Harmonic balance is a method used to calculate the steady-state response of non-linear differential equations, and is mostly applied to electrical circuits  . It is a frequency domain method for calculating the steady state, as opposed to the various time-domain steady state methods. The name "harmonic balance" is descriptive of the method, which starts with Kirchhoff's Current Law written in the frequency domain and a chosen number of harmonics. Effectively the method assumes the solution can be represented by a linear combination of sinusoids, then balances current and voltage sinusoids to satisfy Kirchhoff's law. The method is commonly used to simulate circuits which include nonlinear elements.
Microwave circuits were the original application for harmonic balance methods in electrical engineering. Microwave circuits were well-suited because, historically, microwave circuits consist of many linear components which can be directly represented in the frequency domain, plus a few nonlinear components. System sizes were typically small. For more general circuits, the method was considered impractical for all but these very small circuits until the mid-1990s, when Krylov subspace methods were applied to the problem. The application of preconditioned Krylov subspace methods allowed much larger systems to be solved, both in size of circuit and in numbers of harmonics. This made practical the present-day use of harmonic balance methods to analyze radio-frequency integrated circuits (RFICs).
The algorithm starts with Kirchhoff's current law written in the frequency-domain. To increase the efficiency of the procedure, the circuit may be partitioned into its linear and nonlinear parts, since the linear part is readily described and calculated using nodal analysis directly in the frequency domain.
First, an initial guess is made for the solution, then an iterative process continues:
- Voltages are used to calculate the currents of the linear part, in the frequency domain.
- Voltages are then used to calculate the currents in the nonlinear part, . Since nonlinear devices are described in the time domain, the frequency-domain voltages are transformed into the time domain, typically using inverse fast Fourier transforms. The nonlinear devices are then evaluated using the time-domain voltage waveforms to produce their time-domain currents. The currents are then transformed back into the frequency domain.
- According to Kirchhoff's circuit laws, the sum of the currents must be zero, . An iterative process, usually Newton iteration, is used to update the network voltages such that the current residual is reduced. This step requires formulation of the Jacobian .
Convergence is reached when is acceptably small, at which point all voltages and currents of the steady-state solution are known, most often represented as Fourier coefficients.
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