by extrapolating the output power of the fundamental tone and the third-order product on a log-log scale. These two curves rise at different rates:
- The fundamental increases at +1 dB per 1 dB increase in input
- The third-order products rise at +3 dB per 1 dB increase in input
Their intersection point is the third-order intercept point, and although it cannot usually be reached in practice due to compression or damage, it gives a useful comparative measure of linearity.
IP3 is typically expressed as:
- OIP3: Output-referred IP3 (in dBm)
- IIP3: Input-referred IP3 (in dBm)
The two are related by the gain (G) of the device:
OIP3 = IIP3 + Gain
What IP3 Isn't
IP3 is not the same as dynamic range.
It is not a direct measure of weak-signal performance.
It has nothing to do with noise figure (NF), unless we're evaluating intermod-distortion-limited vs noise-limited behavior in a specific context.
A low-noise amplifier (LNA) can have excellent noise performance (e.g., NF < 1 dB) but a poor IP3, making it easily desensitized in the presence of strong nearby signals. Conversely, a high-IP3 preamp might tolerate strong signals well but introduce more noise.
When IP3 Matters
IP3 becomes critical in environments with multiple strong signals:
- HF contesting stations
- Urban environments with many broadcasters
- VHF/UHF repeater sites
- SDRs or wideband receivers handling large swaths of spectrum
In these cases, third-order products can easily mask weaker wanted signals.
When IP3 Is Less Important
In very quiet RF environments, or in applications where signals are narrow and isolated, the impact of IP3 is minimal. Here, noise figure and sensitivity dominate.
How to Measure IP3
The typical IP3 test setup involves a two-tone test:
- Generate two sinusoidal tones (e.g., 100 kHz apart)
- Inject into the device under test
- Observe the output spectrum
- Measure the power of the fundamental and the third-order products
Extrapolate to find the point where they intersect. IP3 is often measured 20 dB above the third-order product relative to the fundamental (for each dB difference in input power).
Key Takeaways
IP3 matters when you expect many strong signals in-band or nearby. Think contesting, urban HF, VHF repeaters. If you live in a quiet environment or operate below noise floor most of the time, a better NF may serve you more than a high IP3.
You optimize linearity and noise separately.
IP3 is about how clean your system stays under stress. Not how quiet it is.
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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.