1 st semester 1434-1435 Chapter4: Continuous-wave Modulation King Saud University College of Applied studies and Community Service 1301CT By: Nour Alhariqi.

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1st semester 1434-1435

Chapter4: Continuous-wave Modulation King Saud University College of Applied studies and Community Service1301CTBy: Nour Alhariqi

1nalhareqi-2013Communications System Model

2nalhareqi-2013Introduction to Modulation of Analog SignalThe purpose of a communication system is to deliver the input message from the source in a recognizable form to the destination over the channel. The channel is either low-pass or band-pass. A low-pass channel has a bandwidth with frequencies between 0 and . A band-pass channel has a bandwidth with frequency between 1 and 23nalhareqi-2013Introduction Cont.The baseband signals produced by various information sources are not suitable for direct transmission over band-pass channels.A baseband signal or low frequency signal is a signal whose spectrum is positioned close to dc (=0). Examples of baseband signals include speech signals whose spectrum occupies the frequency band 0 to 3.5 kHz and video signals whose spectrum occupies the frequency band 0 to 4.3 kHz.

4nalhareqi-2013Introduction Cont.To perform the transmission, the sender needs to modify the baseband signal into a form that is suitable for transmission over the channel. The adaptation of the baseband signal to the transmission channel called modulation. It can be viewed as a way to change message into suitable form for transmission.At the receiver, the received signal must pass through a reverse process called demodulation in order to reconstruct the baseband signal.

5nalhareqi-2013ModulationIn general there are two main classes of modulation: continuous-wave modulation (CW) and pulse modulation.In CW modulation, the analog baseband signal is transmitted using a high frequency signal, called carrier signal.So, the modulation process involve two waveforms: the baseband signal (called a modulating signal) and the carrier signal which is sinusoid signal.6nalhareqi-2013ModulationThe modulating signal is used to change a parameter of the carrier signal ( amplitude, frequency, or phase) , the modified carrier signal is called the modulated signal. A consequence of modulation is shifting the range of frequencies contained in the message signal into another frequency range suitable for transmission over the channel, centered onto the carrier frequency.

7nalhareqi-2013CW ModulationThe CW modulation classified to : AM, FM, and PMWhen the amplitude of the carrier is varied in accordance with the modulating signal, we have Amplitude Modulation (AM).When the frequency of the carrier is varied in accordance with the modulating signal, we have Frequency Modulation (FM).When the phase of the carrier is varied in accordance with the modulating signal, we have Phase Modulation (PM).

8nalhareqi-2013CW Modulationnalhareqi-20139

DemodulationAs we know, the receiver recreate the original message signal from a degraded version of the transmitted signal after propagation through the channel.This re-construction is a accomplished by using a demodulation. Which is the reverse of the modulation process used in the sender

10nalhareqi-2013Demodulationnalhareqi-201311However, owing to the unavoidable presence of the noise and distortion in the received signal, we find that the receiver cannot recreate the original message signal exactly.The resulting degradation in the overall system performance is influenced by the type of modulation scheme used.Specifically, we find that some modulation schemes are less sensitive to the effects of noise and distortion than othersAnalog Communication SystemThere are two types of communication systems: Analog communication system and digital communication system .The CW modulation is used in the analog communication system.In this system, the transmitter consists of a modulator and the receiver consists of demodulator 12nalhareqi-2013modulatordemodulatorBaseband signalmodulating signalmodulatedsignalmodulatedsignalcarrier signalcarrier signalBaseband signalBenets of Modulationnalhareqi-201313In addition to facilitating transmission, there are some important reasons for modulation:Ease of RadiationMultiplexingReduce Noise and InterferenceEase of RadiationIf the communication channel consists of free space (radio channel), then antennas are needed to radiate and receive the signal. Efficient electromagnetic radiation requires antennas whose dimensions are at least 1/10 of the wavelength of the signal being radiated.14nalhareqi-2013Ease of Radiationnalhareqi-201315 Many baseband signals, including audio signals, have frequency components down to 100 Hz or lower. For these signals, antennas about 300 km long will be necessary if the signal is radiated directly. If modulation is used to impress the baseband signal on a high-frequency carrier, say at 100 MHz, then antennas need be in order of 30 meters.

