MATLAB SIGNAL PROCESSING BLOCKSET 7 Guida Utente Scaricare il pdf (Pagina 557)

Sine Wave

5-400

•Discrete

In discrete mode, the block’s discrete-time output can be generated by

directly evaluating the trigonometric function, by table look-up, or by a

differential method. The three options are explained below.

Discrete Computational Methods

When Discrete is selected from the Sample mode parameter, the secondary

Computation method parameter provides three options for generating the

discrete sinusoid:

•

Trigonometric Fcn

•Table Lookup

•Differential

Trigonometric Fcn. The trigonometric function method computes the sinusoid in

the ith channel, y

i

, by sampling the continuous function

or

with a period of T

s

, where T

s

is specified by the Sample time parameter. This

mode of operation shares the same benefits and liabilities as the

Continuous

sample mode described above.

If the period of every sinusoid in the output is evenly divisible by the sample

period, meaning that 1/(f

i

T

s

)=k

i

is an integer for every output y

i

, then the

sinusoidal output in the ith channel is a repeating sequence with a period of

k

i

samples. At each sample time, the block evaluates the sine function at the

appropriate time value within the first cycle of the sinusoid. By constraining

trigonometric evaluations to the first cycle of each sinusoid, the block avoids

the imprecision of computing the sine of very large numbers, and eliminates

the possibility of discontinuity during extended operations (when an absolute

time variable might overflow). This method therefore avoids the memory

demands of the table look-up method at the expense of many more

floating-point operations.

y

i

A

i

2πf

i

t φ

i

+()sin=

real()

y

i

A

i

e

j 2πf

i

t φ

i

+()

=

complex()

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