1. Chapter 2 Linear Systems
Topics:
Review of Linear Systems
Linear Time-Invariant Systems
Impulse Response
Transfer Functions
Distortionless Transmission
Huseyin Bilgekul
Eeng360 Communication Systems I
Department of Electrical and Electronic Engineering
Eastern Mediterranean University

2. Linear Time-Invariant Systems
An electronic filter or system is Linear when Superposition holds, that is when,
Where y(t) is the output and x(t) = a1x1(t)+a2x2(t) is the input.
l[.] denotes the linear (differential equation) system operator acting on [.].
If the system is time invariant for any delayed input x(t – t0), the output is delayed by just the same amount y(t – t0).
That is, the shape of the response is the same no matter when the input is applied to the system.

3. Impulse Response
x(t)

4. Transfer Function
The output waveform for a time-invariant network can be obtained by convolving the input waveform with the impulse response of the system.
The impulse response can be used to characterize the response of the system in the time domain.
The spectrum of the output signal is obtained by taking the Fourier transform of both sides. Using the convolution theorem,
Where H(f) = ℑ[h(t)] is transfer function or frequency response of the network.
The impulse response and frequency response are a Fourier transform pair:
Generally, transfer function H(f) is a complex quantity and can be written in polar form.

5. Transfer Function The |H(f)| is the Amplitude (or magnitude) Response.
The Phase Response of the network is
Since h(t) is a real function of time (for real networks), it follows
|H(f)| is an even function of frequency and
θ(f) is an odd function of frequency.

6. Output for Sinusoidal and periodic Inputs

7. Power Transfer Function
It is possible to derive the relationship between the power spectral density (PSD) at the input, Px(f), and that at the output, Py(f) , of a linear time-invariant network.
Using the definition of PSD
PSD of the output is
Using transfer function
in a formal sense, we obtain
Thus, the power transfer function of the network is

8. Example 2.14 RC Low Pass Filter

9. Example 2.14 RC Low Pass Filter
3dB Point

10. Distortionless Transmission
No Distortion if y(t)=Ax(t-Td) Y(f)=AX(f)e-j2fTd
Distortionless channel implies that channel output is proportional to delayed version of input
For no distortion at the output of an LTI system, two requirements must be satisfied:
The amplitude response is flat.
|H(f)| = Constant = A ( No Amplitude Distortion)
The phase response is a linear function of frequency.
θ(f) = <H(f) = -2πfTd (No Phase Distortion)
Second requirement is often specified equivalently by using the time delay.
We define the time delay of the system as:
If Td(f) is not constant, there is phase distortion,
Because the phase response θ(f), is not a linear function of frequency.

11. Example 2.15 Distortion Caused By an RC filter
For f <0.5f0, the filter will provide almost distortionless transmission.
The error in the magnitude response is less than 0.5 dB.
The error in the phase is less than 2.18 (8%).
For f <f0,
The error in the magnitude response is less than 3 dB.
The error in the phase is less than (27%).
In engineering practice, this type of error is often considered to be tolerable.

12. Example 2.15 Distortion Caused By a filter
The magnitude response of the RC filter is not constant.
Distortion is introduced to the frequencies in the pass band below frequency fo.

13. Example 2.15 Distortion Caused By a filter
The phase response of the RC filter is not a linear function of frequency.
Distortion is introduced to the frequencies in the pass band below frequency fo.
The time delay across the RC filter is not constant for all frequencies.
Distortion is introduced to the frequencies in the pass band below frequency fo.