deep
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Post by deep on Nov 17, 2004 16:28:35 GMT -5
bandwidth of a base band signal is conisdered as the max frequency component of the signal.Now when we represent base band signal in the frequency domain what is the "negative part". I am told this negative part is a mathematical representation and does not exist. If it does not than how come the bandwidth doubles when the base band signal is modulated into a passband signal
Thanx for the help
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Post by raghava on Dec 7, 2004 6:20:18 GMT -5
in the fourier transform negative frequencies do exist but arent included in bandwidth calc
In pass band the bandwidth doubles as u are shifting the spectrum both sides.This has a 2*W on positive side and 2*W in negative side
Hence u tell that BW has doubled.
BYE
;D
raghava
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Post by charan langton on Dec 7, 2004 12:10:05 GMT -5
The bandwith of a signal is defined as only positive frequencies.
So in low pass it is only half the width of the spectrum. When this signal is shifted to a larger frequency, say fl, through multiplication by a sinusoid of a frequency fl, then the whole spectrum is in the positive frequency domain around the frequncy fl and now the bandwidth is the width of the whole spectrum. This is double the bandwidth of the signal at low pass.
Charan Langton
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Post by Andreas on Jan 20, 2005 22:45:50 GMT -5
There is obviously no such thing as a negative frequency. It's just convenient to think of it that way.
exp(j2 * pi * fo *t) is a generic complex vector rotating at a rate fo counterclock wise in the complex plane.
exp(-j2 * pi * fo *t) is a generic complex vector rotating
at a rate fo clock wise in the complex plane.
They both rotate at the same rate fo.
Let's say you mix these signals up to some RF frequency via a carrier. Let's pick a complex carrier for simplicity.
exp(j2 * pi * fc *t)
thus you get exp(j2 * pi * (fc+fo) *t) and
exp(j2 * pi * (fc-fo) *t)
While at base band the signal existed only at fo, now
is exists at fc + fo and fc - fo. It is the exponential multiplication that makes the clockwise spinning vector into the smaller and the counterclock wise (pos freq) vector into the larger frequency.
Andy
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