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TRUE or FALSE: Quadrature amplitude modulation can combine two real, basebandmessages of absolute bandwidth B in a radio-frequency signal of absolute bandwidth B .

Consider the schematic shown in [link] with the absolute bandwidth of the baseband signal x 1 of 6 MHz and of the baseband signal x 2 ( t ) of 4 MHz, f 1 = 164 MHz, f 2 = 154 MHz, f 3 = 148 MHz, f 4 = 160 MHz, f 5 = 80 MHz, Φ = π / 2 , and f 6 = 82 MHz.

  1. What is the absolute bandwidth of x 3 ( t ) ?
  2. What is the absolute bandwidth of x 5 ( t ) ?
  3. What is the absolute bandwidth of x 6 ( t ) ?
  4. What is the maximum frequency in x 3 ( t ) ?
  5. What is the maximum frequency in x 5 ( t ) ?
The transmission system of Exercise 5-16.
The transmission system of [link] .

Thus the inefficiency of real-valued double-sided AM transmission can be reduced using complex valued quadrature modulation, whichrecaptures the lost bandwidth by sending two messages simultaneously. For simplicity and clarity, the bulk of Software Receiver Design focuses on the real PAM case. There are also other ways of recapturingthe lost bandwidth: both single side band and vestigial sideband (discussed in the document Other Modulations on the website) send a single message, but use only half the bandwidth.

Injection to intermediate frequency

All the modulators and demodulators discussed in the previous sectionsdownconvert to baseband in a single step, that is, the spectrumof the received signal is shifted by mixing with a cosine of frequency f c that matches the transmission frequency f c . As suggested in [link] , it is also possible to downconvert to somedesired intermediate frequency (IF) f I (as depicted in [link] ), and to then later downconvert to baseband by mixing with a cosineof the intermediate frequency f I . There are several advantagesto such a two-step procedure:

  • all frequency bands can be downconverted to the same IF, which allows use of standardized amplifiers, modulators and filterson the IF signals, and
  • sampling can be done at the Nyquist rate of the IF rather than the Nyquist rate of the transmission.

The downconversion to an intermediate frequency (followed by bandpass filtering to extract the passband around theIF) can be accomplished in two ways: by a local oscillator modulating from above the carrierfrequency (called high-side injection) or from below (low-side injection). To see this,consider the double sideband modulation (from "Amplitude Modulation with Suppressed Carrier" ) that creates the transmitted signal

v ( t ) = 2 w ( t ) cos ( 2 π f c t )

from the message signal w ( t ) and the downconversion to IF via

x ( t ) = 2 [ v ( t ) + n ( t ) ] cos ( 2 π f I t ) ,

where n ( t ) represents interference such as noise and spurious signals from other users.By the frequency shifting property (A.33),

V ( f ) = W ( f + f c ) + W ( f - f c ) ,

and the spectrum of the IF signal is X ( f )

= V ( f + f I ) + V ( f - f I ) + N ( f + f I ) + N ( f - f I ) = W ( f + f c - f I ) + W ( f - f c - f I ) + W ( f + f c + f I ) + W ( f - f c + f I ) + N ( f + f I ) + N ( f - f I ) .
Example of high-side and low-side injection to an IF at f_I=455 kHz. (a) transmitted spectrum, (b) low-side injected spectrum, and (c) high-side injected spectrum.
Example of high-side and low-side injection to an IF at f I = 455 kHz. (a) transmitted spectrum, (b) low-side injected spectrum, and(c) high-side injected spectrum.

Consider a message spectrum W ( f ) that has a bandwidth of 200 kHz, an upconversion carrierfrequency f c = 850 kHz, and an objective to downconvert to an intermediate frequency of f I = 455 kHz. For low-side injection, where the frequency of thelocal oscillator is f < f c , the goal is to center W ( f - f c + f ) in [link] at f I , so that f = f c - 455 = 395 . For high-side injection (with f > f c ), the goal is to center W ( f + f c - f ) at f I , so that f = f c + 455 = 1305 . For illustrative purposes, [link] supposes that the interference N ( f ) consists of a pair of delta functions at ± 105 and 1780 kHz. [link] sketches | V ( f ) | and | X ( f ) | for both high-side and low-side injection. In this example, both methods end up with unwanted narrowband interferencesin the passband.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
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Maira Reply
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Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
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Jyoti Reply
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Crow Reply
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RAW Reply
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I think
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Brian Reply
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scanning tunneling microscope
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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
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
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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
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Damian Reply
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Stoney Reply
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Adin Reply
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Smarajit 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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