2 Overview Communication systems Analog Modulation Digital Modulation AMFMDigital ModulationASKFSKModems
3 Communication systems DigitalAnalogThe block diagram on the top shows the blocks common to all communication systems
4 Remember the components of a communications system: Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicatingTransmitter: The device that sends the transduced signalTransmission channel: The physical medium on which the signal is carriedReceiver: The device that recovers the transmitted signal from the channelOutput transducer: The device that converts the received signal back into a useful quantity
5 Analog ModulationThe purpose of a communication system is to transmit information signals (baseband signals) through a communication channelThe term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducerFor example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of HzThe “hello” wave is a baseband signal:
6 Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulationRemember the radio spectrum:For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band)The FM radio system must operate with frequencies in the range of MHz (VHF band)AM radioFM radio/TV
7 Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorilyThis process is accomplished by a device called a modulatorThe transmitter block in any communications system contains the modulator deviceThe receiver block in any communications system contains the demodulator deviceThe modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrumFor example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrumFor AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum)The demodulator extracts the original baseband signal from the received modulated signalTo Summarize:Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signalModulation is done to bring information signals up to the Radio Frequency (or higher) signal
8 Basic analog communications system Baseband signal (electrical signal)EM waves (modulated signal)TransmitterInput transducerTransmission ChannelModulatorEM waves (modulated signal)CarrierBaseband signal (electrical signal)ReceiverOutput transducerDemodulator
9 Types of Analog Modulation Amplitude Modulation (AM)Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constantFrequency Modulation (FM)Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constantPhase Modulation (PM)Another form of analog modulation technique which we will not discuss
11 Amplitude Modulation Uses a higher frequency carrier Most efficient use of frequencyTime and Frequency DomainSusceptible to Noise
12 Sine Wave Peak Amplitude (A) Frequency (f) Phase () General Sine wave maximum strength of signalvoltsFrequency (f)Rate of change of signalHertz (Hz) or cycles per secondPeriod = time for one repetition (T)T = 1/fPhase ()Relative position in time, from 0-2*piGeneral Sine wave
13 S(t) = Ac(1 + Ka m(t))cos(2fct) Where Ka is a constant.m(t) is the baseband message. For an audio signal, the spectrum of m(t) would typically be in the range of 300 Hz - 3 kHz.fc is the carrier frequencym(t) = Am cos(2fmt)Where fc >> fm.
22 Frequency Domain Unmodulated Modulated watts Carrier Signal frequency BasebandwattsModulatedCarrierSignalfrequencyBasebandBaseband
23 The AM WaveformThe three components that form the AM waveform are listed below:1. The lower-side frequency (fc - fi)2. The carrier frequency (fc)3. The upper-side frequency (fc + fi)If a radio signal of frequency with1000KHz is mixed with an audio signal of 1 KHz the output will beA signal at 1000 KHz (Carrier wave)A signal at 1001 KHz (upper sideband)A signal at 999 KHz (lower sideband)
25 Upper and Lower Sidebands In most systems the intelligence signal is a complex waveform containing components from roughly 200Hz to 3KHz.If this complex waveform is used to modulate the carrier there would be a whole band of side frequencies.The band of frequencies above the carrier is term the upper sideband.The band of frequencies below the carrier is called the lower sideband.
26 ExampleA 1.4MHz carrier is modulated by a music signal that has frequency components from 20Hz to 10kHz. Determine the range of frequencies generated for the upper and lower sidebands.
32 Points to be Remembered Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constantThe function of the carrier in AM is simply to provide a signal to heterodyne (mix) with the modulated audio, to convert all the AF components to a higher frequency.The bandwidth of an AM signal is equal to twice the highest freq.[ In commercial AM broadcast tx’s the freq of the modulating audio is permitted to be as high as 4.5 KHz, and since a double side system is used , commercial stations therefore have a bandwidth of 9 KHz]The bandwidth does not depend on the power of the modulating signal.