Efficient per-element distortion contribution analysis via Harmonic Balance adjoints

We propose a new metric for quantifying per-element distortion that is simple, intuitive and well-defined for both small- and large-signal excitations. Traditional distortion concepts, based on polynomial expansions and Volterra series, can be viewed as an approximation of our new metric. Although computing this metric exactly is quadratic in circuit size, we devise a novel approximation that (unlike polynomial/Volterra) also makes sense for large distortions and can be computed efficiently using adjoint Harmonic Balance. We validate our new approximate metric on a differential pair and the 741 op-amp, comparing it against the full metric and demonstrating order-of-magnitude speedups. Unlike polynomial/Volterra based methods, our proposed per-element distortion computation technique is easy to implement in any modern simulator that features Harmonic Balance or similar steady-state/RF analyses.