rive the circuit are not neglected during operat- ing point computations. Hence, operating point voltages and currents may vary periodically with time. 3. Linearize the circuit equations in the neighborhood of the periodic operating point. For stable systems, input small-signal variations cause the circuit's volt- ages and currents to deviate slightly from their operating point values. The small- ness of these deviations justifies linearization. This yields a system of linear equations that relates the small-signal variations of the voltages and currents to the input small-signal variations. This system of equations describes an LPTV system. 4. Using the results described in section 3.4.4, the linearized circuit equations can be converted into HTMs. The procedure above is very similar to the traditional small-signal analysis of an opamp. There, one computes a DC operating point by discarding all small input signals. Lin- earizing the opamp's behavior in the neighborhood of this operating point then yields a small-signal model. The behavior of this small-signal model can be captured by scalar transfer functions. As far as LPTV systems are concerned, e.g. mixers and PLLs, the main difference with opamp small-signal modeling lies in the fact that the operat- ing point is periodically time-varying rather than constant. Hence, we need a HTM, i.e. a matrix of transfer functions, to describe their small-signal behavior rather than a scalar transfer function. In this sense, HTMs generalize LTI transfer functions to handle LPTV systems as well. Prior art LPTV system analysis dates back as far as the 19th century. In 1883, G. Floquet [Floq83] published his study on linear differential equations with periodically time- varying coefficients. Since then, his approach was further elaborated while alternative techniques have emerged. This section briefly introduces prior art on LPTV system analysis. 3.2.1 Floquet theory Originally, Floquet developed his theory to study the solutions an n-th order single- input single-output dif