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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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24.13 Some properties of the d.f.t. 793

(b)

which usingthe Cartesian form can be written

1 1 1

1 − 1 √

3

⎝ 2 −j − 1 √

3

2 2 +j 2

1 − 1 √

3

2 +j − 1 √ ⎟

⎠ 3

2 2 −j 2

⎛ ⎞ 1 1 1

F[0]

⎝F[1]

⎠ = ⎜

1 − 1 √

3

F[2]

⎝ 2 −j − 1 √ ⎛ ⎞ ⎛ ⎞

3

2 2 +j 4 8

2

1 − 1 √

3

2 +j − 1 √ ⎟ ⎝−7⎠ = ⎝2 +15.5885j⎠

⎠ 3

2 2 −j 11 2 −15.5885j

2

So, the required d.f.t. isthe sequence

8,2 +15.5885j,2 −15.5885j

EXERCISES24.12

1 Using the definition ofthe d.f.t. show thatthe matrix

whichcan be used to implement the d.f.t. ofa

four-point signal is

1 1 1 1

W = ⎜1 −j −1 j

⎝1 −1 1 −1⎠

1 j −1 −j

Use thismatrix to findthe d.f.t. ofthe sequences

f[n] = {1,2,0, −1} andg[n] = {3,1, −1,1}.

Solutions

1 F[k]=2,1−3j,0,1+3j.G[k]=4,4,0,4

24.13 SOMEPROPERTIESOFTHED.F.T.

24.13.1 Periodicity

Thed.f.t.F[k] isperiodic with periodN, thatis

F[k+N] =F[k]

This is why we take onlyN terms in the frequency domain. Calculating further values

will only reproducethe earlier ones.Theinverse d.f.t. isalsoperiodicwith periodN.

24.13.2 Linearity

The d.f.t. is a linear transform. This means that the d.f.t. of the sequenceaf[n] +bg[n]

whereaandbareconstants isaF[k] +bG[k].

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