Spurious-Free Dynamic Range (SFDR)

The primary regulatory metric for spectral regrowth, measuring the ratio of power in the main channel to power leaking into adjacent channels. While SFDR focuses on the worst-case spurious component anywhere in the spectrum, ACLR specifically quantifies integrated power spillage into neighboring frequency allocations. A system with excellent ACLR may still have poor SFDR if a single dominant spur exists far from the carrier.

Nonlinear mixing of two or more signals generating products at sum and difference frequencies. Third-order intermodulation products (IMD3) at frequencies 2f₁-f₂ and 2f₂-f₁ fall close to the original carriers and are the dominant SFDR limiters in multi-tone environments. IP3 (Third-Order Intercept Point) is the theoretical extrapolation used to predict IMD levels and thus SFDR at arbitrary power levels.

The two fundamental nonlinear transfer characteristics of a power amplifier. AM-AM distortion is gain compression where output amplitude deviates from linear proportionality with input. AM-PM distortion is phase shift variation with instantaneous envelope amplitude. Both mechanisms convert amplitude variations in modulated signals into spectral regrowth. SFDR captures the aggregate effect of these distortions as discrete spurious components in the frequency domain.

A phenomenon where a power amplifier's current output depends on past input states due to thermal inertia, bias circuit impedance, and semiconductor trapping. Memory effects cause frequency-dependent nonlinear behavior that cannot be corrected by memoryless predistortion alone. In SFDR measurements, memory effects manifest as asymmetry in spectral regrowth sidebands and complicate the cancellation of spurious components across wide modulation bandwidths.

A comprehensive modulation quality metric measuring the vector difference between ideal constellation points and actual transmitted symbols. While SFDR quantifies out-of-band spectral purity, EVM captures in-band signal fidelity degradation. Nonlinear distortion simultaneously degrades both: spectral regrowth increases out-of-band spurs (worsening SFDR) while constellation distortion increases EVM. The two metrics together provide a complete picture of transmitter linearity.

Regulatory-defined limits on unwanted transmitter emissions at frequencies beyond the adjacent channels, protecting distant spectrum users. SEM masks specify absolute power limits (dBm) rather than relative ratios, distinguishing them from ACLR. SFDR provides the dynamic range headroom needed to ensure that when the fundamental signal is at maximum power, all spurs—including those in the SEM frequency ranges—remain below regulatory thresholds.