Campbell 21X Manual do Operador descarregar pdf (Página 117)

TABLE 10-1. Example of FFT Power

Spectra Bin Averaging

(Assuming

1024 time series

values

starting

in Location

1)

SECTION 10. PROCESSING

INSTRUCTIONS

For

example,

given

that the

power

spectra

result

shows that the

energy

peak

of a

signal falls

in

bin

32

when

it is

sampled at a frequency

of 10

Hzlor 1024

samples

and that the

bin averaging

specified

is

4, the frequency

of the

signal in

bin

i

is:

31

*

10 " 4

/

1024 <li<32"

10*

4 I 1024

'l.21

Hz

<

f,

<

1 .25 H2

POWER

SPECTRA

The result of the FFT

with

A

bins averaged,

are

(N/2A)-1

bins of average

spectral energy

(APSn)

representing frequencies

from 0 Hzto 112lhe

sampling frequency.

The

value

of ivaries

from

1

to

(N/2A)-1.

The

results are found in

consecutive input

locations starting with

the first

one specified by Parameter

4.

The value

for

average bin n

(APS.)

is related to the

spectral

bin values

(PS;

see

previous

section)

by the

following equation:

No

BIN

NO.

0

1

2

3

4

5

6

7

I

9

10

11

502

503

504

505

506

507

508

509

510

511

Bin Averaging

LOC. REPRESENTATIVE

FREQUENCY

APS'=(X

PS;+O.5(PSn4_4y2+PSn4*ed),ln

[1

where

i

goes

from

nA-(A/2-1) to nA+(A/2-1)

The following table illustrates how

bin

is

done

given

a time series

of

1024 values

at one

per

second with

the

resulting

512

bins averaged in

groups

of 4

(Parameter

3

=

base 2

of

4

=

2l

to

produce

'127

averaged

bins.

In

the

following

example, averaging

produces

bins with representative frequencies

that are

same as if

256

samples had been used with no

averaging.

Note

that when

averaging

is done,

information from the first

few and

last few

bins

is

not included so that the averages can better

represent frequencies

that correspond to

of fewer

original

samples.

Examples of the use of

the

FFT are

given

in

Section 8,

Averaged in

Groups

of

4

AVERAGED

LOC. REPRESENT

BIN NO.

FREQUENCY

1 411024 or 1

1

2

3

4

5

6

7

8

9

10

11

12

DC

111024

211024

3/1024

411024

511024

611024

7/1024

811024

9t1024

10t1024

11t1024

502/1024

50311024

50411024

s05/1

024

506/1024

507/1024

50811024

509t1024

510/1024

51',|11024

--

126

2 8/1024

or

126 50411024 or 1

10-10

----

127 127

508fi024 or

127,

1 2 ... 112 113 114 115 116 117 118 119 120 121 122 ... 190 191

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