One of the many contributions of Prof. Fettweis was the invention of wave digital filters. These filters are obtained from classical RLC filters, in particular doubly terminated lossless two-ports, by using some transformations. Namely, the voltages and currents in the circuit elements are transformed into wave-variables and then a bilinear transformation is performed. If the wave transformation is performed appropriately it results in a realizable digital filter structure which furthermore enjoys a number of robustness properties such as low passband sensitivity, low roundoff noise, and freedom from limit cycle oscillations. Prof. Fettweis and his colleagues also showed that these properties are due to the inheritence of passivity properties from the continuous-time domain into the digital filter domain. Subsequent to this landmark work, a number of researchers worked on the problem of obtaining robust digital filter structures without starting from continuoustime circuits. One of these is the structurally bounded or structurally passive class of digital filters. These structures are based on inducing structural passivity directly into the implementation and are therefore simpler, both conceptually and from a practical viewpoint. They are also more general and lead to new structures which have no natural connection to electrical circuits. This paper gives an overview of some of these developments.

中文翻译：

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稳健的数字滤波器结构：直接方法

Fettweis 教授的众多贡献之一是波数字滤波器的发明。这些滤波器是通过使用一些转换从经典 RLC 滤波器获得的，特别是双端接无损二端口。即，将电路元件中的电压和电流变换为波变量，然后进行双线性变换。如果适当地执行波变换，则会产生可实现的数字滤波器结构，该结构还具有许多鲁棒性特性，例如低通带灵敏度、低舍入噪声和无极限循环振荡。Fettweis 教授和他的同事还表明，这些特性是由于从连续时间域到数字滤波器域的无源特性的继承。在这项具有里程碑意义的工作之后，许多研究人员致力于在不从连续时间电路开始的情况下获得稳健的数字滤波器结构的问题。其中之一是结构上有界或结构上无源的数字滤波器类。这些结构基于将结构被动性直接引入到实现中，因此在概念上和从实践的角度来看都更简单。它们也更通用，并导致与电路没有自然联系的新结构。本文概述了其中一些发展。这些结构基于将结构被动直接引入到实现中，因此在概念上和从实践的角度来看都更简单。它们也更通用，并导致与电路没有自然联系的新结构。本文概述了其中一些发展。这些结构基于将结构被动性直接引入到实现中，因此在概念上和从实践的角度来看都更简单。它们也更通用，并导致与电路没有自然联系的新结构。本文概述了其中一些发展。