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

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22.4 Block diagram representation of difference equations 691

x[n]

x[n]

S

3

y[n]

T

3

–x[n – 1]

1

— T

–1

Figure22.8

Block diagram ofadifferentiator.

s[n]

y[n]

s[n]

3

y[n]

3

a[n]

T 3

—T

1

3

—T

1

–s[n – 1] –y[n – 1]

Figure22.9

Two differentiators in series.

–1 –1

problemistoobtainadifferenceequationfortheprocessoffindingasecondderivative.

Given that

s[n] −s[n −1]

v[n] = (22.2)

T

and

v[n] − v[n −1]

a[n] = (22.3)

T

then substituting Equation (22.2) into Equation (22.3) gives

a[n] =

(s[n] −s[n −1]) − (s[n −1] −s[n −2])

T 2

s[n] −2s[n −1] +s[n −2]

a[n] =

T 2

TheblockdiagramforthisdifferenceequationisshowninFigure22.10.Notethatthe

difference equation is non-recursive and so there are no feedback paths in the block

diagram.Theoutputsequenceisa[n]andtheinputsequenceiss[n].Theindependent

variable isn.

–2

T

3

–2s[n – 1]

s[n]

T

T

s[n]

S

s[n – 2]

3

1—

T 2

a[n]

Figure22.10

a[n] is obtained by

calculating the second

derivative ofs[n].

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