Telecommunications and the Information Age ET108B LM#2

Transcription

1 Telecommunications and the Information Age ET108B LM#2 Characteristics of Sound Frequencies used by people Sound Waves versus Electrical Waves Basic Electricity as it pertains to Circuits Electromagnetic Spectrum

2 Characteristics of Sound Definition Periodic variation in air pressure As we speak, vibrations created by our vocal cords travel in the medium of air These vibrations travel through the air and cause sound wave just like a boat moving through water causes waves in the water Wave in either air or water emanating from a source From: Introduction to Telecommunications, 2/e By M.A.Rosengrant Pictures Copyright (c) 2007 by Pearson Education, Inc.

3 Characteristics of Sound Distinguishing Characteristics Frequency Definition: Number of occurrences during a specific period of time Examples: Frequency of the seasons, full moons, etc. As used in Communications Used to distinguish between the different sounds wave of that sound will occur in one second The period of a wave is the length of time required for one wave Five cps analog waveform From: Introduction to Telecommunications, 2/e By M.A.Rosengrant Pictures Copyright (c) 2007 by Pearson Education, Inc.

4 Characteristics of Sound Distinguishing Characteristics Frequency From: Introduction to Telecommunications, 2/e By M.A.Rosengrant Pictures Copyright (c) 2007 by Pearson Education, Inc.

5 Characteristics of Sound Distinguishing Characteristics Wavelength In sound the standard unit of time is a second Definition The physical length of one waveform From: Introduction to Telecommunications, 2/e By M.A.Rosengrant Pictures Copyright (c) 2007 by Pearson Education, Inc.

6 Characteristics of Sound Distinguishing Characteristics Amplitude Term used to describe the strength of a signal Voltage is used to measure the signal strength of various amplitudes. The greater the amplitude, the louder the sound and the stronger the signal. From: Introduction to Telecommunications, 2/e By M.A.Rosengrant Pictures Copyright (c) 2007 by Pearson Education, Inc.

7 Sound and Electrical Waves Frequencies Peoples use in Communications The full range of frequencies that can be heard is 20 20kHz Most of the information in human speech can be represented with Hz Telephones use this smaller range as a historical compromise that balances the information content with reliable transmission distance Compromised developed during the late 1800 s and early 1900 s Sound Waves verses Electrical Waves Within the frequency limitations of the telephone The sound waves and resulting electrical wave track They have different speeds

8 Sound and Electrical Waves Sound Waves verses Electrical Waves Within the frequency limitations of the telephone The sound waves and resulting electrical wave track Different wavelengths due to the different speed» 250 Hz sound wave travels through the air at m/s and has a wavelength of meters» A 250 Hz analog electrical signal leaveing a telephone on a pair of wires has a wavelength of 3x10 8 m/ 250 = 1.2 x 10 6 m However both still have the same frequency and period» 250 cycles per second or 250Hz» And each cycle takes seconds to complete

9 Basic Electrical Theory Voltage and Current Voltage The difference in potential energy per charge in electrical terms is called voltage Voltage is defined as energy or work (W) per unit charge (Q). W is expressed in joules (J) and Q is in coulombs (C) Where joule is the energy measurement for a unit of work Example V W Q kg m m J Nm m kg 2 2 s s If 50 J of energy are available for every 10 C of charge, what is the voltage? V W 50J 5V C 10C 2

10 Basic Electrical Theory Voltage and Current Current 6.25 X electrons passing a point in a wire in one second due to a difference of potential at the two ends is a current of one ampere (1A) One coulomb: the total charge possessed by 6.25 X electrons A single electron has a charge of 1.6 X C Power The voltage across a device times the current through the device equals the power being used as measured in Watts Resistance Resistance is the opposition to current

11 Basic Electrical Theory Voltage and Current Resistance One ohm (1 ) of resistance exists if there is one ampere (1A) of current in a material when one volt (1V) is applied across the material Impedance Resistance is a critical component of impedance and in AC circuits. It s effect doesn t very with frequency of the AC voltage Capacitive and Inductive Reactance As the frequency of the signal decreases the value of capacitive reactance increases and it has a bigger effect on the impedance to an AC signal As the frequency of the signal increases the value of inductive reactance increases and it has a bigger effect on the impedance to an AC signal

12 Basic Electrical Theory Decibels and measuring Signal Strength The bel and decibel units Derived from how a received signal is perceived Specifically by the human ear It, the human ear, is logarithmic not linear» With two sounds, one perceived as being twice as loud» The louder sound actually has approx. four times the power The original unit, the bel, was name in honor of Alexander Graham Bell the decibel = 1/10 bels and is the most commonly used unit Relative Power Gain = (Power Output)/(Power Input) A p = P o /P i Where P o & P i are specified in Watts

13 Basic Electrical Theory Decibels and measuring Signal Strength Relative Power Gain = (Power Output)/(Power Input) A p = P o /P i The gain can be the same for different values of P o & P i Usually expressed in terms of Decibels {A P(dB) } A P(dB) = 10 log A p or A P(dB) = 10 log (P o /P i ) Absolute Power Gain and dbm Previous discussion of dbs were relative measures i.e, an infinite sets of powers in a ratio that can yield a 10 db change Communications industry developed a standard Load resistance from the set 50 s, 600 s, or 900 s Reference power level = 1 mw Symbol = dbm

14 Basic Electrical Theory Decibels and measuring Signal Strength Absolute Power Gain and dbm Formulas A P(dBm) = 10log P/(1mW) Milliwatt values compared with dbm values. For example mw and 60 dbm represent the same value. Using 60 dbm to identify the strength of a signal is much simpler than using mw. From: Introduction to Telecommunications, 2/e By M.A.Rosengrant Pictures Copyright (c) 2007 by Pearson Education, Inc.

15 Electromagnetic Spectrum Characteristics Broad range of frequencies, Referred to in terms of wavelength instead or of Hertz (aka, cycles per second) Depends upon the part of the spectrum and traditional references

16 Electromagnetic Spectrum Characteristics Another view by Carl Rod Nave of Georgia State University Converting from frequency to wavelengths Note: the speed of light in a material depends upon the electrical and magnetic characteristics of the material Speed of light = Frequency of the waveform * wavelength 8 c f * c 3.0 *10 m / s