lecture_4

# lecture_4 - LECTURE#4 AM MODULATION PART I INTRODUCTION TO...

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LECTURE #4 AM MODULATION PART I: INTRODUCTION TO AMPLITUDE MODULATION (AM) BACKGROUND INFORMATION 1. AM is the oldest method of impressing information onto a carrier signal. 2. What are the some of the applications of AM? a) local broadcast (535 – 1620 kHz in the USA) b) aircraft communications in the 118-138 MHz band c) shortwave broadcasts in the HF bands (3-30 MHz), which affords worldwide coverage d) analog television, in which an AM carrier is used for the picture and a separate FM carrier frequency is used to carry the sound e) data communications, in which AM and PM (phase modulation) are used together in high-speed modems. 3. Radio uses a high-frequency sine wave called a carrier to move information from the transmitter to the receiver. Intelligence can be impressed onto a carrier by AM, FM and PM. AM SIGNALS IN THE TIME DOMAIN 1. We use an oscilloscope to determine what type of information is modulating the transmitter as well as the percentage of modulation. 2. In an AM signal, the information is carried on top of the RF carrier. The actual shape of the carrier is altered by the addition of the information during the process of modulation.

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3. c m V V m = m = modulation index V m = modulation of index V c = carrier voltage min max min max V V V - V m + = V max = maximum waveform voltage (the peak) V min = minimum waveform voltage (the trough) 4. Modulation can be as a percent or decimal % mod = 100 × m, where m is a decimal Example 1 Compute the modulation index as a decimal and percent when V m is 40 V and V c is 70 V. 0.57 V 70 V 40 V V m c m = = = m = 0.57 × 100 = 57 % 5. Instantaneous Modulating Voltage Signal V(t) = (V c + V m sin ω m t) sin ω c t and V(t) = V c [1 + m(sin ω m t] sin ω c t ω m = modulating angular frequency (rad/s) ω c = carrier angular frequency (rad/s) V(t) = instantaneous voltage
Example 3 A carrier wave with an RMS voltage of 2 V and a frequency of 1.5 MHz is modulated by a sine wave with a frequency of 500 Hz and amplitude of 1 V RMS. Write the equation for the modulating voltage. f m = modulating electrical frequency f m = 500 Hz f c = carrier electrical frequency f c = 1.5 × 10 6 Hz We need peak voltages and radian frequencies.

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