Modulation What is modulation? What is modulation? a process of changing one or more properties of...
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ModulationWhat is modulation?a process of changing one or more properties of the analog carrier in proportion to the information signal.One of the characteristics of the carrier signal is changed according to the variations of the modulating signal.AM amplitude, EFM frequency , PM - phase ,
Modulation (contd)Why modulation is needed?To generate a modulated signal suited and compatible to the characteristics of the transmission channel.For ease radiation and reduction of antenna sizeReduction of noise and interferenceChannel assignmentIncrease transmission speed
Noise, interference and distortionNoiseunwanted signals that coincide with the desired signals. Two type of noise:internal and external noise.Internal noiseCaused by internal devices/components in the circuits.External noisenoise that is generated outside the circuit. E.g. atmospheric noise,solar noise, cosmic noise, man made noise.
Noise, interference and distortion (Contd)InterferenceContamination by extraneous signals from human sources.E.g. from other transmitters, power lines and machineries.Occurs most often in radio systems whose receiving antennas usually intercept several signals at the same timeOne type of noise.
Noise, interference and distortion (Contd)DistortionSignals or waves perturbation caused by imperfect response of the system to the desired signal itself.May be corrected or reduced with the help of equalizers.
Limitations in communication systemA) Technological problemsIncludes equipment availability, economic factors, federal regulations and interaction with existing systems.Problem solved in theory but perfect solutions may not be practical.
Limitations in communication system (contd)B) Physicals limitationsi) Bandwidth limitationMeasure of speedThe system ability to follow signal variations depends on the transmission bandwidth.Available bandwidth determines the maximum signal speed.
Limitations in communication system (contd)ii) Noise limitationUnavoidable.The kinetic theory.Noise relative to an information signal is measured in terms of signal to noise ratio (SNR).
Communication system designCompromise within:Transmission time and powerSNR performanceCost of equipmentsChannel capacityBandwidth
FREQUENCY AND WAVELENGTHCycle - One complete occurrence of a repeating wave (periodic signal) such as one positive and one negative alternation of a sine wave.Frequency - the number of cycles of a signal that occur in one second.Period - the time distance between two similar points on a periodic wave.Wavelength - the distance traveled by an electromagnetic (radio) wave during one period.W2-2
timePERIOD AND FREQUENCY COMPAREDFrequency = f = 1/T
+0timedistanceFrequency and wavelength comparedf = 1/T
CALCULATING WAVELENGTHAND FREQUENCY = wavelength in metersf = frequency in MHz = 300/ff = 300/C = f where C = 3 x 108 m/s (speed of light)
ELF103 m107 m104 m105 m106 m10 m1 m10-1 m10-2 m10-3 m10-4 m102 m300 Hz30 Hz30 kHz3 kHz300 kHz30 MHz3 MHz300 MHz3 GHz300 GHz30 GHzTHE ELECTROMAGNETIC SPECTRUM FROM 30 HZ TO 300 GHZUHFVHFHFMFLFVLFVFSHFEHFMillimeterwaves( = 300/f)(f = 300/)
LOW AND MEDIUM FREQUENCIESExtremely Low Frequencies - 30 to 300 HzVoice Frequencies - 300 to 3000 Hz Very Low Frequencies - 3 kHz to 30 kHzLow Frequencies - 30 kHz to 300 kHzMedium Frequencies - 300 kHz to 3 MHz
HIGH FREQUENCIESHigh Frequencies - 3 MHz to 30 MHz
Very High Frequencies - 30 MHz to 300 MHz
Ultra High Frequencies - 300 MHz to 3 GHz (1 GHz and above = microwaves)
Super High Frequencies - 3 GHz to 30 GHz
Extremely High Frequencies - 30 GHz to 300 GHz
< 3 Hz> 100,000 km
Extremely low frequency
330 Hz100,000 km 10,000 km
Communication with submarines
Super low frequency
30300 Hz10,000 km 1000 km
Communication with submarines
Ultra low frequency
3003000 Hz1000 km 100 km
Communication within mines
Very low frequency
330 kHz100 km 10 km
Submarine communication, avalanche beacons, wireless heart rate monitors, geophysics
30300 kHz10 km 1 km
Navigation, time signals, AM longwave broadcasting
3003000 kHz1 km 100 m
AM (Medium-wave) broadcasts
330 MHz100 m 10 m
Shortwave broadcasts, amateur radio and over-the-horizon aviation communications
Very high frequency
30300 MHz10 m 1 m
FM, television broadcasts and line-of-sight ground-to-aircraft and aircraft-to-aircraft communications
Ultra high frequency
3003000 MHz1 m 100 mm
television broadcasts, microwave ovens, mobile phones, wireless LAN, Bluetooth, GPS and Two-Way Radios such as FRS and GMRS Radios
Super high frequency
330 GHz100 mm 10 mm
microwave devices, wireless LAN, most modern Radars
Extremely high frequency
30300 GHz10 mm 1 mm
Radio astronomy, high-speed microwave radio relay
Above 300 GHz< 1 mm
Longwave AM Radio = 148.5 - 283.5 kHz (LF) Mediumwave AM Radio = 535kHz - 1605kHz (MF) TV Band I (Channels 2 - 6) = 54MHz - 88MHz (VHF) FM Radio Band II = 88MHz - 108MHz (VHF) TV Band III (Channels 7 - 13) = 174MHz - 216MHz (VHF) TV Bands IV & V (Channels 14 - 69) = 470MHz - 806MHz (UHF)
Public Broadcast Frequencies
FCC Channel Bandwidth AllocationsFCC : Federal Communications Commission, United States
AM broadcast 535 kHz 1605 kHz 106 10kHz
Citizen band 26.965 MHz 27.405 MHz 40 10kHz
FM broadcast 88 MHz 108 MHz 100 200kHz
TV broadcast 54 MHz 806 MHz 67 6MHzDesignation Frequency Range No. of Channels Bandwidth
10-3 m10-4 m300 GHzMillimeterwavesTHE ELECTROMAGNETIC SPECTRUM ABOVE 300 GHZWavelength0.8 x 10-6 m0.4 x 10-6 mInfraredVisibleUltravioletX-raysGamma raysCosmic rays10-5 m
OPTICAL FREQUENCIESInfrared - 0.7 to 10 micronVisible light - 0.4 to 0.8 micronUltraviolet - Shorter than 0.4 micronNote: A micron is one millionth of a meter. Light waves are measured and expressed in wavelength rather than frequency.
