f new systems. Most of the time, they describe systems that they would like to have but that are not there yet. In [Hest87], such descriptions would be called fictitious mod- els. Since this viewpoint puts unequal emphasis on objects that already exist, this text prefers a different, more balanced definition: Definition (Model-Engineering): A model is a conceptual representation of a system that you want to realize (specification model) or that you have realized (implementation model). This definition puts equal weight on both reasons for using models in engineering prac- tice: the specification of a system's behavior involves constructing a model for the sys- tem as we would like to have it; the verification of a particular implementation involves the construction of a model for a physically realized system as it is given to us. The first kind of models occur as we go down in the design hierarchy while the second kind occurs when going up. Example (Low-noise amplifiers models): Consider the example in Fig. 2.1 where two people construct models for a low-noise amplifier (LNA). The system designer is responsible for the realization of a complete frontend architecture. He or she will construct a specification model that describes the LNA behavior as (s)he wants it. The circuit designer, on the other hand, realizes an LNA in terms of transistors, coils, ca- pacitors and resistors. The resulting netlist represents an implementation model that describes the LNA as it is or will be physically available. Of course, the goal is to realize an LNA that meets the specifications. Phrased in the terminology introduced above: it must be possible to map the implementation model realized by the circuit designer on the specification model provided by the system designer. 2.1.1 Models: what you want or what you have System designer Specification E{n(t)2}<0.01 f1 f2 x+ax 2 a<0.02 f <0.9 GHz & f2>1.1 GHz Low noise amplifier models 0.3nH 0.13nH Implementation Circuit designer Figure 2.1: A system designer constructs LNA models as (s)