1. What is modulation?• Modulation is the process of putting information onto a high frequency carrier for transmission (frequency translation).
• Once this information is received, the low frequency information must be removed from the high frequency carrier. This process is known as “ Demodulation”.
2. What are the reasons for modulation?1. Frequency division multiplexing (To support multiple transmissions via a single channel) To avoid interference
M1(f) M(f) Multiplexed f signal0 + M2(f) 0 f1 f2 f f0
2. Practicality of Antennas Transmitting very low frequencies require antennas with miles in wavelength
ANALOG AND DIGITALAnalog-to-analog conversion is the representation ofanalog information by an analog signal. One may askwhy we need to modulate an analog signal; it isalready analog. Modulation is needed if the medium isbandpass in nature or if only a bandpass channel isavailable to us.Topics discussed in this section:Amplitude ModulationFrequency ModulationPhase Modulation
Figure Types of analog-to-analog modulation
Figure Amplitude modulation
Note The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.
Figure AM band allocation
NoteThe total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.
4. What are the Basic Types of Analogue Modulation Methods ?Consider the carrier signal below: sc(t ) = Ac(t) cos( 2πfc t + θ ) 1. Changing of the carrier amplitude Ac(t) produces Amplitude Modulation signal (AM) 2. Changing of the carrier frequency fc produces Frequency Modulation signal (FM) 3. Changing of the carrier phase θ produces Phase Modulation signal (PM)
5. What are the different Forms of Amplitude Modulation ?1. Conventional Amplitude Modulation (DSB-LC) (Alternatively known as Full AM or Double Sideband with Large carrier (DSB-LC) modulation2. Double Side Band Suppressed Carrier (DSB-SC) modulation3. Single Sideband (SSB) modulation4. Vestigial Sideband (VSB) modulation
Conventional Amplitude Modulation (Full AM)
6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)
6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)1 The carrier signal is sc (t ) = Ac cos(ω c t ) where ω c = 2πf c2 In the same way, a modulating signal (informationsignal) can also be expressed as sm (t ) = Am cos ωm t
3 The amplitude-modulated wave can be expressed as s (t ) = [ Ac + sm (t )] cos(ω c t )4 By substitution s (t ) = [ Ac + Am cos(ω mt )] cos(ω c t )5 The modulation index. Am m = Ac
6 Therefore The full AM signal may be written as s (t ) = Ac (1 + m cos(ω mt )) cos(ω c t ) cos A cos B = 1 / 2[cos( A + B ) + cos( A − B )] mAc mAcs (t ) = Ac (cos ω ct ) + cos(ω c + ω m )t + cos(ω c − ω m )t 2 2
7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth fc-fm fC fc+fm 2fm
8. Draw Frequency Spectrum for a complex input signal with AM fc-fm fc fc+fm
Frequency Spectrum of an AM signalThe frequency spectrum of AM waveform containsthree parts: 1. A component at the carrier frequency fc 2. An upper side band (USB), whose highest frequency component is at fc+fm 3. A lower side band (LSB), whose highest frequency component is at fc-fmThe bandwidth of the modulated waveform is twice theinformation signal bandwidth.
• Because of the two side bands in the frequency spectrum its often called Double Sideband with Large Carrier.(DSB- LC)• The information in the base band (information) signal is duplicated in the LSB and USB and the carrier conveys no information.
Modulation Index (m) 9. What is the significance of modulation index ?• m is merely defined as a parameter, which determines the amount of modulation.• What is the degree of modulation required to establish a desirable AM communication link? Answer is to maintain m<1.0 (m<100%).• This is important for successful retrieval of the original transmitted information at the receiver end.
• If the amplitude of the modulating signal is higher than the carrier amplitude, which in turn implies the modulation index m ≥ 1.0(100%) . This will cause severe distortion to the modulated signal.
10. Calculate the power efficiency of AM signals• The ratio of useful power, power efficiency : sidebands power m2 / 2 m2 = = total power 1 + m / 2 2 + m2 2• In terms of power efficiency, for m=1 modulation, only 33% power efficiency is achieved which tells us that only one-third of the transmitted power carries the useful information.