Amplitude modulation

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What is AM?

Modulation Scheme, have been widely used in Analog Radio.used by e.g. by analog radio.

An Amplitude Modulator modulates the Amplitude of an sinoid Carrier signal using the incoming signal.

The Carrier signal is sinusoid with a fixed Frequency, usually much higher than the highest frequency occuring in the spectrum of the modulating signal.

Mathematical Setting


The first formula desribes what happens in time domain. It's basically multiplying of two cosine signals leading to the time domain picture in the example below.

The second formula describes the frequency response, the frequency spectrum consists of three spectrum lines at each time t. The first term is the carrier, for shure the carrier is in the spectrum, the second and third termes describe two spectrum lines around the carrier, and their amplitude is dependend on the modulation index m. As the modulating sognakll variies the lines around the carrier frequency are variing.

we should add an frequeny response picture below in the example

An Example

Carrier Signal in time domain
File:AM picture4.jpg
Carrier Signal in frequency domain (to be added)
Modulating Signal in time domain
File:AM picture5.jpg
Modulating Signal in frequency domain (to be added)
AM Signal
File:AM picture6.jpg
AM Signal in frequency domain (to be added)

Special Kinds of AM

Aquired Bandwidth

An AM Signal shows up as single Peak in the Frequency Spectrum.

it would be nice to have a picture showing how AM signals in the frequency spectrum behave and how much bandwidth they allocate. A short overview of AM-related problems would be nice, too. Has anybody of you this info handy?

Noise Sensitivity

Echoes, multipath signaly, athmospheric and meteorologic phenomenons like lightings and electrostatic discharges contribute noise that simply gets added to the transmitted signal. Most other Modulation Schemes are less sensitive to noise.