Analog Communication

UNIT – I

Made by: Hridesh Vishwdewa

Definition of Analog Signal

• Unlike a digital signal, which has a discrete value at each sampling point, an analog signal has constant fluctuations.

• The illustration in the above figure shows an analog pattern (represented as the curve) alongside a digital pattern (represented as the discrete lines).

•An analog signal is a continuous signal that contains time-varying quantities.

• An analog signal can be used to measure changes in some physical phenomena such as light, sound, pressure, or temperature.

• For instance, an analog microphone can convert sound waves into an analog signal.

• Even in digital devices, there is typically some analog component that is used to take in information from the external world, which will then get translated into digital form (using an analog-to-digital converter).

Systems

• A System, is any physical set of components that takes a signal, and produces a signal. In terms of engineering, the input is generally some electrical signal X, and the output is another electrical signal (response) Y.

• However, this may not always be the case. Consider a household thermostat, which takes input in the form of a knob or a switch, and in turn outputs electrical control signals for the furnace.

Classification of Systems

• Continuous vs. Discrete

• Linear vs. Nonlinear

• Time Invariant vs. Time Varying

• Causal vs. Noncausal

• Stable vs. Unstable

Continuous vs. Discrete

• One of the most important distinctions to understand is the difference between discrete time and continuous time systems.

• A system in which the input signal and output signal both have continuous domains is said to be a continuous system. One in which the input signal and output signal both have discrete domains is said to be a continuous system.

• Of course, it is possible to conceive of signals that belong to neither category, such as systems in which sampling of a continuous time signal or reconstruction from a discrete time signal take place.

Linear vs. Nonlinear

• A linear system is any system that obeys the properties of scaling (first order homogeneity) and superposition (additivity) further described below. A nonlinear system is any system that does not have at least one of these properties.

• To show that a system obeys the scaling property is to show that

Block Diagram

of Comm. system

A: The information signal to be transmitted (Voice, music, DC to represent rudder position in Radio Control).B: Digitally compressed data. Analogue filtering and compression is possible too.C: The low power radio frequency carrier signal.D: The modulated carrier. The carrier has been modified in proportion to the signal to be transmitted.E: High power signal, usually radio, but light and ultrasound are common too.F: Attenuated signal. Energy is lost in the transmission medium.G: Amplified signal.H: A low power copy of the original compressed signal.I: An exact copy of the original information signal.

Types Of Signals

• Continuous-Time vs. Discrete-Time

• Analog vs. Digital

• Periodic vs. Aperiodic

• Finite vs. Infinite Length

• Causal vs. Anticausal vs. Noncausal

• Even vs. Odd

• Deterministic vs. Random

Continuous-Time vs. Discrete-Time

• As the names suggest, this classification is determined by whether or not the time axis isdiscrete (countable) or continuous (Figure 1). A continuous-time signal will contain a value for all real numbers along the time axis. In contrast to this, a discrete-time signal, often created by sampling a continuous signal, will only have values at

equally spaced intervals along the time axis.

Analog vs. Digital• The difference between analog and digital is similar to the

difference between continuous-time and discrete-time. However, in this case the difference involves the values of the function.

• Analog corresponds to a continuous set of possible function values, while digital corresponds to a discrete set of possible function values.

• A common example of a digital signal is a binary sequence, where the values of the function can only be one or zero.

Periodic vs. Aperiodic• Periodic signals repeat with some period T, while aperiodic, or

non-periodic, signals do not (Figure 3). We can define a periodic function through the following mathematical expression, where t can be any number and T is a positive constant: f(t)=f(T+t) ----Eq.(1)

• The fundamental period of our function, f(t), is the smallest value of T that the still allows Eq(1) to be true.

Finite vs. Infinite Length• As the name implies, signals can be characterized as to

whether they have a finite or infinite length set of values. Most finite length signals are used when dealing with discrete-time signals or a given sequence of values. Mathematically speaking, f t is afinite-length signal if it is nonzero over a finite interval t 1<f t< t 2 where t 1 >−∞ and t 2 <∞ . An example can be seen in Fig.4.Similarly, an infinite-length signal, f t , is defined as nonzero over all real numbers: ∞≤f t≤−∞

Causal vs. Anticausal vs. Noncausal• Causal signals are signals that are zero for all negative time,

while anticausal are signals that are zero for all positive time.Noncausal signals are signals that have nonzero values in both positive and negative time Fig.5(a-c)

Even vs. Odd

• An even signal is any signal f such that f (t) = f (−t). Even signals can be easily spotted as they are aymmetric around the vertical axis. An odd signal, on the other hand, is a signal f such that f (t) = − f (−t) , Fig.6

• Using the definitions of even and odd signals, we can show that any signal can be written as a combination of an even and odd signal. That is, every signal has an odd-even decomposition. To demonstrate this, we have to look no further than a single equation.

f (t) = ½ (f (t) + f(−t) ) + ½ ( f (t) −f (−t))

Deterministic vs. Random• A deterministic signal is a signal in which each value of the signal is fixed

and can be determined by a mathematical expression, rule, or table. Because of this the future values of the signal can be calculated from past values with complete confidence. On the other hand, a random signal has a lot of uncertainty about its behavior. The future values of a random signal cannot be accurately predicted and can usually only be guessed based on the averages of sets of signals (Figure 8).

Example

• Consider the signal defined for all real t described by -

F (t) = { sin (2πt) / t } 0 t ≥ 1 t < 1

•This signal is continuous time, analog, aperiodic, infinite length, causal, neither even nor odd, and, by definition, deterministic

Important Continuous Time Signals

• Sinusoids• Complex Exponentials• Unit Impulses• Unit Step

Sinusoids• One of the most important elemental signal that you will deal

with is the real-valued sinusoid. In its continuous-time form, we write the general expression as -

Complex Exponentials

• As important as the general sinusoid, the complex exponential function will become a critical part of your study of signals and systems. Its general continuous form is written as -

A est

• where s = σ + jω is a complex number in terms of σ, the attenuation constant, and ω the angular frequency.