MultiplexingConsider a case of several senders want to send at the same time. If they send the baseband signals directly, all signals at same frequencies from different senders would be mixed up.The solution of allowing one sender to send at time is wasteful because the channels bandwidth my be much larger than that of the signal.16nalhareqi-2013Multiplexingnalhareqi-201317One way to solve this problem is to use modulation. modulation is used to translate different signals to different frequency range by assign the carriers to different frequencies that are sufficiently far apart of each other. This way of multiplexing called the frequency division multiplexing (FDMA).At the received side, a tunable bandpass filter is used to select the desired signal

Reduce Noise and InterferenceThe effect of noise and interference cannot be eliminated in a communication system. A brute-force method for combating noise and interference is to increase the signal power until it overwhelms the contaminations.But increasing power is costly and my damage equipments.18nalhareqi-2013Reduce Noise and Interferencenalhareqi-201319Other way to minimize the effects of noise and interference is using modulation . certain types of modulation schemes have valuable property of suppressing both noise and interference.These schemes generally require a transmission bandwidth much larger than the bandwidth of the modulating signal.

Amplitude Modulation (AM)nalhareqi-201320As the name suggests, in AM, the modulating signal varies the amplitude of the carrier sinusoid wave.The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal.The carrier frequency and phase remain constant during the modulation process, just its amplitude varies in accordance with the modulating signal.Amplitude Modulation (AM)nalhareqi-201321

Types of Amplitude Modulation (AM)nalhareqi-201322 There are several different ways of amplitude modulating the carrier signal by the baseband signal ( modulating signal), each of which results in different spectral characteristics for the transmitted signal (modulated signal):Double-sideband suppressed carrier (DSB-SC) modulationAmplitude modulation (AM). Single-sideband modulation (SSB). Vestigial-sideband modulation (VSB). Each of these schemes has its own distinct advantages, disadvantages, and practical applications.Double-sideband suppressed carrier (DSB-SC) modulation ModulationSpectrumUpper and Lower SidebandsBandwidthExample Demodualtionnalhareqi-201323Modulationnalhareqi-201324The modulating signal (information or baseband signal)

The carrier signal is

The modulated signal

Modulationnalhareqi-201325

Modulationnalhareqi-201326The DSB-SC modulated signal undergoes a phase reversal whenever the message signal m(t) cross zero.

Consequently, the envelop of a DSB-SC modulated signal is different from the message signal

Envelopphase reversalSpectrumnalhareqi-201327The process of DSB-SC modulation shifts the spectrum of the modulating signal m(t) to the left and the right by c (carrier frequency).

if

then

Spectrumnalhareqi-201328

28Upper and Lower Sidebands nalhareqi-201329The modulated signal spectrum centered at c is composed of two parts: a portion that lies above c , known as the upper sideband (USB), and a portion that lies below c , known as the lower sideband (LSB).Similarly, the spectrum centered at - c has upper and lower sidebands.Hence, this is a modulation scheme with double sidebandsUpper and Lower Sidebands nalhareqi-201330

c +Mc - MBandwidthnalhareqi-201331If the bandwidth of the modulating signal is B Hz, then the bandwidth of the modulated signal is 2B Hz.

2B2B2BBandwidthnalhareqi-201332In order to avoid the overlap of the spectra centered at c and c , the c should be M.If c < M, the LSB of c will overlap with LSB of -c Which make it impossible to get back the m(t) from the modulated signal m(t) cos (ct).

Examplenalhareqi-201333Suppose that the modulating signal m(t) is a sinusoid of the form

And the carrier signal is :

Determine The DSB-SC signal SDSB(t) (modulated signal)The upper and lower sidebandsThe bandwidthSketch the spectrum of the m(t) and SDSB(t)

Solutionnalhareqi-201334The DSB-SC signal

By using the relation

The SDSB(t) is

Solutionnalhareqi-201335The upper and lower sidebands

USB = 106+103=1001KHz LSB = 106-103=999KHz

The bandwidth of SDSB(t)

BW = 2 * 103 =2000 Hz = 2 KHz

Solutionnalhareqi-201336Sketch the spectrum of the m(t) and SDSB(t)

M()103-103

SDSB()ff1000Demodulationnalhareqi-201337The DSP-SC demodulation consists of multiplication of the incoming modulated signal m(t) cos (ct ) by a carrier cos (ct) followed by a low pass filter.

The receiver need to generate a local carrier that has the same frequency and the same phase of the carrier used for modulation.

