Digital Modulation Techniques

Topics: Modulation, Phase-shift keying, Data transmission Pages: 9 (2154 words) Published: November 25, 2012


Topic: Digital Modulation Techniques

Course code: ELE102

Course title: Electrical science-II

Submitted to:Submitted by:

Subrahmanyam Tanala Sirvishwajeet kumar




• 1 Aim
• 2 Analog modulation methods
• 3 Digital modulation methods
o 3.1 Fundamental digital modulation methods
o 3.2 Modulator and detector principles of operation o 3.3 List of common digital modulation techniques
• 4 Digital baseband modulation or line coding
• 5 Pulse modulation methods
• 6 Miscellaneous modulation techniques
• 7 References

In electronics, modulation is the process of varying one or more properties of a high frequency periodic waveform, called the carrier signal, with respect to a modulating signal. This is done in a similar fashion as a musician may modulate a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also occur. Aim

The aim of digital modulation is to transfer a digital bit stream over an analog passband channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band. The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal, over an analog passband channel, for example a limited radio frequency band or a cable TV network channel.

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2 Analog modulation methods
In analog modulation, the modulation is applied continuously in response to the analog information signal. [pic]
A low-frequency message signal (top) may be carried by an AM or FM radio wave. Common analog modulation techniques are:
• Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) o Double-sideband modulation (DSB)

▪ Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band) ▪ Double-sideband suppressed-carrier transmission (DSB-SC) ▪ Double-sideband reduced carrier transmission (DSB-RC) o Single-sideband modulation (SSB, or SSB-AM),

• Angle modulation
o Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) o Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) The accompanying figure shows the results of (amplitude-)modulating a signal onto a carrier (both of which are sine waves). At any point along the y-axis, the amplitude of the modulated signal is equal to the sum of the carrier signal and the modulating signal amplitudes. [pic]

Simple example of amplitude modulation.
3 Digital modulation methods
In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet). [pic]

Schematic of 4 baud (8 bps) data link.
A simple example: A telephone line is designed for transferring...
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