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Even within the frequency range of the message signal, the medium can affect different frequencies in different ways. (These are called frequency selective effects .) For example, a signal may arrive at the receiver, and a momentlater a copy of the same signal might arrive after having bounced off a mountain or a nearby building.This is called multipath interference , and it can be viewed as a sum of weighted and delayed versions

Analog AM communication system
Analog AM communication system

of the transmitted signal. This may be familiar to the (analog broadcast) TV vieweras “ghosts,” misty copies of the original signal that are shifted and superimposed over the main image.In the simple case of a sinusoid, a delay corresponds to a phase shift,making it more difficult to reassemble the original message. A special filter called the equalizer is often added to the receiver to help improve the situation.An equalizer is a kind of “deghosting” circuit, We refrain from calling these ghost busters. and equalization is addressed in detail in [link] .

Sampling at the receiver

Because of the proliferation of inexpensive and capable digital processors, receivers often contain chipsthat are essentially special purpose computers. In such receivers, many of the functions that are traditionally handled bydiscrete components (such as analog oscillators and filters) can be handled digitally.Of course, this requires that the analog received signal be turned into digital information (a series of numbers) that acomputer can process. This analog-to-digital conversion (A/D) is known as sampling .

Sampling measures the amplitude of the waveform at regular intervals, and thenstores these measurements in memory. Two of the chief design issues in a digital receiverare the following:

  • Where should the signal be sampled?
  • How often should the sampling be done?

The answers to these questions are intimately related to each other.

When taking samples of a signal, they must be taken fast enough so that important information is not lost.Suppose that a signal has no frequency content above f * Hz. The widely known Nyquist reconstruction principle (see [link] ) says that if sampling occurs at a rate greater than 2 f * samples per second, it is possible to reconstruct the original signal fromthe samples alone. Thus, as long as the samples aretaken rapidly enough, no information is lost. On the other hand, when samples are taken too slowly,the signal cannot be reconstructed exactly from the samples, and the resulting distortion is called aliasing .

Accordingly, in the receiver, it is necessary to sample at least twice as fast as the highest frequency present in the analog signal being sampledin order to avoid aliasing. Because the receiver contains modulators that change thefrequencies of the signals, different parts of the system have different highest frequencies.Hence the answer to the question of how fast to sample is dependent on where the samples will be taken.

The sampling

  1. could be done at the input to the receiver at a rate proportional to the carrier frequency,
  2. could be done after the downconversion, at a rate proportional to the rate of the symbols, or
  3. could be done at some intermediate rate.

Questions & Answers

what is the stm
Brian Reply
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Rafiq
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Damian
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LITNING Reply
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LITNING
scanning tunneling microscope
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Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
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Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
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what king of growth are you checking .?
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yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
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Kyle
biomolecules are e building blocks of every organics and inorganic materials.
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research.net
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sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
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nano basically means 10^(-9). nanometer is a unit to measure length.
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NANO
how can I make nanorobot?
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s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
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Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
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Source:  OpenStax, Software receiver design. OpenStax CNX. Aug 13, 2013 Download for free at http://cnx.org/content/col11510/1.3
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