3.19  Comments and Further Reading

We have explained that an impulse response $h(t)$ or $h[n]$ is capable of fully representing the behavior of a LTI system because this is the case in current engineering problems. But as discussed in [Vog12] and references therein, it is possible to define abstract examples where the LTI cannot be described by an impulse response.

The physical dimension, unit and interpretation of the impulse response are discussed in [GS06, PTP10].

The group delay can assume negative values and, more than that, the channel output signal may look like as anticipated with respect to the input signal, as if the group delay was “negative”. This issue is discussed online, e. g. [ url3grp], and also in [MC97], where an interesting example is provided.

It should be noted that the powerful Matlab function yulewalk can be used to find an IIR filter that approximates an arbitrary magnitude response.

Some useful links about analog filter design can be found at [ url3fid].

There are several topologies for analog filters. For example, at [ url3fto] one can find a discussion about a better topology than Sallen-Key.