Also known as: IP3, IIP3, OIP3, TOI, third-order intercept point
Third-order intercept (IP3, or TOI) is a theoretical power level — found by extrapolation, never actually reached — at which a device’s third-order intermodulation products would rise to equal the wanted signal.1 It is the standard single-number grade of an amplifier’s or mixer’s linearity: the higher the IP3, the more strong-signal punishment the device tolerates before it manufactures troublesome spurs. IP3 directly sets a receiver’s spurious-free dynamic range and is a key term in every strong-signal-handling specification.
How it works
A weak non-linearity in an amplifier or mixer can be written as a power series; the cubic term is responsible for third-order products. Feed two equal tones f₁ and f₂ and the cubic term produces spurs at 2f₁ − f₂ and 2f₂ − f₁, close enough to the wanted signals that no filter removes them. Their growth rate is the whole point:
- The fundamental output rises 1 dB per dB of input (slope 1).
- The third-order product rises 3 dB per dB of input (slope 3).
Two lines of different slope must eventually cross. Extend the measured slope-1 and slope-3 lines and their intersection is the third-order intercept point. Referred to the input it is IIP3; referred to the output, OIP3; they differ by the device gain (OIP3 = IIP3 + G). The device never actually operates there — it compresses and saturates long before — but the extrapolated intercept is a clean, gain-independent linearity figure that lets you predict the spur level at any real operating power:
IMD3 level below each tone (dBc) = 2·(IIP3 − P_in)
Back off the input 10 dB and the third-order spurs drop 30 dB — a 3:1 payoff that is the engineer’s main lever against intermod.
Variants
- IIP3 vs OIP3 — same intercept, referenced to input or output. Receivers usually quote IIP3 (how strong an input they tolerate); power amplifiers often quote OIP3.
- IP3 vs the 1 dB compression point — both measure linearity, but IP3 comes from a two-tone intermod test while P1dB comes from a single-tone gain test. As a rough rule of thumb IP3 sits about 10–15 dB above P1dB for many amplifiers, though it is always better to measure than assume.
Relevance to SDR
IP3 is the first thing to check when picking an LNA or evaluating an SDR for a busy RF environment. A low-noise amplifier with a great noise figure but poor IP3 will improve weak-signal reception in a quiet band yet make things worse near strong transmitters, because it generates intermod that the bare receiver would not. The best front-end choice balances low noise figure (bottom of the dynamic range) against high IP3 (top of it). This is why serious scanner and SDR users add high-IP3 amplifiers and, crucially, front-end filtering to strip strong out-of-band signals before they can drive the non-linearity.
GopherTrunk is a decoder and cannot see or undo intermodulation once it is in the samples — a 2f₁ − f₂ spur looks like a real carrier to any DSP. When decode quality collapses only in the presence of strong nearby signals, IP3 (and the SFDR it sets) is the limitation, addressed upstream with better front-end linearity, filtering, or reduced gain, not in software.
Sources
-
Third-order intercept point — Wikipedia, definition of IP3/IIP3/OIP3 and the slope-1/slope-3 extrapolation. ↩