Causality and stability of analog systems
The transfer function of an analog system is the Laplace transform of the impulse-response function . A causal system is one for which is one sided — that is, for . Suppose that we look at the impulse response of a causal system as . If
we say that the system is stable.
In Example A, the prototype causal analog signal is the causal exponential
For our purposes, we assume that the parameter a is a real number. The Laplace transform is
This integral converges for . That is, the region of convergence is the half-plane to the right of the point a. Case A1 is for . The region of convergence includes the axis (Figure 8a), and the causal signal is stable. Case A2 is for . The region of convergence does not include the axis (Figure 8b), and the causal signal is unstable.
In Example B, the prototype anticausal analog signal is the anticausal exponential
Its Laplace transform is
which converges for . That is, the region of convergence is the half-plane to the left of point a. Case B1 is for . The region of convergence includes the axis (Figure 9a), and the anticausal signal is unstable. Case B2 is for . The region of convergence includes the axis (Figure 9b), and the anticausal signal is stable. As is the case with digital signals, we usually would choose stability at the expense of causality in the case of analog signals.
|Causality and stability of digital systems
|Frequency response of a digital system
Also in this chapter
- Digital linear time-invariant systems
- Analog linear time-invariant systems
- Digital transfer functions
- Analog transfer functions
- Causality and stability of digital systems
- Frequency response of a digital system
- Digital prediction
- Digital prediction error
- Analog prediction error