What Is Intermodulation Distortion?
When two or more signals (f₁, f₂) pass through a nonlinear RF device, they mix to create new spurious products at frequencies not present in the input. These intermodulation (IM) products can fall within the receiver's passband, causing interference and reducing system sensitivity.
IM Product Frequencies
Two tones at f₁ and f₂ (close together, Δf = f₂ − f₁): 2nd order: f₁ ± f₂, 2f₁, 2f₂ (far from carriers) 3rd order: 2f₁ − f₂, 2f₂ − f₁ (close to carriers — CRITICAL!) 5th order: 3f₁ − 2f₂, 3f₂ − 2f₁ (also in-band) Example: f₁=900 MHz, f₂=901 MHz, Δf=1 MHz IM3 at: 2×900−901=899 MHz, 2×901−900=902 MHz Both within 1 MHz of carriers → inside receiver passband!
IM3 Level and IIP3
P_IM3 = 3·P_in − 2·IIP3 [all in dBm, linear extrapolation] P_IM3 slope: +3 dB per +1 dB in P_in P_fund slope: +1 dB per +1 dB in P_in IIP3 = P_in + (P_fund − P_IM3)/2 [input IP3, measured] OIP3 = IIP3 + G₀ [output IP3] Typical values: LNA (SiGe, 1–4 GHz): IIP3 = −5 to +5 dBm Mixer (passive Gilbert): IIP3 = +10 to +20 dBm PA (GaN 2.4 GHz): OIP3 = +40 to +50 dBm
Note on S-Parameters and IMD
IMD cannot be measured or predicted from linear S-parameters. S-parameters assume linear (small-signal) operation. IMD characterization requires two-tone testing with a signal generator and spectrum analyzer. S-parameters from a VNA represent the linear device model; IMD represents the nonlinear effects above P1dB.
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