A carrier wave is a pure wave of constant frequency, a bit like a sine wave. To include speech information or data information, another wave needs to be imposed, called an input signal, on top of the carrier wave. This process of imposing an input signal onto a carrier wave is called modulation.
In other words, modulation changes the shape of a carrier wave to somehow encode the speech or data information that we were interested in carrying. Modulation is like hiding a code inside the carrier wave. Recall that any wave has three basic properties: 1 Amplitude — the height of the wave 2 Frequency — a number of waves passing through in a given second 3 Phase — where the phase is at any given moment.
There are different strategies for modulating the carrier wave. First, a user can tweak the height of the carrier. This is called amplitude modulation or AM. Frequency of an input signal can also be changed. If this input signal is added to the pure carrier wave, it will thereby change the frequency of the carrier wave.
In that way, users can use changes of frequency to carry speech information. This is called frequency modulation or FM. Obviously, a vertical antenna of this size is impractible. On the other hand, for a frequency of 1 MHz, this height is reduced to 75 m.
This shows that for the same antenna length, power radiated is large for shorter wavelength. Thus, our signal which is of low frequency must be translated to the high frequency spectrum of the electromagnetic wave. This is achieved by the process of modulation. Other answers have described modulation as it applies to communications applications, where the information to be conveyed is superimposed onto a carrier.
My answer will apply the concept of modulation to power electronics applications, particularly in DC-AC inverters. Refer to Figure 1. This shows a typical 3-phase inverter bridge. Six electronic switches are arranged in three-phase bridge configuration, and are supplied by a DC voltage source. These switches are high-power electronic devices. The most suitable devices will be governed by conversion power, voltage level and switching frequencies requirements.
The load is shown as an induction motor, but may be the primary of a three-phase transformer or any other A. Figure 1: Three-phase Inverter Switches and Waveforms If each of the three phases are switched in this manner, but with the switching of each phase electrical degrees delayed from the previous phase, the three waveforms of the centre points VAO, VBO, VCO are as shown in the figure.
If the difference in voltage between any two phases is measured, VAB in the figure , the result is a 6-step or quasi-square wave. Thus the output of the three-phase inverter bridge is a three-phase waveform.
The frequency of the waveform is set by the switching frequency. The RMS voltage is controlled by modulating the waveform. In practice, the waveform is pulse width modulated with chops.. These chops are of varying size. The purpose is twofold: to make the inverter output currents more closely resemble a sine wave, and to enable adjustment of the RMS output voltage. Figure 2: Sinusoidal Modulation of Inverter Output Waveform A method of modulation often used is sinusoidal modulation.
Refer to Figure 2. A triangle waveform is compared with a sine waveform. When the sine wave exceeds the triangle wave, the top switch of the inverter phase is turned on. When the triangle wave exceeds the sine wave, the bottom switch is turned on. The resultant current waveform to the load is nearly sinusoidal with very little harmonic distortion. If the load is an induction motor, no further filtering is required. If the inverter is fixed frequency, such as in a UPS, then further filtering is needed.
The harmonic distortion content is a function of the modulation frequency of the carrier wave. At a high modulation frequency, the first significant harmonic will be at high frequency and low magnitude, which means that harmonics are relatively easy to filter out with a low-pass LC filter.
In the process of modulation, the baseband signal is translated i. This frequency shift is proportional to the frequency of carrier. Skip to content. Change Language. Related Articles. Table of Contents. Save Article. There are other sets of signals, such that every signal in the set is orthogonal to every other signal in the set. A simple orthogonal set is time multiplexed division TDM -- only one transmitter is active at any one time.
Other more complicated sets of orthogonal waveforms—Walsh codes and various pseudo-noise codes such as Gold codes and maximum length sequences—are also used in some communication systems. The process of combining these waveforms with data signals is sometimes called "modulation", because it is so very similar to the way modulation combines sine waves with data signals.
From Wikibooks, open books for an open world. There is 1 pending change awaiting review. Communication Systems. Category : Book:Communication Systems. Namespaces Book Discussion.
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