DemodulationModulationDemodulationnalhareqi-201338The multiplication will shift the modulated signal spectrum to the left and to the right by c and multiply it by one-half.Use a low pass filter to get the desired baseband spectrum and suppress the unwanted spectrum at + c and - c

nalhareqi-201339

modulated signal spectrum

c-c2c-2cAfter multiplicationfilterDemodulationnalhareqi-201340This method of recovering the baseband signal is called synchronous detection or coherent detection, because the receiver uses a carrier of exactly the same frequency and phase as the carrier used for modulation

Standard Amplitude Modulation (AM)nalhareqi-201341Standard Amplitude Modulation (AM)nalhareqi-201342In the DSP-SC demodulation, a receiver must generate a local carrier in frequency and phase synchronism with the carrier used for modulation.This call for a sophisticated receiver and could be quit costly.The other alternative is for the transmitter to transmit a carrier Ac cos (ct) along with the modulated signal m(t) cos (ct). so there is no need to generate a carrier at the receiver and this is the idea of the standard AM.Modulationnalhareqi-201343The modulating signal (information or baseband signal)

The carrier signal is

The modulated signal

Modulationnalhareqi-201344

Modulation case 1nalhareqi-201345When ( is nonnegative) for all value of t.The Envelop has the same shape as m(t).Hence, we can recover m(t) from this envelop.At the receiver, the detection is extremely simple and inexpensive operation, which doesnt require generation of a local carrier for demodulation.

Modulation case 2nalhareqi-201346The condition is not satisfied. The Envelop shape is not as m(t).So, we cant recover m(t) from this envelop.We cant build a simple receiver.

Modulation Indexnalhareqi-201347Let mp the peak amplitude of m(t) and A is the carrier amplitude.We define modulation index as:

When : this mean that for all t and the SAM(t) (modulated signal) can be demodulated by the envelop detector. When (overmodulation): this mean that A + m(t) is not 0 for all t and the option of envelop detection is no longer viable.

47Demodulationnalhareqi-201348In the standard AM modulation, the modulation index should be in order to demodulate the received modulated signal by using an envelop detector.

Spectrumnalhareqi-201349The spectrum of the modulated signal SAM(t) is the same as that of m(t) cos (ct) plus tow additional impulses at + c and - c .If

then

Spectrumnalhareqi-201350

USBLSBSpectrumnalhareqi-201351The modulated signal spectrum centered at c is composed of three parts:An impulse at the carrier frequency c The upper sideband (USB), a portion that lies above c whose highest frequency component is at c + M The lower sideband (LSB), a portion that lies below c whose lowest frequency component is at c - MThe bandwidth of the modulated waveform is twice the information signal bandwidth.

Transmission Powernalhareqi-201352The advantage of envelop detection in the standard AM has its price.Recall, the transmission power and the channel bandwidth are the two primary communication resources and should be used efficiently.The transmission of the carrier wave represent a waste of power.In the standard AM, only a fraction of the total transmitted power is actually for m(t). Single-Sideband (SSB) Modulationnalhareqi-201353Introductionnalhareqi-201354Standard AM and DSB-SC modulations has two sidebands: LSB and USB. These two methods waste the channel bandwidth because they both require a transmission bandwidth equal to twice the message bandwidth.

Introductionnalhareqi-201355Note that the USB and the LSB are symmetric about the carrier frequency.Hence, given the spectra of either sideband, we can determine the other.So, the transmission of either sideband is sufficient to reconstruct the message signal m(t) at the receiver.Thus, the bandwidth of the transmitted signal ( modulated signal) will be the bandwidth of the modulating signal ( baseband signal)Single-Sideband (SSB) Modulationnalhareqi-201356In single-sideband (SSB)modulation just only one sideband is transmitted.

Generation of SSB Signalsnalhareqi-201357One way to generate an SSB signal is to:generate a DSB signal first, and then suppress one of its sidebands by filtering ( band-pass filter designed to pass one of the sidebands of the modulated signal)

Generation of SSB Signalsnalhareqi-201358

The filter must have sharp cutoff characteristics to eliminate the undesired sidebandDemodulationnalhareqi-201359Demodulation of SSB signals can be achieved easily by using the coherent detector as used in the DSB demodulation, that is, by multiplyingby a local carrier and passing the resulting signal through a low-pass filter.

Demodulationnalhareqi-201360

Vestigial-Sideband (VSB) modulationnalhareqi-201361Vestigial-Sideband (VSB) modulationnalhareqi-201362Vestigial-Sideband (VSB) modulation is a compromise between SSB and DSB modulations. In this modulation scheme, one sideband is passed almost completely, whereas just a trace, or vestige, of the other sideband is retained.The typical bandwidth required to transmit a VSB signal is about 1.25 that of SSB. VSB is used for transmission of the video signal in commercial television broadcasting.Generation of VSB Signalsnalhareqi-201363A VSB signal can be generated by passing a DSB signal through a sideband-shaping filter [VSB filter].

nalhareqi-201364

Demodulation of VSBnalhareqi-201365The m(t) can be recovered by synchronous or coherent demodulation , that is, by multiplyingby a local carrier and passing the resulting signal through a low-pass filter.

nalhareqi-201366

ANGLE MODULATIONnalhareqi-201367Introductionnalhareqi-201368Another class of modulation methods are frequency and phase modulation which referred to as angle-modulation methods.In frequency-modulation (FM), the frequency of a carrier wave is changed by the message signal. In phase-modulation (PM) , the phase of the carrier is changed according to the variations in the message signal.

