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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

find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
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how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
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I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
is it a question of log
Commplementary angles
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algebra 2 Inequalities:If equation 2 = 0 it is an open set?
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or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
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Differences Between Laspeyres and Paasche Indices
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In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
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after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
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Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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the Beer law works very well for dilute solutions but fails for very high concentrations. why?
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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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