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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
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
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.
what is the actual application of fullerenes nowadays?
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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Berger describes sociologists as concerned with
Mueller Reply
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Mapo Reply
The formulation of new memories is sometimes called ________, and the process of bringing up old memories is called ________.
Mapo Reply
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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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