Introduction to Signals and Systems: Properties of systems

• Difficulty Level : Hard
• Last Updated : 30 Sep, 2022

Signal is an electric or electromagnetic current carrying data, that can be transmitted or received.

Mathematically represented as a function of an independent variable e.g. density, depth, etc. Therefore, a signal is a physical quantity that varies with time, space, or any other independent variable by which information can be conveyed. Here independent variable is time.

Types of time signals:

1. Continuous time signals x(t)- defined at every point in time
2. Discrete time signals x[n] – defined only at a discrete set of values of time (integer).

A System is any physical set of components or a function of several devices that takes a signal in input, and produces a signal as output.

Calculating Energy and Power of signals:

Energy– Square of amplitude/magnitude(if complex) over entire time domain.

for a continuous time signal-

for a discrete time signal-

Power- Rate of change of energy.

for a continuous time signal.

for a discrete time signal-

Classes of signals on the basis of their power and energy:

1. Energy signal– generally converging signals, aperiodic signals or signals that are bounded.

2. Power signal– generally periodic signals, as they encompass infinite area under their graph and extend from to .

3. Neither energy nor power signal

Transformation of the independent variable:

1. Shifting- the signal can be delayed ( x(t-T) ) or advanced ( x(t+T) ) by incrementing or decrementing the independent variable (time here).

The shape of the graph remains same only shifted on the time axis.

2. Scaling- the signal can be compressed ( x(at), a>1 ) or expanded ( x(t/a), a>1 or x(at), 1>a>0 ).

Here the shape/behaviour of the graph of the signal changes as the fundamental time period changes. In compression the time period decreases and in expansion the time period increases.

3. Reversal- also called folding as the graph is folded about the Y-axis or T if given x(T-t).

Properties of systems:

1. Periodicity- the signal’s behavior/graph repeats after every T. Therefore,

here T is the fundamental period
So we can say signal remains unchanged when shifted by multiples of T.

2. Even and Odd- an even signal is symmetric about the Y-axis.
x(t)=x(-t) even
x(t)=-x(-t) odd
A signal can be broken into it’s even and odd parts to make certain conversions easy.

3. Linearity- constitutes of two properties-

if x1(t) -> y1(t)
and x2(t) -> y2(t)

(ii) Property of scaling-
if x1(t) -> y1(t)
then

If both are satisfied, the system is linear.

4. Time invariant- Any delay provided in the input must be reflected in the output for a time invariant system.

here x2(t) is a delayed input.
We check if putting a delayed input through the system is the same as a delay in the output signal.

5. LTI systems- A linear time invariant system. A system that is linear and time-invariant.
6. BIBO stability- The bounded input bounded output stability.
We say a system is BIBO stable if-

7. Causality- Causal signals are signals that are zero for all negative time.
If any value of the output signal depends on a future value of the input signal then the signal is non-causal.

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