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6 - A M PL I T UD E M O D UL AT IO N

B A S I C P R I N C I P L E S

PRINCIPLES OFCOMMUNICATIONS

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AMPLITUDE MODULATION

Amplitude Modulation Amplitude modulation is defined as a system of modulation in which the amplitude of the carrier is made proportional to theinstantaneous amplitude of the modulating voltage.

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AM WAVE

Amplitude modulated wave and equation:

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AM PARAMETERS

Modulating Index and PercentModulation

The modulating index shouldnot exceed 1 (100% for thepercent modulation)

Greater than 1 modulating index will cause over-modulation ordistortion in the modulated waveform, which may affect theintelligence of the signal.

Assignment: What is adjacent channel interference due to over-modulation? How can over-modulation be prevented? (Explain indetail)

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SIDE BAND AND BANDWIDTH CALCULATIONS

AM signal:

Upper Sideband:

Lower Sideband:

Bandwidth Requirement:

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AM WAVEFORMS AND SPECTRUM

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TIME DOMAIN AND FREQUENCY DOMAIN WAVEFORMS

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AM Broadcast Standard

Highest ModulatingFrequency: 4.5kHz

Channel Bandwidth:

9kHz [10kHz USStandard]

Station Spacing: 4Channels

AM BroadcastBand: 535kHz to1605kHz

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AM BROADCAST BAND

Philippine AM Radio Broadcasting

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POWER RELATIONS IN THE AM WAVE

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SINGLE TONE MODULATION

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MULTITONE MODULATION

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PROBLEM SET 1

1. The tuned circuit of the oscillator in a simple AM transmitter employs a 50- Hcoil and a 1-nF capacitor. If the oscillator output is modulated by audiofrequencies up to 10kHz, what is the frequency range occupied by the sidebands?

2. The antenna current of an AM transmitter is 8A when only the carrier is sent, but

it increases to 8.93 A when the carrier is sinusoidally modulated. Find thepercentage modulation. Determine the antenna current when the depth of modulation is 0.8.

3. A 400W carrier is modulated on a depth of 75 percent; calculate the total powerin the modulated wave of standard AM signal.

4. A certain transmitter radiates 9kW with the carrier un-modulated, and 10.125 kW when the carrier is sinusoidally modulated. Calculate the modulation index. If another sine wave, corresponding to 40 percent modulation, is transmittedsimultaneously, determine the total radiated power.

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PROBLEM SET 2 FROM FREZNEL BOOK

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