Tài liệu Lecture 13: Communication Basics: pdf

Amplitude, frequency and phase are the attributes of a signal that changes with time.. But to send that signal far we need to convert that signal to the electrical form... Transducers th

Communication Basics:

Signals:

Communication happens in the form of signals

Signals are transmission of energy (mechanical, electrical or light) through appropriate media

A signal that is constant and changed once conveys single information

The more changes in the signal, the more information that the signal can convey

For example, a push can mean one thing and removing it can mean another thing

But how do we send more information than just these two?

We can do that by varying the amplitude, i.e., a strong push or a light push could have two

different meaning

Or, a rate of change of push, i.e 10 pushes per minute or 1 push per minute could have different

meaning too

Or, even phase, i.e., pushing at a different angle could have different meaning too

Amplitude, frequency and phase are the attributes of a signal that changes with time

This pushing, transfer of mechanical energy has a limit How many different amplitude, rate and phase of pushing can we transmit and distinguish (receive)? God gave us organs like vocal cord which can create higher rate of pushing and ears to sense that

Higher the rate of pushing the more information it can convey and even the mixing of these different rate could mean different information And our vocal cord is able to do that as well

We have even media (air) which will carry that signal far, so we don't have to stay close to communicate But that has a limit too

Then we came out with a solution of converting this pressure wave to electrical signals and then transmitting over long distances

Signals can exists in different forms and transmit over different media For example,

acoustic (pressure waves), electrical, electromagnatic and light signals

Transducers:

Different signals and media have their own limitations So according to need we change signals

to different forms

The devices that convert signals from one form to another are called transducers

In general, human do not generate electrical signals, nor do they respond to that or any other form of signals except acoustic signals

Human generate acoustic signals (speech) for communication But to send that signal far we need to convert that signal to the electrical form

Transducers that convert acoustic signals into electrical signals are called microphones

Transducers that convert electric signals into acoustic (sound) signals are called speakers

Microphones:

Carbon microphone - used in telephones

Moving coil microphones - public address systems

Moving magnet microphones - public address systems

Piezoeletric microphones -

Capacitor microphones - Newer telephones, home audio systems

Speakers:

Moving coil speakers - public address systems

Piezoeletric speakers - Newer telephones, toys

Transducers converting electrical signals to light lignals:

Infrared diode - generates infrared light signals from electrical light signals, used in infrered

transmitters (remote control uinit)

Photo diode - generates electrical signals from infrared light signals, used in infrered

receivers (controled uinit, e.g., TV)

We also use these two in a pair to isolate two electrical circuits to reduce noise, and they are called couplers

Similar devices are laser diodes and detectors

The Electromagnetic signals:

Information (voice, data, image, video) can be represented by electrical or electromagnetic signals and transmitted over a suitable transmission medium

Information can be transmitted on wires by varying some physical properties such as voltage or current

Transmitter/

Receiver

Transmitter/

Receiver

Information

Transmission medium

An electromagnetic signal is a function of time, but it can also be expressed as a function of frequency

The frequency-domain view of a signal is far more important to an understanding of data

transmission than a time-domain view

The characteristics and quality of a data transmission are determined by both the characteristics

of the medium and the characteristics of the signal

Time-domain concept:

Viewed as a function of time, an electromagnetic signal can be either analog or digital

An analog signal is one in which the signal intensity varies in a smooth fashion over time There

are no breaks or discontinuities in the signal

A digital signal is one in which the signal intensity maintains a constant level for some period of

Information

Electromagnetic signal

time and then changes to another constant (discrete) level

Analog

Time, t

Signal

Amplitude,

A

Digita

Time, t

l

Signal

Amplitude,

A

These signals can be of two type:

1 periodic, in which the signal pattern repeats over time e.g., Sine wave and square wave

2 non-periodic, in which the signal pattern does not repeat over time

The sine wave is the fundamental analogue signal and it has three parameters:

1 Amplitude (A), strength of the signal over time, volts, current

2 Frequency (f), is the rate at which the signal repeats Cycles per second, or Hertz

An equivalent parameter is period (T), is the amount of time for one repetition, ( T = 1/f )