Introductionnalhareqi-201369ForallAM modulationschemesModulated spectrum isthefrequency translated message spectrumTransmission BWneverexceeds twicethemessage BWIn Anglemodulation Modulatedspectrumisnot a translated copy of the message spectrum.TransmissionBWisusuallymuchgreater than twice the message BW.The major benefit of the FM and PM modulation is their high degree of noise immunity.Introduction nalhareqi-201370Consider a sinusoid, Ac cos (ct+) where Ac is the (constant) amplitude, c is the (constant) frequency and is the initial phase.

In the AM modulation, the condition that Ac be a constant is relaxing and the amplitude become a function of the message signal m(t).the frequency and the phase remain constant and dont change or effect by the m(t).

Introductionnalhareqi-201371In the FM and PM modulation, Ac is a constant but ct+, instead of being constants it will be a function of m(t).We must extend the concept of a sinusoid to a generalized function whose frequency vary with time

Generalized Sinusoidal Signalnalhareqi-201372Let us consider a generalized sinusoidal signal given asAc cos ((t)) where the (t) is the instantaneous angle and is a function of t.

The generalized angle for the conventional sinusoid Ac cos (ct+) is (t) = ct+ Angle Modulated signalnalhareqi-201373So for angle modulation, the modulated carrier represented by Sangle_mod(t) = Ac cos ((t) ) where Ac is a constant amplitude and (t) is a function of the message signal m(t).We define the instantaneous radian frequency of the angle modulated wave i(t) as:

nalhareqi-201374

Phase Modulation (PM)nalhareqi-201375In phase modulation the angle is varied linearly with the message signal m(t) as(t) = ct + kp m(t)where kp is the phase deviation or sensitivity constant.Thus the phase modulated signal is defined as:SPM(t) = Ac cos (ct + kp m(t) ) The instantaneous radian frequency of SPM(t) is

Examplenalhareqi-201376If the message signal m(t) = a cos (mt) is used to phase modulate the carrier Ac cos (ct) Find the PM modulated signal

SPM(t) = Ac cos (ct + a kp cos (mt) )Frequency Modulation (FM)nalhareqi-201377In frequency modulation the angle is varied linearly with the integral of message signal m(t) aswhere kf is the frequency deviation or sensitivity constant.Thus the frequency modulated signal is defined as:

The instantaneous radian frequency of SFM(t) is

Examplenalhareqi-201378If the message signal m(t) = a cos (mt) is used to phase modulate the carrier Ac cos (ct) Find the FM modulated signal

Examplenalhareqi-201379In a frequency modulation process m(t) be a periodic triangular wave with mmax(t) =1 and mmin(t)=-1the carrier frequency is 100 kHzkf = 104 Hz/voltfind the maximum and minimum values of the instantaneous frequencyfi_max(t) = 100 *103 + 104 =110 kHzfi_min(t) = 100 *103 - 104 =90 kHz

The Relationship Between FM and PM nalhareqi-201380There is a close relation between FM and PM modulations.An FM modulated wave can be generated by first integrating the message signal m(t) with respect to time t and thus using the resulting signal as the input to a phase modulation.A PM modulated wave can be generated by first differentiating m(t) with respect to time t and then using the resulting signal as the input to a frequency modulatornalhareqi-201381

Bandwidth nalhareqi-201382The FM modulated wave is not band-limited. It has an infinite bandwidth and is not related to the modulating signal spectrum in any simple way, as was the case in AM modulation.Although the theoretical bandwidth of an FM wave is infinite, the most of the modulated signal power resides in a finite bandwidth.There are two distinct possibilities in terms of bandwidth: narrow-band FM and wide-band FM. Single-Tone Frequency Modulationnalhareqi-201383Consider a sinusoidal modulating signal defined asm(t) = Am cos( 2 fm t)

So, the instantaneous frequency (in Hertz) of the FM signal is fi(t) =fc + kf Am cos( 2 fm t) = fc + f cos( 2 fm t) where f is called the frequency deviation given by f =kf Am

The resultant FM signal is

is the modulation index

83Single-Tone Frequency Modulationnalhareqi-201384The frequency deviation factor indicates the amount of frequency change in the FM signal from the carrier frequency fc on either side of it. Thus FM signal will have the frequency components between (fc - f ) to (fc +f ). The modulation index, represents the phase deviation of the FM signal and is measured in radians. Depending on the value of , FM signal can be classified into two types: 1. Narrow band FM ( > 1).nalhareqi-201385