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Chapter 2 amplitude_modulation

by Hattori Sidek, student at suksis on Dec 03, 2011

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Chapter 2 amplitude_modulationPresentation Transcript

• CHAPTER 2: AMPLITUDE MODULATION
• TOPICS
• Need for Modulation
• Principles of AM
• Modulation Index and Signal Power
• AM Circuits
• Single Sideband Suppressed Carrier (SSBSC)
• SSB Circuits
• NEED FOR MODULATION
• It is because modulation makes the information signal more compatible with the medium.
• Modulation = Imposing information at low frequency onto a higher frequency signal.
• A technique for transmitting information efficiently from one place to another.
• Simplest form of modulation is the amplitude modulation.
• PRINCIPLES OF AMPLITUDE MODULATION
• PRINCIPLES OF AM
• AM is defined as:
• Amplitude of carrier frequency change proportionately to the value of the modulation signal.
• Simple modulator circuits
• Cheap :low-quality form of modulation used for commercial broadcasting of audio & video signal.
• Poor performance due to noise
• Inefficient use of transmitter power.
• Application:
• AM modulators are nonlinear devices
• 2 input and 1 output: modulating signal and carrier signal.
• Several types of amplitude modulation
• AM DSBFC
• DSB-SC
• SSB
• AM generation is shown in Figure 2.1
• Modulated wave = AM envelope as shown in Figure 2.2
Figure 2.1: Block diagram of Amplitude Modulation Information, V m (t) Carrier, V c (t) Modulator V AM (t)
• Figure 2.2 AM signal with the envelope
• AM IN ACTION
• AM begins with carrier v c ,  a sine wave with frequency  c & amplitude V c :
• Modulating signal:
• Then AM is:
DERIVATION OF AM EQUATION
• Where m (modulation index) is defined as V m / V c , hence:
• The voltage resulting AM wave envelope at any instant is:
• This yield, the upper and lower sidebands – frequency & amplitude.
Using Trigo ID Carrier LSB USB
• AM FREQUENCY SPECTRUM & BANDWIDTH
• AM modulators are non-linear device => non-linear mixing occurs.
• Output envelope is complex wave made up of DC voltage, carrier frequency, the sum ( f m + f c ) & difference ( f c – f m ) frequencies.
• AM spectrum contains frequency component spaced f m Hz on either side of the carrier.
• Figure 2.3 shows the frequency spectrum of AM wave.
• Figure 2.3: Frequency spectrum for AM wave f c f c + f m f c - f m Bandwidth = 2f m V c V m /2 V m /2
• SPECTRUM PARAMETERS
• Center frequency = Carrier frequency =
• Upper sideband freq. = carrier freq. + modulating freq.
• Lower sideband freq. = carrier freq. - modulating freq.
• Center frequency peak amplitude:
• Upper and lower sideband voltages:
• Bandwidth = Maximum freq. - minimum freq.
• EXAMPLE 2.1
• Q. Modulating signal f m =3 kHz frequency and a carrier frequency f c =1 M Hz. What is the upper & lower sideband frequency? Then find the bandwidth of the modulated signal.
• A. 997 kHz, 1003 kHz, 6 kHz.
• EXAMPLE 2.2
• Q. A 1.4 MHz carrier is modulated by a signal with frequencies from 20Hz & 10KHz. Determine the range of frequencies generated for the upper and lower sidebands?
• USB = 1.400020Hz, 1.410000Hz,
• LSB = 1.390000Hz, 1.399980Hz