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Amplitude Modulation and Single-Sideband Modulation

Amplitude Modulation Principles Top

Information signals such as voice, video, or binary data are sometimes transmitted directly from one point to another over some communications medium.

AM Signal

	Modulation and Demodulations Defined. Top
	The previous section described transmission using analog and digital signals. If either analog or digital signals were used exclusively, communications would be simplified and this section would not be needed.

The solution to this problem is devices a PC’s digital signal to analog signal: a modem (short formodulating/demodulating).

Computer data Transmitted over telephone line

example of analog device communicating using digital signal in the telephone systems

Voice information transmitted digitally

Amplitude Modulation. Top

Amplitude modulation is defined as a carrier frequency whose amplitude is varied in proportion to the instaneous amplitude of modulating voltage. The modulation voltage is referred to as intelligence.

	Amplitude Modulation Defined.
	Modulation is the process of modifying the characteristic of one signal in accordance with some characteristic of another signal. In most cases, the information signal, be it voice, video, binary data, or some other information.

	Amplitude Modulation with Sine Wave.
	In one modulation technique, known as amplitude modulation, or Am, the amplitude, or height ,of the sine wave varies to transmit the ones and zeroes. Another name for amplitude modulation is amplitude shift keying, or ASK, since data is transmitted by shifting amplitudes.

The bit pattern 00101001 is being transmitted using amplitude modulation .

Amplitude Modulation.

The AM Waveform. Top

Relationships unmodulated and modulated have a relate forsignals RF carrier RF modulating signal Modulated RF carrier

The AM Waveform

Amplitude Modulation with Signals. Top

When signals other than a sine wave are used to amplitude modulate the carrier frequency, the envelope still reflects the original shape of the modulating waveform. In addition, the spectral (frequency) content of the modulation wave is preserved in hte LSB and USB > Figure 1.5 depicts various AM wave forms and their respective modulation voltages.

Digital,usually binary,signals may also be used to amplitude modulate a carrier.

Amplitude modulation of a carrier with a binary signal.

Some digital signal have more than two levels. As long as a signal varies in discretesteps. it is considered digital

Multilevel digital AM.

Single Sideband Communications. Top

	Single Sideband
	An inherent problem with AM or , more specifically , double - sideband full carrier (DSBFC) is the enormous amount of power containted in the carrier frequency.

	Double Sideband
	Double sideband support carrier. Suppressing the carrier component of the AM signal so that the transmitted wave only consists of the upper and lower sidebands is called double-sideband-suppressed carrier (DSBSC). This Figure shows the time and frequency-domain representations a DSBSC signal with sine-wave modulation.

	Double and Single Sideband
	Amplitude modulation generates two sets of sidebands, each containing the same information. The information is redundant in an AM or a DSB signal. Therefore, all the information can be conveyed in just one sideband. Eliminating one sideband produces a single-sideband (SSB) signal. Eliminating the carrier and one sideband produces a more efficient AM signal.

A typical DSB signal.

A time-domain display of a DSB AM signals.

Single Sideband Benefits. Top

In a DSB signal the basic information is transmitted twice, once in each sideband. Since the sidebands are the sum and difference of the carrier and modulating signals, the information must be contained in both of them. As it turns out, there is absolutely no reason to transmit both sidebands in order to covey the information. One sideband may be suppressed. The remaining sideband is called a single- sideband suppressed carrier (SSSC or SSB) signal.

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