CHAPTER4: CONTINUOUS-WAVE (CW) MODULATION First semester King Saud University College of Applied studies and Community Service 1301CT

Communications System Model A typical communication system can be modeled as Input Transducer Transmitter ChannelReceiver Output Transducer Distortion And Noise Source Input message Input signal Transmitted signal Received signal Output signal Output message Destination Sender/ Transmitter Receiver 2

Introduction to Modulation The purpose of a communication system is to deliver the input message from the source in a recognizable form to the destination over the channel. To do this, the transmitter modifies the message signal into a form suitable for transmission over the channel. This modification is achieved by means of a process known as modulation. Modulation involves varying some parameter of a carrier signal in accordance with the message signal. 3

Introduction to Modulation The receiver re-create the original message signal from a degraded version of the transmitted signal after propagation through the channel. This re-creation is accomplished by using a process known as demodulation, which is the reverse of the modulation process used in the transmitter. 4

Introduction to Modulation However, 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 others 5

Modulation In general there are two main classes of modulation: continuous-wave modulation (CW) and pulse modulation. In CW modulation, a sinusoidal signal is used as the carrier. When 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). 6

Modulation In pulse modulation, the carrier consists of a periodic sequence of rectangular pulses. Pulse modulation can itself be of an analog or digital type. 7

Analog Communication System There are two types of communication systems: analog communication system and digital communication system. The CW modulation is used in the analog communication system. In CW modulation, the analog baseband signal is transmitted using a high frequency sinusoidal signal. In this system, the transmitter consists of a modulator and the receiver consists of demodulator 8

CW Modulation The channels of communication system are either be low-pass or band-pass channels. A low-pass channel has a bandwidth with frequencies between 0 and. A band-pass channel has a bandwidth with frequency between 1 and 2 9

CW Modulation 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 (ω=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. 10

CW Modulation To perform the transmission, the sender needs to modify the baseband signal into a form that is suitable for transmission over the channel. The modulation process cause a shift in the range of frequencies contained in the message signal into another frequency range suitable for transmission over the channel, centered onto the carrier frequency. So, the baseband signal can be transmitted through a band-pass channel 11

Benefits of Modulation In addition to facilitating transmission, there are some important reasons for modulation: Ease of Radiation Multiplexing Reduce Noise and Interference 12

Ease of Radiation If 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. 13

Ease of Radiation 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. 14

Multiplexing Consider 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 channel’s bandwidth my be much larger than that of the signal. 15

Multiplexing One 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 16

Reduce Noise and Interference The 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. 17

Reduce Noise and Interference Other 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. 18

Amplitude Modulation (AM) As 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. 19

Amplitude Modulation (AM) 20

Types of Amplitude Modulation (AM) 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) modulation Amplitude modulation (AM). Single-sideband modulation (SSB). Vestigial-sideband modulation (VSB). Each of these schemes has its own distinct advantages, disadvantages, and practical applications. 21

DOUBLE-SIDEBAND SUPPRESSED CARRIER (DSB-SC) MODULATION Modulation Spectrum Upper and Lower Sidebands Bandwidth Example Demodualtion 22

Modulation The modulating signal (information or baseband signal) The carrier signal is The modulated signal 23

Modulation 24

Modulation The 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 25 Envelop phase reversal

Spectrum The process of DSB-SC modulation shifts the spectrum of the modulating signal m(t) to the left and the right by ω c ( carrier frequency). if then 26

Spectrum 27

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

Upper and Lower Sidebands 29 ω c +ω M ω c - ω M

Bandwidth If the bandwidth of the modulating signal is B Hz, then the bandwidth of the modulated signal is 2B Hz. 30 2B 2πB2πB

Bandwidth In order to avoid the overlap of the spectra centered at ω c and –ω c, the ω c should be ≥ ω M. If ω c < ω M, the LSB of ω c will overlap with LSB of -ω c Which make it impossible to get back the m(t) from the modulated signal m(t) cos (ω c t). 31

Example Suppose that the modulating signal m(t) is a sinusoid of the form And the carrier signal is : Determine The DSB-SC signal S DSB (t) (modulated signal) The upper and lower sidebands The bandwidth Sketch the spectrum of the m(t) and S DSB (t) 32

Solution The DSB-SC signal By using the relation The S DSB (t) is 33

Solution The upper and lower sidebands USB = =1001KHz LSB = =999KHz The bandwidth of S DSB (t) BW = 2 * 10 3 =2000 Hz = 2 KHz 34

Solution Sketch the spectrum of the m(t) and S DSB (t) 35 M( ω ) S DSB ( ω ) f f 1000

Demodulation The DSP-SC demodulation consists of multiplication of the incoming modulated signal m(t) cos (ω c t ) by a carrier cos (ω c t) 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. 36 DemodulationModulation

Demodulation The multiplication will shift the modulated signal spectrum to the left and to the right by ω c and multiply it by one-half. Use a low pass filter to get the desired baseband spectrum and suppress the unwanted spectrum at + ω c and - ω c 37

38 modulated signal spectrum ωcωc -ωc-ωc 2ωc2ωc -2ω c After multiplication filter

Demodulation This 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 39