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01:
02:
amount of
at
the different frequencies but
do not
any phase information.
originaltime varying
signalcan
be
by taking
the Inverse Fourier
magnitude ahd phase results.
PROGRAMMING
The FFT Instruction is a Processing Instruction
and
will not dutput data to final memory or any
data storage
device. FFT
results are transferred
to Final Meniory by setting the Output Flag and
sampling
thQ
input locations which contain the
FFT
results.
Referring to
Table
5,
the thirteenth
through
the $eventeenth instructions illustrate
one method bf transferring
the data from input
memory to
fifial
memory.
PAR. DATA
NO. TYPE
DESCRIPTION
Log base 2 of N,
where N
is
the
number of
samples in the original
data; e.9.,
this parameter is
11
for
2048 samples
or
12
for 4096
samples.
Maximum
is
13
or 8192.
FFT Option
X0
= No Taper
0X
= Power
Spectra
1X
= Pss; g
X1
=Taper
lmaginary
2X=Yag6ilude&Phase
Log base 2 of
the number of
adjacent bins to average; e.9.,
this
parameter is
3
for averaging
8
adjacent bins.
Input location containing the
first
value
in
the
originaltime series.
After
this program has executed,
this location will contain the first
spectral bin or bin average
of
the
result.
Multiplier
for scaling FFT results.
Input
storage
locations:
Greater
than or equal
to
the number of values
in
the
original time
series data.
lntermediate
storage locations: 2 +
(.125.
N)
SECTION
10.
PROCESSING INSTRUCTIONS
Execution
time in seconds:
REAL
AND IMAGINARY
= .00065*N*LOG2 of
N
+
.58
MAGNITUDE
AND PHASE
= .00077*N*LOG2 of N + 1.66
POWER SPECTRA
=.00066*N*LOG2 of
N
+
.45
PARAMETER
1
is
an
integerfrom
1
to
13.
The
number
2, raised to the power of Parameter 1,
is
equal to
the number of data values
in
the
originaltime
series data, N. For
example,
if
Parameter 1=13,
then
N=8192.
Maximum value
for Parameter
1
is 13 (8192).
PARAMETER
2,
the FFT Option, is
a
two digit
parameter. The first digit specifies which results
are
to be generated and the second digit
specifies whether
or not a "taper"
is
applied.
First
Digit: A
"0"
in
the
first digit specifies that
the FFT result be expressed as the power
spectra.
Bin
averaging
is
allowed with this
option. A
"1"
in
the
f
irst digit specifies that
the
FFT result be expressed as
the real and
imaginary
components. Bin averaging is not
allowed with this
option. A
"2"
in
the first digit
specifies that
the FFT result be expressed as
the magnitude and phase
components.
Bin
averaging is not allowed with
this option.
Second
Digit: A
"0"
in
the second digit specifies
that
no
taper be applied, whereas
a "1" specifies
that
a
taper be
applied.
lf
the original data set
is
not known
to be periodic with an integral number
of periods
in
the data set,
then it is necessary to
apply
a
taper to
the beginning and end of the
data.
The taper
that is applied
is a
four term
Blackman-Harris as
specified by Fredric
J.
Harris, Proceedings of
the IEEE, Vol. 66, No.
1,
January
1978.
When the
taper is used, the 21X
applies
a
correction
by multiplying the results by
the ratio of the variance before the taper to the
variance after
the taper.
The
variance ratio is saved only when the power
spectra option is
selected. The variance ratio
is
saved in input location (N/2)+B, where B
is
the
location (Parameter 4) that contained the first
value
of
the original time series data.
PARAMETER 3 specifies
how many
of
the
resulting spectral bins (if any) are
to
be
averaged.
Bin averaging
is
only allowed when
FP
03:
04:
05:
10-7
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