# Chapter 3 Digital Mod Part2

• Published on
15-Oct-2014

• View
29

• Download
5

Embed Size (px)

Transcript

<p>Chapter 35</p> <p>DIGITAL MODULATION TECHNIQUES</p> <p>Digital ModulationPart 2 a. Information capacity, Bits, Bit Rate, Baud, M-ary Encoding b. Digital Modulation Techniques - ASK, FSK, PSK, QAM</p> <p> Information capacity, Bits &amp; Bit Rate Represents the number of independent symbols that can be carried through a system in a given unit of time. Binary digit or bit is the basic unit of information in communication. It is the amount of information stored by a digital device Bit rate is the number of bits that are conveyed or processed per unit of time - Express the information capacity</p> <p>Hartleys Law</p> <p>I w BvtWhere I = information capacity (bps) B = bandwidth (Hz) t = transmission time (s)</p> <p>From the equation, Information capacity is a linear function of bandwidth and transmission time and directly proportional to both.</p> <p>Shannons FormulaS I ! B log 2 (1 N )</p> <p>or</p> <p>I ! 3.32 B log10 (1 </p> <p>S N</p> <p>)</p> <p>Where I = information capacity (bps) B = bandwidth (Hz)</p> <p>S N = signal to noise power ratio (unitless)</p> <p>The higher S/N the better the performance and the higher the information capacity Shannon-Hartley theorem tells the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise</p> <p>ExampleBy using the Shannons Formula, calculate the information capacity if S/N = 30 dB and B = 2.7 kHz.</p> <p>Baud &amp; Minimum BW Baud refers to the rate of change of a signal on the transmission medium after encoding and modulation have occurred. "Baud" is the name for an information "symbol."</p> <p>1 baud ! tsWhere baud = symbol rate (symbol per second) ts = time of one signaling element @ symbol (seconds)</p> <p>Contd Minimum Bandwidth Using multilevel signaling, the Nyquist formulation for channel capacity</p> <p>f b ! 2 B log 2 MWhere fb= channel capacity (bps) B = minimum Nyquist bandwidth (Hz) M = number of discrete signal or voltage levels</p> <p>ContdFor B necessary to pass M-ary digitally modulated carriers</p> <p> fb B! log M 2 </p> <p> fb ! N ! baud </p> <p>Where N is the number of bits encoded into each signaling element.</p> <p>M-ary Encoding It is often advantageous to encode at a level higher than binary where there are more then two conditions possible. The number of bits necessary to produce a given number of conditions is expressed mathematically as</p> <p>N ! log 2 MWhere N = number of bits necessary M = number of conditions, level or combinations possible with N bits.</p> <p>Contd Each symbol represents n bits, and has M signal states, where M = 2N.</p> <p>ExampleFind the number of voltage levels which can represent an analog signal witha. 8 bits per sample b. 12 bits per sample</p> <p>There are several digital modulation techniques used to modulate digital signal or data, depending on the applications, the rate of transmission required, allocated bandwidth and cost. Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM)</p> <p>Forms of Digital Modulation</p> <p>v(t ) ! V sin( 2Tft U )If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK) If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)</p> <p>Contd If the phase, of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK) If both the amplitude and the phase, of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)</p> <p>Contd...</p> <p>Amplitude Shift Keying (ASK)In ASK, a carrier wave is switched ON and OFF by the input data or binary signals. During a mark (binary 1), a carrier wave is transmitted and during a space (binary 0), the carrier is suppressed. Also known as ON-OFF Keying (OOK)</p> <p>ASK Contd...</p> <p>ASK Waveform</p> <p>ASK Contd...Applications of ASK</p> <p>It is used in multichannel telegraph system. Simple ASK is no longer used in digital communication systems due to noise problems</p> <p>Frequency Shift Keying (FSK) Two different carrier frequencies are used and they are switched ON and OFF by the binary signals. Binary 1 or mark switches one carrier ON while the other carrier is OFF Binary 0 or space switches the second carrier ON while the first carrier is OFF At the Tx, the pulse generated will shift the frequency to either 2 frequencies used, producing 2 tones This signal then transmitted through the telephone line</p> <p>FSK contd The Rx the detect the tone by differentiating the 2 frequencies &gt; converts to appropriate voltage level.s1 f1 Data s2 f2 FSK signal</p> <p>FSK contd The Rx the detect the tone by differentiating the 2 frequencies &gt; converts to appropriate voltage level.s1 f1 Data s2 f2 FSK signal</p> <p>FSK Waveform</p> <p>FSK contdApplications of FSK Used mainly for low-speed digital data transmission Keyboard-type computer terminal modems that connect the terminal to a main computer dial up lines Advantages FSK over ASK ASK need automatic gain control (AGC) to overcome fading effects Relatively easy for FSK generation The constant amplitude property of the carrier does not waste power and does produce some immunity to noise</p> <p>Phase Shift Keying (PSK) Another form of angle-modulated, constant amplitude digital modulation. Binary digital signal input &amp; limited number of output phases possible.Data</p> <p>PSK Modulator</p> <p>PSK signal</p> <p>Carrier</p> <p>PSK contd Binary signals are used to switch the phase of a carrier wave between 2 values which are normally 00 and 1800 For binary 1, the carrier has one phase For binary 0, the carrier is reversed by 1800 Also known as Phase-Reversal Keying (PRK)</p> <p>PSK Waveform</p> <p>ExampleFor the digital message 1101 1100 1010, sketch the waveform for the following: a. ASK b. FSK c. PSK</p>