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) Multiplexed signal M(f) f 0 + M2(f) 0 f1 f2 f f 0
2. Practicality of Antennas Transmitting very low frequencies require antennas with miles in wavelength
3. What are the Different of Modulation Methods? 1.Analogue modulation- The modulating signal and carrier both are analogue signals Examples: Amplitude Modulation (AM) , Frequency Modulation (FM) , Phase Modulation (PM) 2. Pulse modulation- The modulating signal is an analogue signal but Carrier is a train of pulses Examples : Pulse amplitude modulation (PAM), Pulse width modulation (PWM), Pulse position modulation (PPM)
3.What are the Different of Modulation Methods? 3. Digital to Analogue modulation- The modulating signal is a digital signal , but the carrier is an analogue signal. Examples: Amplitude Shift Keying (ASK), FSK, Phase Shift Keying (PSK) 4. Digital modulation - Examples: Pulse Code Modulation, Delta Modulation,Adaptive Delta Modulation
ANALOG AND DIGITAL Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us. Topics discussed in this section: Amplitude ModulationFrequency ModulationPhase Modulation
Note The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.
Note The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.
Note The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal:BPM = 2(1 + β)B.
4. What are the Basic Types of Analogue Modulation Methods ? • Consider the carrier signal below: • sc(t ) = Ac(t) cos( 2fc t+ ) • 1. Changing of the carrier amplitude Ac(t)produces • Amplitude Modulation signal(AM) • 2. Changing of the carrier frequencyfc produces • Frequency Modulation signal (FM) • 3. Changing of the carrier phase 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) modulation 2. Double Side Band Suppressed Carrier(DSB-SC) modulation 3. Single Sideband(SSB) modulation 4. Vestigial Sideband(VSB) modulation
6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC) 1 The carrier signal is 2 In the same way, a modulating signal (information signal) can also be expressed as
3 The amplitude-modulated wave can be expressed as 4 By substitution 5 The modulation index.
7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth
7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth fC fc-fm fc+fm 2fm
8. Draw Frequency Spectrum for a complex input signal with AM fc-fm fc fc+fm
Frequency Spectrum of an AM signal • The frequency spectrum of AM waveform contains three parts: • 1. A component at the carrierfrequency 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-fm • The bandwidth of the modulated waveform is twice the information 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.
Example We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM?
Example We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM? Solution An AM signal requires twice the bandwidth of the original signal: BW = 2 x 5 KHz = 10 KHz
9. What is the significance of modulation index ? Modulation Index (m) • 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.
9. What is the significance of modulation index ? Modulation Index (m)
If the amplitude of the modulating signal is higher than the carrier amplitude, which in turn implies the modulation index . This will cause severe distortion to the modulated signal.
10. Calculate the power efficiency of AM signals Power distribution in full AM
10. Calculate the power efficiency of AM signals • The ratio of useful power, power efficiency : • 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.
Double Side Band Suppressed Carrier (DSB-SC) Modulation • The carrier component in full AM or DSB-LC does not convey any information. Hence it may be removed or suppressed during the modulation process to attain higher power efficiency. • The trade off of achieving a higher power efficiency using DSB-SC is at the expense of requiring a complex and expensive receiver due to the absence of carrier in order to maintain transmitter/receiver synchronization.
11. Derive the Frequency Spectrum for Double Sideband Suppressed Carrier Modulation (DSB-SC) 1 Consider the carrier 2 modulated by a single sinusoidal signal 3 The modulated signal is simply the product of these two