TYPES OF COMMUNICATIONSTXRXTXTXRXRXSimplex:One-wayDuplex:Two-wayHalf duplex:Alternate TX/RXFull duplex:SimultaneousTX/RXChannelChannel(s)
COMMUNICATIONS SIGNAL VARIATIONSBaseband - The original information signal such as audio, video, or computer data. Can be analog or digital.Broadband - The baseband signal modulates or modifies a carrier signal, which is usually a sine wave at a frequency much higher than the baseband signal.
Frequency Spectrum &Bandwidth
The frequency spectrum of a waveform consists of all frequencies contained in the waveform and their amplitudes plotted in the frequency domain.The bandwidth of a frequency spectrum is the range of frequencies contained in the spectrum. It is calculated by subtracting the lowest frequency from the highest.
Frequency Spectrum &Bandwidth(contd)Bandwidth of the information signal equals to the difference between the highest and lowest frequency contained in the signal.Similarly, bandwidth of communication channel is the difference between the highest and lowest frequency that the channel allow to pass through it
Gain and Loss Electronic SystemPinVinIinPoutVoutIoutIf ratio Pout /Pin > 1 , power gain.If ratio Pout /Pin < 1 , power loss.If ratio Vout /Vin > 1 , voltage gain.If ratio Vout /Vin < 1 , voltage loss.If ratio Iout /Iin > 1 , current gain.If ratio Iout /Iin < 1 , current loss.W2-3
Signal GainVoltage amplifier: Av= Vo/Vi.Transistors current gain: = ic/ib,Step-up voltage transformer, Vsecondary > VprimaryStep-up current transformer, Isecondary > IprimaryIn some cases, signal gain may be >1 eventhough the power gain is < 1.
Occasionally, a signal loss or power loss is also called as signal attenuation or power attenuation.
Power GainIt is the ratio of output power over input power. Ap = Po/Pi.
In amplifiers, the power gain may be more than one. The signal power is amplified. DC power supply is transformed into signal power.
Power and voltage gains in communicationThe term decibels (dB) is a relative unit of measurement used frequently in electronic communication to describe gain or loss.
Power gain in dB = 10 log (Po/Pi)
Voltage gain in dB = 20 log (Vo/Vi)
Power ratio Po/Pi = 10,000 = 40 dB
Voltage ratio Vo/Vi = 100 = 40 dB.
See that Po/Pi = (Vo/Vi)2
hence, (Po/Pi) dB = 2(Vo/Vi)dB
Alternatively: Power gain ratio = 10 (gain in dB/10)
Voltage gain ratio = 10 (gain in dB/20)
Example 1:Convert a power gain of 64 dB into ratio.
64 dB = 10(64/10) = 10(6.4) =2.5212 x 106
In this case, Pout= 0.01 Pin
Therefore, Power gain (dB)= 10 log(0.01)= -20 dB
When the answer is negative, indicates a loss orattenuation.
Example 2 :What is the dB gain or loss for a certain system when thePower out of a certain system is 0.01 times the power in?
Example 3:Let there be two amplifiers in cascade as shown. Their gains are 13 dB and 10 dB respectively.
The overall gain at the output is 13+10 = 23 dB.A1A213dB gain10dB gainOutput
In terms of ratio: 23 dB = 10(23/10)= 200 13 dB = 10(13/10)= 20 10 dB = 10(10/10)= 10 Again 20 x 10 = 200.
Relative dBIt is convenient to express signals with some reference such as 1mW power or, 1 V voltage level. This permits input- and output- signals to be expressed in terms of relative dB.When referenced to 1mW, it is written dBmWhen referenced to 1 V, it is written as dBVWhen referenced to 1W, it is written dBWWhen referenced to 1 V, it is written as dBV
Relative dB is not an absolute gain but is termed as gain with respect to a reference unit. Examples :
a) For a 5 watts signal in reference to 1mW, the relative dB; 10 log(5W/1mW) = 36.99 dBm
b) For a 500 V signal in reference to a 1 V , the relative dB; 20 log(500 V /1 V ) = 53.98dB V