Communication Part 2

ece-1109 ECE-1109 Introduction to ECE

Concept of Bandwidth in Communication

Definition and Basics

• Bandwidth is the portion of the electromagnetic spectrum occupied by a signal, or the frequency range over which an information signal is transmitted .
• It is calculated as the difference between the upper and lower frequency limits of the signal .
• Example: For a music signal ranging from $20\text{ Hz}$ to $15\text{ kHz}$ ($15,000\text{ Hz}$), the bandwidth (BW) is calculated as: $BW=f_2-f_1=15000-20=14980\text{ Hz}$ .

Bandwidth of Different Signal Types Different signals require specific frequency ranges :

Signal Type	Frequency Range	Bandwidth
Voice signal (telephony)	$300-3400\text{ Hz}$	$3100\text{ Hz}$
Music Signal	$20-20000\text{ Hz}$	$19980\text{ Hz}$
TV signal (picture)	$0-5\text{ MHz}$	$5\text{ MHz}$
Digital data (via telephone line)	$300-3400\text{ Hz}$	$3100\text{ Hz}$

Bandwidth Requirements

Analog Communication Requirements To transmit analog signals over long distances wirelessly, modulation is required. The bandwidth depends on the modulation scheme used , :

1. Amplitude Modulation (AM):
   • The requirement is twice the frequency of the modulating (information) signal ($f_m$) .
   • $B{W}_{AM}=2\times f_m$ .
   • The spectrum consists of a Carrier, a Lower Sideband (LSB), and an Upper Sideband (USB). The bandwidth is the difference between USB and LSB ($f_c+f_m$) - ($f_c-f_m$) .
2. Frequency Modulation (FM):
   • Bandwidth is high, approximately 10 times the modulating signal frequency .
   • $B{W}_{FM}=10\times f_m$ .
3. Phase Modulation (PM):
   • Bandwidth is lower than FM but still high, approximately 6 times the modulating signal frequency .
   • $B{W}_{PM}=6\times f_m$ .

Digital Communication Requirements

• The minimum bandwidth requirement is half the bit rate: $BW=\text{Bit rate}/2$ .
• The relationship between channel capacity ($C$), bandwidth ($B$), and modulation levels ($M$) is expressed as $C=B{\log }_{2}M$ or $B=C/{\log }_{2}M$ .

Types of Bandwidth Classifications

• Narrow Band: Used for relatively slow data volumes or limited data transmission .
• Wide Band: Supports medium capacity data transmission rates .
• Broad Band: High-speed, high-capacity mediums supporting a wide frequency range and multiple signals simultaneously (e.g., Internet, Cable TV, Satellite) .

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Transmission Impairments

In communication, the received signal is rarely identical to the transmitted signal due to transmission impairments . These cause signal degradation in analog systems and bit errors in digital systems .

1. Attenuation

• This is the loss of signal strength as it travels over a distance, happening exponentially .
• It affects electrical circuits, optical fibers, and air propagation .
• If attenuation is too high, signals become unintelligible, requiring repeaters at regular intervals .
• Attenuation often increases with frequency, leading to attenuation distortion, particularly in analog signals .

2. Distortion

• Distortion is the alteration of the original signal’s shape or waveform due to the medium’s properties .
• It typically occurs in guided media like cables and fibers .
• Types include amplitude, harmonic, and phase distortion .
• It is critical in digital data as it can cause bit errors, though equalizing circuits can help solve this .

3. Noise

• Noise is random, unpredictable electrical signaling from internal or external sources that interferes with the required signal .
• It is a major limiting factor in system performance .

Categories of Noise:

• Thermal Noise (White Noise): Caused by the thermal agitation of electrons. It is present in all electronic devices, is a function of temperature, cannot be eliminated, and increases with bandwidth .
• Intermodulation Noise: Occurs when signals at different frequencies share a medium. The mixing of signals ($f_1$ and $f_2$) creates energy at sum or difference frequencies (e.g., $f_1+f_2$) .
• Crosstalk: Unwanted electrical coupling between nearby signal paths (e.g., twisted pairs). It is classified as NEXT (Near-end Crosstalk) or FEXT (Far-end Crosstalk) .
• Impulse Noise: Non-continuous, irregular pulses or spikes of high amplitude caused by external disturbances like lightning. It is a minor annoyance for analog data but a primary source of error in digital communication .

Sources of Noise Classification:

• External Noise: Atmospheric Noise, Extraterrestrial Noise, Man-made/Industrial Noise [21 in source list images, text describes structure].
• Internal Noise: Thermal Noise, Shot Noise .

