MR8740、MR8741_user_manual_eng_20191016H.pdf - 第298页

12.8 FFT Analysis Modes 286 W aveform Example  Emphasizing only a Specific Portion (Highlighted Display) A specific portion of a phase spectrum can be empha sized and displayed. See: " 12.3.8 Emphasizing Analysis R…

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12.8 FFT Analysis Modes
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Chapter 12 FFT Function
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Waveform Example
Coherence Function
This function gives a measure of the correlation (coherence) between input and output signals. Val-
ues obtained are between 0 and 1.
Main uses:
• To evaluate transfer functions
• In a system with multiple inputs, to inspect the effect of each input on the output
See: "12.8.2 Analysis Mode Functions" (p.293)
Axis Display Type Description
X axis
Linear Frequency is displayed with equal spacing
Log Frequency display of logarithm interval
Y axis Lin-Mag
Displays the causal relationship and degree of relationship between two input
signals, as a value between 0 and 1 (dimensionless units).
Normal display
X axis: Log
Y axis: Lin-Mag
• With a single measurement, the coherence function gives a value of one for all
frequencies. Spectrum (frequency-domain) averaging should always be per-
formed before measurement (analysis is not available with time-domain aver-
aging).
• The coherence function has two general definition formulas. For the definition
formulas, see"12.8.2 Analysis Mode Functions" (p.293).
12.8 FFT Analysis Modes
286
Waveform Example
Emphasizing only a Specific Portion (Highlighted Display)
A specific portion of a phase spectrum can be emphasized and displayed.
See: "12.3.8 Emphasizing Analysis Results (phase spectra only)" (p.259)
Phase Spectrum
Shows the phase characteristics of the input signal.
Main uses:
• To inspect the phase spectrum of channel 1. Displays the phase of a cosine waveform as a reference (0º).
• To inspect the phase difference between channels 1 and 2.
See: About the Functions"12.8.2 Analysis Mode Functions" (p.293)
1 Ch FFT: Displays the phase of the signal on channel 1. Displays the phase of a cosine waveform as a ref-
erence (0º). Unless the waveform is synchronous, phase values are unstable.
2 Ch FFT: Displays the phase difference between channels 1 and 2. Positive values indicate that the phase
of channel 2 is leading.
Axis Display Type Description
X axis
Linear Frequency is displayed with equal spacing
Log Frequency display of logarithm interval
Y axis Lin-Mag Analysis values are displayed linearly.
1chFFT
X axis: Log
Y axis: Lin-Mag
2chFFT
X axis: Log
Y axis: Lin-Mag
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Chapter 12 FFT Function
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Waveform Example
Auto Correlation Function
Shows the correlation of two points on the input signal at time differential t.
Main uses:
• To detect periodicy in irregular signals (improving and detecting SNR)
• To inspect periodic components in a noisy waveform.
See: About the Functions"12.8.2 Analysis Mode Functions" (p.293)
Axis Display Type Description
X axis Linear
Time display
The center (
t = 0) is the reference. To the right is lag time (+t), and to the left is
lead time (-t)
Y axis Lin-Mag
+1 to -1 (dimensionless units)
The closest correlation at time differential
t is +1, and the least correlation is 0.
-1 indicates completely reversed polarity.
Because of the characteristics of the function, t = 0 becomes +1.
X axis: Linear
Y axis: Lin-Mag
This instrument provides a circular auto-correlation function. Analysis results are
normalized to the maximum value.