3 Phase (φ), is a measure of the relative position in time within a single period of a signal

The general sine wave can be written as:

S(t) = A sin(2πft + φ)

Amplitude: Frequency:

Phase:

Space-domain concept:

Consider the wavelength of an electromagnetic sine wave:

The wavelength (λ) of a signal is the distance occupied by a single cycle in space

Or, the distance between two points of corresponding phase of two consecutive cycles

The electromagnetic wave travels at the speed of light, v = 3 x 108 m/sec

So, the wavelength is related to time as follows:

Wavelength = Velocity of the wave x Time period

λ = vT

λ = v/f when f = 1 kHz, λ = 300 km λ = v/f = (3 x 108

m/sec) /(1000 cycles/sec) when f = 10 kHz, λ = 30 km = 3 x 105 m/cycles

when f = 1 MHz, λ = 300 m = 300,000 m

when f = 100 MHz, λ = 3 m = 300 km

when f = 1 GHz, λ = 0.3 m

Note: An antenna with a length of the signal wavelength will radiate electromagnetic wave

efficiently, or if it is submultiple of the wave length, such 0.5λ, 0.25λ

Frequency-domain concept:

A sine wave with a frequency of 1 kHz will look like this in the frequency domain:

Signal

Amplitude,

A

Frequency, f

1 kHz

A square wave of 1 kHz is composed of a 1 kHz sine wave and its odd multiples (Harmonics) with diminishing amplitudes:

A Sin(2π(f1)t) + A/3 sin(2π(3f1)t) + A/5 sin(2π(5f1)t) + A/7 sin(2π(7f1)t)

Reed organ is also called Harmonica, as the reeds are tuned to vibrate at harmonics of the

fundamental signal

Fourier Theorem:

In the early 19th century, the French mathematician Jean-Baptiste Fourier states that:

Any periodic waveform can be expressed as the sum of sine waves with frequencies at integer or harmonic multiples of the fundamental frequency of the waveform and with appropriate

maximum amplitudes and phases

Voice signal:

Voice signal is an analog signal with frequency components in the range of 20Hz to 20kHz:

Non-periodic

Signal

Amplitude,

A

Time, t Voice signal

Signal

Amplitude,

A

Frequency, f

1 kHz

Spectrum, of a signal is the range of frequencies that it contains The intelligent voice signal

spectrum extends from 300 Hz to 3300 Hz

Bandwidth, of a signal is the width of the spectrum The bandwidth of the intelligent voice

signal is 3000 Hz

Telephone equipment allows the voice a bandwidth of 4000Hz, which includes a guard band at top (700Hz) and bottom (300Hz) to prevent interference This limit on bandwidth is imposed by the telephone and the switching equipment used in the telephone network

Video signal:

Video signal is an analog signal with frequency components in the range of 60Hz to 4MHz The bandwidth is almost 4MHz With guard bands the standard bandwidth for colour video signaling is 6MHz

Telephone speech 3kHz

Hi-fi stereo 20kHz

AM radio station 10kHz

FM radio station 200kHz

Filters:

Filters are electronic circuits, deliberately designed to have non flat frequency response

A filter that passes only low frequencies is called a low-pass filter

Frequency, f

Signal

Amplitude,

A

1 kHz

0 Hz

One that passes only high frequencies is called a high-pass filter

Signal

Amplitude,

A

Frequency, f

5 kHz

0 Hz

A filter that passes only frequencies in the middle of a range is called a bandpass filter

Signal

Amplitude,

A

Frequency, f

0 Hz

The inverse of a bandpass filter that passes low and high frequencies and stops in the middle is

called a bandstop filter

Signal

Amplitude,

A

Frequency, f

0 Hz

The copper cables has resistance, inductance and capacitance Thus these cables normally work like lowpass filters

What will happen to a voice signal if passed through a bandpass filter whose cutoff frequencies are 0.9kHz and 1.1kHz?