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Comparison of Systems and Technologies

Analog vs. Digital Communication

Feature	Analog Communication	Digital Communication
Signal Type	Analog information signal	Digital information signal
Noise Effect	Highly affected by noise	Less affected by noise
Multiplexing	Uses FDM (Frequency Division Multiplexing)	Uses TDM (Time Division Multiplexing)
Bandwidth	Requires low bandwidth	Requires high bandwidth
Power	High power consumption	Low power consumption
Privacy/Security	Low privacy/security	High privacy/security
Synchronization	Difficult	Easier
Error Probability	High	Low

Communication Technologies Overview Key technologies include Telephone, Radio, Television, Mobile, Internet, Satellite, Radar, and Optical systems .

Electronics Devices Used in Communication Systems

The primary electronic devices utilized include:

1. Carbon Microphone
2. Speaker
3. Amplifier
4. Filter

1. Carbon Microphone

Structure and Function

• Primary Components: It consists of a thin moving diaphragm acting as a primary transducer and a capsule filled with carbon granules acting as a secondary transducer .
• Power Source: The device requires an external constant DC voltage source to operate .

Working Principle

1. Sound waves strike the thin diaphragm, causing it to displace by a certain distance .
2. This displacement compresses the carbon granules within the capsule .
3. The compression changes the orientation and contact area of the granules, leading to a change in electrical resistance .
4. This resistance change alters the current flowing through the output coil of the transformer, producing an output voltage proportional to the sound pressure .

Pros and Cons

• Advantages:
  • Low cost and simple construction .
  • Robust and good for rough handling .
  • Does not deteriorate due to heat or cold .
  • Provides relatively high power output compared to other microphones .
• Disadvantages:
  • Requires an external power source .
  • Suffers from high sound distortion compared to other types .
  • Limited frequency response (cannot work effectively above 5 kHz) .
  • Issues with low accuracy and linearity .

Applications

• Telephone transmission circuits .
• Radio broadcasting .
• Recording devices .

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2. Speakers

Definition and Operation

• A speaker (or loudspeaker) is a transducer that converts electrical signals back into sound waves .
• Transduction Process: It converts electrical energy into mechanical energy (vibrating sound waves), which the human brain interprets as sound .
• Mechanism: Magnetic fields are generated inside the speaker cabinet. The speaker cone responds to these magnetic signals by moving back and forth, creating air pressure that we perceive as sound .

Key Components

• Magnet and Voice Coil: These components convert the electrical energy into mechanical energy (motion) .
• Speaker Cone and Surround: Responsible for pushing air to create sound waves .
• Spider: Holds the speaker cone in place in a suspended configuration, allowing it some movement .

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3. Amplifiers

Definition and Categories

• An amplifier is an electronic circuit or device that increases the amplitude or strength of a transmitted signal .
• Categories:
  1. Small Signal (Voltage) Amplifiers: Used to increase the voltage level of small input signals .
  2. Large Signal (Power) Amplifiers: Convert DC power into AC power to produce a large signal at the output .

Power Amplifier Classifications Power amplifiers are classified into several types :

• Class A
• Class B
• Class AB
• Class C
• Class D

Important Terms and Formulas

• Gain: The ratio of the output electrical quantity to the input quantity .
• Frequency Response: The curve representing the relationship between voltage gain and signal frequency. Gain varies with frequency because circuit capacitor reactance changes with frequency .
• Resonant Frequency ($f_r$): The frequency at which the gain becomes maximum .
• Bandwidth: The range of frequencies over which the voltage gain is equal to or greater than $70.7$ of the maximum gain . It is defined by the difference between the upper cut-off frequency ($f_2$) and the lower cut-off frequency ($f_1$) .

Calculations (Examples)

• Power Gain (Decibels): Calculated using the formula: $\text{Power gain (dB)}=10{\log }_{10}(P_{out}/P_{in})$ .
  • Example: If output power is $1.5\text{ W}$ and input is $10\text{ mW}$ ($0.01\text{ W}$): $10{\log }_{10}(1.5/0.01)=21.76\text{ dB}$ .
• Voltage Gain (Decibels): Calculated using the formula: $\text{dB voltage gain}=20{\log }_{10}(V_{out}/V_{in})$ .
  • Example: For a gain of $15\text{ dB}$ and input voltage of $0.8\text{ V}$, the output voltage is calculated as $4.5\text{ V}$ .

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4. Filters

• Function: Filters are essential building blocks that alter the amplitude and/or phase characteristics of a signal with respect to frequency .
• Purpose: They are primarily linear circuits used to remove unwanted signal components such as Noise, Interference, and Distortion from the input signal .
• Placement: Typically used at the receiving side of a communication system .