4-8
FFT Mode
• T.R.F. (Transfer Function)
This function expresses the frequency characteristics of the input and output of a
transfer system. It can be used to obtain the ratio of the signal input's power
spectrum to the cross spectrum of the signal input and signal output. Similar to
the Cross Spectrum setting, the T.R.F. setting conducts averaging to reduce the
impact of noise mixing into the system's signal input. Because the transfer
function is measured throughout the entire range of the FFT analysis frequency
specified by the Frequency setting, however, you must input signals that will
contain the frequency components of the entire bandwidth of the FFT analysis
frequency.
Display
Y axis
Amplitude
LIN.
20 log
LOG.
PhasePhase
FFT result of signal input : X(i) = X
R
(i) + jX
I
(i)
FFT result of signal output : Y(i) = Y
R
(i) + jY
I
(i)
S
R
(i) = X
R
(i)Y
R
(i) + X
I
(i)Y
I
(i)
S
I
(i) = X
R
(i)Y
I
(i) - X
I
(i)Y
R
(i)
S
R
(i)
2
+ S
I
(i)
2
X
R
(i)
2
+ X
I
(i)
2
[dB]
tan
—1
[deg]
S
R
(i)
2
+ S
I
(i)
2
X
R
(i)
2
+ X
I
(i)
2
S
I
(i)
S
R
(i)
–When the Y (axis) setting is LOG., the reference value is 0 dBV = 1.
– The Phase value is identical to that of Phase Value of the Cross Spectrum
setting shown in the preceding table.
– In the WR300's FFT mode, the CH.A and CH.B parameters correspond to X(i)
and Y(i), respectively.
• Coherence (Coherence function)
The Coherence function is obtained by calculating the ratio of [the product of the
input and output power spectra] to the [the amplitude squared of the cross
spectrum of the signal input and signal output]. The result expresses the causal
relationship between the system's input and output signals. The value of the
coherence function ranges from 0 to 1. "1" indicates that all signal output derives
its power from the signal input, and "0" indicates that the power of the signal output
is totally unrelated to the signal input. By measuring the coherence function, you
can check the reliability of measuring the T.R.F. or other functions. When
averaging is not performed, all values of the coherence function will become 1.
When using the coherence function, be sure to also perform averaging.
Display
Y axis
Amplitude
LIN.
FFT result of signal input : X(i) = X
R
(i) + jX
I
(i)
FFT result of signal output : Y(i) = Y
R
(i) + jY
I
(i)
S
R
(i) = X
R
(i)Y
R
(i) + X
I
(i)Y
I
(i)
S
I
(i) = X
R
(i)Y
I
(i) - X
I
(i)Y
R
(i)
S
R
(i)
2
+ S
I
(i)
2
( X
R
(i)
2
+ X
I
(i)
2
)(Y
R
(i)
2
+ Y
I
(i)
2
)
– In this recorder's FFT mode, the CH.A and CH.B parameters correspond to X(i)
and Y(i), respectively.
CH.A/B ........................... This parameter specifies the channels for each analysis method.
TIP
Aliasing may cause the input frequency to differ from the analyzed result.
< Aliasing >
Because the WR300 conducts measurement by using an analog-to-digital converter to
sample signals, if the frequency of the signal input becomes greater than 50% of the
frequency of the sampling interval, a phenomenon occurs in which theWR300
inadvertently measures a frequency that has not been input. This phenomenon is called
"aliasing".
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