Understanding IP3: What It Is and What It Isn't
What IP3 Is
IP3 stands for Third-Order Intercept Point. It’s an extrapolated figure of merit used to describe the linearity of RF systems — especially amplifiers, mixers, and receivers. In practical terms, it indicates how well a system can handle strong signals without generating third-order intermodulation distortion (often written as IM3).
Third-order intermodulation products arise when two strong, closely spaced signals (at frequencies f1 and f2) are applied to a nonlinear device. The nonlinearity produces unwanted signals at 2f1 - f2 and 2f2 - f1. These fall close to the original signals and are hard to filter. Unlike many second-order products (which often land farther away and can be easier to filter, depending on the system), third-order products can land inside your band of interest.
IP3 is found by extrapolating the output power of the fundamentals and the third-order products on a dB scale (output power versus input power). In the small-signal region, these two lines rise at different rates:
- The fundamental increases at +1 dB per 1 dB increase in input
- The third-order products increase at +3 dB per 1 dB increase in input
The point where those extrapolated lines intersect is the third-order intercept point. You usually cannot reach this point in practice (real devices compress and may be damaged first), but it provides a useful way to compare linearity between devices.
IP3 is typically expressed as:
- OIP3: Output-referred IP3 (in dBm)
- IIP3: Input-referred IP3 (in dBm)
The two are related by the (linear) gain of the device:
OIP3 = IIP3 + Gain
What IP3 Isn't
IP3 is not the same thing as dynamic range, and it is not a direct measure of weak-signal sensitivity.
It also has nothing to do with noise figure (NF) by itself — although IP3 and NF together determine metrics like spurious-free dynamic range (SFDR) for a given bandwidth.
A low-noise amplifier (LNA) can have excellent noise performance (e.g., NF < 1 dB) but a poor IP3, making it prone to desensitization in the presence of strong nearby signals. Conversely, a high-IP3 preamp might tolerate strong signals well but add more noise than an LNA.
When IP3 Matters
IP3 becomes critical in environments with multiple strong signals in-band or nearby, for example:
- 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 rise quickly and mask weaker wanted signals.
When IP3 Is Less Important
In very quiet RF environments — or in applications where signals are narrow, isolated, and well-filtered — the impact of intermodulation products is usually minimal. In those scenarios, noise figure and overall system sensitivity tend to dominate.
How to Measure IP3
The typical IP3 test setup uses a two-tone test:
- Generate two equal-amplitude sinusoidal tones (e.g., 100 kHz apart)
- Inject them into the device under test
- Observe the output spectrum
- Measure the power of the fundamental tones and the third-order products
To calculate IP3 at a given test level (while staying below compression), measure the output power of one fundamental tone (PFUND) and its corresponding third-order product (PIM3). Let the difference be Δ = PFUND − PIM3 (in dB). Then:
- OIP3 ≈ PFUND + (Δ / 2)
- IIP3 ≈ PIN + (Δ / 2) (where PIN is the input power per tone)
This works because the IM3 products grow 2 dB faster than the fundamentals for each 1 dB increase in input (3 dB vs 1 dB), so the intercept is half the measured separation above the fundamentals. Note that measured IP3 can vary with frequency, tone spacing, bias, and the impedance environment, so it’s best practice to document the test conditions.
Key Takeaways
IP3 matters when you expect many strong signals in-band or nearby (contesting, urban HF, VHF repeater sites, wideband SDR use). If you operate in a quiet environment or you’re usually below the noise floor, a better NF may help more than chasing very 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.