Analog signal

Shahad hussian && Ahmed shamel

supervised by:Dr.En.Muyad sadik

Outline

Communication Systems Multiplexing Analog communication Types of Analog Modulation Noise

Communication Systems Communication system aims to transmit information

signals (baseband signals) through a communication channel.

The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer

For example, the voice signal from a microphone is a baseband signal ,and contains frequencies in the range of 0-3000 Hz.

Baseband Communications• Signal strength attenuates with distance. Needs repeaters to amplify the signals in stages • Received signal is corrupted by noise – R(t)=A S(t)+ n(t) • Received signal quality depends on channel noise and noise between repeaters.• To transmit a signal with bandwidth B, we need >= B Hz in channel bandwidth • If the signal is low-pass (0-B), must the channel operate at 0-B range of frequency? • How do we send multiple signals over the channel?

Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation.

Why do we need “modulation” A communication channel only operates at a certain frequency

range. telephone cables, terrestrial (over the air broadcast), Ethernet, optical

fiber, etc. Modulation translates a signal from its baseband to the operating

range of the channel In other words we can say that modulation is used because the

some data signals are not always suitable for direct transmission, but the modulated signal may be more suitable.

By modulating different signals to different frequency bands, they can be transmitted simultaneously over the same channel.

frequency division multiplexing

Modulation = Frequency Shifting

Multiplexing When required to transmit several signals on

the same channel, Baseband transmission fails.

To encounter this problem Multiplexing is employed.

Multiplexing is a way of sending multiple signals or streams of information over a communications link at the same time in the form of a single; the receiver recovers the separate signals, in a process called demultiplexing.

Tow types of Multiplexing:

Frequency Deviation Multiplexing (FDM) : is the method of transmitting several channels simultaneously

Time Deviation Multiplexing (TDM) : is the method of allotting a time slot for each signal and it gets repeated cyclically.

Analog communication Information is encoded in a continuous amplitude, continuous time signal.

Analog systems are less tolerant to noise.

make good use of bandwidth.

easy to manipulate mathematically.

However, analog signals require hardware receivers and transmitters that are designed to perfectly fit the particular transmission. If you are working on a new system, and you decide to change your analog signal, you need to completely change your transmitters and receivers.

Types of Analog Modulation Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation (PM)

The choice of the type of modulation is based on several factors, such as :

the amount of bandwidth allocated.

the types of noise and interference that the signal encounters in transmission over the channel.

Amplitude Modulation (AM)

In AM, the information signal varies with the amplitude of the carrier sine wave.

The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal.

The carrier frequency remains constant during the modulation process, but its amplitude varies in accordance with the modulating signal.

An increase in the amplitude of the modulating signal causes the amplitude of the carrier to increase.

Normal (Am) modulation

Single-Sideband (SSB) Modulations

Vestigial-Sideband (VSB) Modulations

Frequency Modulation (FM) Frequency modulation is a form of analog

angle modulation in which the baseband information carrying signal, typically called the message or information signal m(t), varies the frequency of a carrier wave.

the frequency of the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier frequencies.

The amount by which the signal frequency varies is very important. This is known as the deviation and is normally quoted as the number of kiloHertz deviation.

AM vs. FM AM requires a simple circuit, and is very easy

to generate.

It is simple to tune, and is used in almost all short wave broadcasting.

The area of coverage of AM is greater than FM (longer wavelengths) (lower frequencies) are utilized.

However, it is quite inefficient, and is susceptible to static and other forms of electrical noise.

AM vs. FM The main advantage of FM is its audio quality

and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals.

The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation.

The main disadvantage of FM is the larger bandwidth it requires.

Phase Modulation Frequency modulation (FM) and Phase modulation

(PM) have much in common and they are usually concluded as angle modulation, i.e., it is impossible to tell them apart without knowledge about the modulation function.

Phase modulation is the process of varying the of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant .

A strong motivation for FM and PM is that the amplitude is constant and the transmitter’s power amplifier can work at a constant high amplification level.

Noise Noise is unwanted signal that affects wanted

signal. Noise is random signal that exists in

communication systems

Noise: Internal: due to random movement of electrons in

electronic circuits Major sources are resistors, diodes,

transistors etc. Thermal noise or Johnson noise for e.g shot

noise .

External Man- made and natural resources . Sources over which we have no control .Examples are Motors, generators, atmospheric sources. Noise level in system is proportional to: Temperature and bandwidth Amount of current Gain of circuit Resistance of circuit Effect of noise Degrades system performance (Analog and

digital) Receiver cannot distinguish signal from

noise Efficiency of communication system reduces

Thank you