Definition
The third-order intercept point (IP3) is an extrapolated figure-of-merit for RF device linearity. When two tones at f₁ and f₂ are applied, third-order intermodulation products appear at 2f₁−f₂ and 2f₂−f₁. These fall close to the wanted signals and cannot be filtered out.
IM3 power slope: +3 dB per 1 dB increase in input power Fundamental slope: +1 dB per 1 dB input power IIP3 (dBm) = Pin + ½·(P_fund − P_IM3) [input referred] OIP3 = IIP3 + G₀ [output referred] OIP3 ≈ OP1dB + 9.6 dB (Class A rule of thumb)
Cascaded IIP3 (Friis for Linearity)
1/IIP3_total = 1/IIP3₁ + G₁/IIP3₂ + G₁·G₂/IIP3₃ + … [linear power] Key rule: The last high-gain stage dominates cascaded IIP3. Unlike noise figure (first stage dominates), linearity is worst at the output of high-gain stages where signal levels are highest.
Typical IIP3 Values
| Device | IIP3 (Input) | OIP3 (Output) |
|---|---|---|
| LNA (SiGe 1–4 GHz) | −5 to +5 dBm | +10 to +25 dBm |
| GaAs driver amp | +25 dBm | +35 dBm |
| Active mixer | +10 dBm | 0 dBm (after CL) |
| CMOS PA (mobile, 28nm) | +30 dBm | +40 dBm |
Two-Tone IIP3 Test Procedure
- Apply two equal-power tones at f₁ and f₂ (Δf = 1–10 MHz apart)
- Measure fundamental output power P_fund and IM3 product P_IM3
- IIP3 = Pin + (P_fund − P_IM3)/2
- Verify at multiple power levels — the gap P_fund−P_IM3 should increase by 2 dB per 1 dB reduction in input power
RF View: RF View analyzes linear S-parameter data. For comprehensive amplifier characterization, combine RF View's S-parameter analysis (gain, match, stability) with bench P1dB and IIP3 measurements.