Publisher Summary This chapter discusses the modeling aspect of differential-algebraic equations (DAEs). In computational engineering, the network modeling approach forms the basis for computer-aided analysis of time-dependent processes in multibody dynamics, process simulation, or circuit design. Its principle is to connect compact elements via ideal nodes, and to apply some kind of conservation rules for setting up equations. The mathematical model, a set of so-called network equations, is generated automatically by combining network topology with characteristic equations describing the physical behavior of network elements under some simplifying assumptions. Interconnects and semiconductor devices (i.e., transistors) are modeled by multi-terminal elements (multi-ports), for which the branch currents entering any terminal and the branch voltages across any pair of terminals are well-defined quantities. Interconnects and semiconductor devices (i.e., transistors) are modeled by multiterminal elements (multi-ports), for which the branch currents entering any terminal and the branch voltages across any pair of terminals are well-defined quantities.

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Modelling and discretization of circuit problems

Homotopy/continuation methods are attractive for finding dc operating points of circuits because they offer theoretical guarantees of global convergence. Existing homotopy approaches for circuits are, however, often ineffective for large mixed-signal applications. In this paper, we describe a robust homotopy technique that is effective for solving large metal-oxide-semiconductor (MOS)-based mixed-signal circuits. We demonstrate how certain common circuit structures involving turning-point nesting can lead to extreme inefficiency, or failure, of conventional probability-one homotopy methods. We also find that such situations can lead to numerical ill-conditioning and homotopy paths that fold back upon themselves, leading to algorithm failure. Our new homotopy model for MOS devices, dubbed Arc-tangent Schichman-Hodges (ATANSH), features decoupled continuation parameters that are instrumental in avoiding these problems. ATANSH-based homotopy methods in production use have led to the routine solution of large previously hard-to-solve industrial circuits, several examples of which are presented.

Delivering global DC convergence for large mixed-signal circuits via homotopy/continuation methods

In this paper, we present a parallel harmonic-balance approach, applicable to the steady-state and envelope-following analyses of both driven and autonomous circuits. Our approach is centered on a naturally parallelizable preconditioning technique that speeds up the core computation in harmonic-balance-based analysis. As a coarse-grained parallel approach by algorithm construction, the proposed method facilitates parallel computing via the use of domain knowledge and simplifies parallel programming compared with fine-grained strategies. The proposed parallel preconditioning technique can be combined with more conventional parallel approaches such as parallel device model evaluation, parallel fast Fourier transform operation, and parallel matrix-vector product to further improve runtime efficiency. In our message-passing-interface-based implementation over a cluster of workstations and multithreading-based implementation on a shared-memory machine, favorable runtime speedups with respect to the conventional serial approaches and the serial implementations of the same parallel algorithms are achieved.

A Parallel Harmonic-Balance Approach to Steady-State and Envelope-Following Simulation of Driven and Autonomous Circuits

As a widely adopted frequency-domain method, harmonic balance (HB) provides efficient steady-state circuit analysis for analog and RF circuits. The conventional matrix-implicit Krylov subspace technique with the block-diagonal (BD) preconditioner has made it possible to compute the steady-state responses of large-scale circuits. However, not all HB problems, particularly strongly nonlinear circuit problems, can be solved reliably or efficiently using the standard BD-preconditioning technique. In this paper, hierarchical HB methods are proposed wherein robust preconditioning is provided via solution of a set of approximate linearized HB problems of progressively smaller size across multiple levels of the problem hierarchy. These subproblems are constructed using the same matrix-implicit formulation to retain the memory efficiency of Krylov subspace methods. Moreover, the number of allocated Krylov subspace matrix solvers, hence the memory usage, is significantly reduced via a recently introduced solver-sharing technique. The efficiency of our hierarchical preconditioning technique is further improved by adopting a one-step correction to the standard BD preconditioner and a multigrid-motivated iterative scheme. It has been shown that the proposed approaches can achieve up to 10 runtime speedup over the popular BD preconditioner and robust convergence even for strongly nonlinear circuits for which the BD preconditioner fails to converge.

Hierarchical Harmonic-Balance Methods for Frequency-Domain Analog-Circuit Analysis

Shooting, finite difference, or harmonic balance techniques in conjunction with a damped Newton method are widely employed for the numerical calculation of limit cycles of (free-running, autonomous) oscillators. In some cases, however, nonconvergence occurs when the initial estimate of the solution is not close enough to the exact one. Generally, the higher the quality factor of the oscillator the tighter are the constraints for the initial estimate. A 2-D homotopy method is presented in this paper that overcomes this problem. The resulting linear set of equations is underdetermined, leading to a nullspace of rank two. This underdetermined system is solved in a least squares sense for which a rigorous mathematical basis can be derived. An efficient algorithm for solving the least squares problem is derived where sparse matrix techniques can be used. As continuation methods are only employed for obtaining a sufficient initial guess of the limit cycle, a coarse grid discretization is sufficient to make the method runtime efficient.

Homotopy Method for Finding the Steady States of Oscillators

We present a new technique, based on a continuation method, for oscillator analysis using harmonic balance. With the use of Krylov subspace iterative linear solvers, harmonic balance has become a very powerful method for the analysis of general nonlinear circuits in the frequency domain. However, application of the harmonic balance method to the oscillator problem has been difficult due to the very small region of convergence. The main contribution of the paper is a robust and efficient continuation method that overcomes this problem.

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Simulation of high-Q oscillators

A robust and efficient oscillator analysis technique using harmonic balance

A new adaptive approach to solving large-dimension harmonic balance (HB) problems in RF circuit simulation is presented. The method is based on adjusting the order of the equation system according to the degree of nonlinearity of each node in the circuit. A block-diagonal preconditioner is used to construct an algorithm for order reduction during the iterative HB process.

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M. Zharov

Papers

Simulation of high-Q oscillators

A robust and efficient oscillator analysis technique using harmonic balance

New methods for speeding up computation of Newton updates in harmonic balance