Unit Impulses

• The unit impulse function, also known as the Dirac delta function, is a signal that has infinite height and infinitesimal width. However, because of the way it is defined, it integrates to one.

• While this signal is useful for the understanding of many concepts, a formal understanding of its definition more involved. The unit impulse is commonly denoted  δ( t ).

• For now, it suffices to say that this signal is crucially important in the study of continuous signals, as it allows the sifting property to be used in signal representation and signal decomposition.

Unit Step

• Another very basic signal is the unit-step function that is defined as -

• The step function is a useful tool for testing and for defining other signals. For example, when different shifted versions of the step function are multiplied by other signals, one can select a certain portion of the signal and zero out the rest.

Transmission Media• A transmission medium (plural transmission media) is a

material substance (solid, liquid, gas, or plasma) that can propagate energy-waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.

• The term transmission medium also refers to a technical device that employs the material substance to transmit or guide waves. Thus, an optical fiber or a copper cable is a transmission medium. Not only this bt also is able to guide the transmission of networks.

A transmission medium can be classified as a:

• Linear medium, if different waves at any particular point in the medium can be superposed;

• Bounded medium, if it is finite in extent, otherwise unbounded medium;

• Uniform medium or homogeneous medium, if its physical properties are unchanged at different points;

• Isotropic medium, if its physical properties are the same in different directions.

Transmission Media - Guided & Unguided

• Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only.

• Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media.

• There 4 basic types of Guided Media:1. Open Wire  2.Twisted Pair  3. Coaxial Cable  4.Optical Fiber

Open Wire

• Open Wire is traditionally used to describe the electrical wire strung along power poles. There is a single wire strung between poles. No shielding or protection from noise interference is used.

• We are going to extend the traditional definition of Open Wire to include any data signal path without shielding or protection from noise interference.

• This can include multi-conductor cables or single wires. This media is susceptible to a large degree of noise and interference and consequently

not acceptable for data transmission except for short distances under 20 ft. 

 Twisted Pair • The wires in Twisted Pair cabling are twisted together in pairs. Each pair would consist

of a wire used for the +ve data signal and a wire used for the -ve data signal.

• Any noise that appears on 1 wire of the pair would occur on the other wire. Because the wires are opposite polarities, they are 180 degrees out of phase (180 degrees - phasor definition of opposite polarity). When the noise appears on both wires, it cancels or nulls itself out at the receiving end.

• Twisted Pair cables are most effectively used in systems that use a balanced line method of transmission: polar line coding (Manchester Encoding) as opposed to unipolar line coding (TTL logic).

Twisted Pair

• The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference.

• To further improve noise rejection, a foil or wire braid shield is woven around the twisted pairs.

• This "shield" can be woven around individual pairs or around a multi-pair conductor (several pairs).

• Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP. Cables without a shield are called Unshielded Twisted Pair or UTP. Twisting the wires together results in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable.

• UTP or Unshielded Twisted Pair cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc..)

• STP or Shielded Twisted Pair is used with the traditional Token Ring cabling or ICS - IBM Cabling System. It requires a custom connector. IBM STP (Shielded Twisted Pair) has a characteristic impedance of 150 ohms.

Coaxial Cable • Coaxial Cable consists of 2 conductors. The inner conductor is held inside

an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.

The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet. The excellent control of the impedance characteristics of the cable allow higher data rates to be transferred than Twisted Pair cable.

Optical Fibre • Optical Fibre consists of thin glass fibres that can carry information at

frequencies in the visible light spectrum and beyond. The typical optical fibre consists of a very narrow strand of glass called the Core. Around the Core is a concentric layer of glass called the Cladding. A typical Core diameter is 62.5 microns (1 micron = 10-6 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.

Optical Fibre

• An important characteristic of Fibre Optics is Refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other. An example of this is when we look into a pond of water.

• Optical Fibres work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops

light from escaping through it - it bounds the media!

Optical Fiber

Radio Waves• Radio waves are a type of electromagnetic radiation with wavelengths in

the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, they travel at the speed of light.

• Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications

Micro Waves

• Microwaves are radio waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.

• This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries.[2] In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Infrared Transmission

• Infrared transmission refers to energy in the region of the electromagnetic radiation spectrum at wavelengths longer than those of visible light, but shorter than those of radio waves. Correspondingly, infrared frequencies are higher than those of microwaves, but lower than those of visible light.

• Scientists divide the infrared radiation (IR) spectrum into three regions. • The wavelengths are specified in microns (symbolized µ, where 1 µ = 10-

6 meter) or in nanometers (abbreviated nm, where 1 nm = 10-9 meter = 0.001 5).

• The near IR bandcontains energy in the range of wavelengths closest to the visible, from approximately 0.750 to 1.300 5 (750 to 1300 nm).

• The intermediate IR band (also called the middle IR band) consists of energy in the range 1.300 to 3.000 5 (1300 to 3000 nm). The far IR band extends from 2.000 to 14.000 5 (3000 nm to 1.4000 x 104 nm).

Infrared is used in a variety of wireless communications, monitoring, and control applications. Here are some examples:

1. Home-entertainment remote-control boxes2. Wireless (local area networks)3. Links between notebook computers and desktop computers4. Cordless modem5. Intrusion detectors6. Motion detectors7. Fire sensors8. Night-vision systems9. Medical diagnostic equipment10.Missile guidance systems11.Geological monitoring devices

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