MR8740、MR8741_user_manual_eng_20191016H.pdf - 第270页

12.3 Setting FFT Analysis Conditio ns 258  T rigger Modes and A veraging If the trigger mode is [Single] or the calculation se tting is [Once] Measurements continue until the specified number of averaging points is acqu…

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12.3 Setting FFT Analysis Conditions
257
11
Chapter 12 FFT Function
12
FFT Analysis Modes and Averaging
When averaging time-domain waveform values:
Waveforms are acquired and averaged within the time domain. After averaging, FFT
calculation is performed.
When the trigger mode is [Auto]: Data is acquired when measurement is started, even
if trigger criteria are not met after a certain interval. So if averaging is applied to an asyn-
chronous signal, the resulting data is meaningless.
Synchronous signals have better SNR (signal-to-noise ratio) and are more suitable for
analysis.
When averaging spectrum values:
Acquired data is first subject to FFT analysis. After analysis, averaging is performed
within the frequency range, and the result is displayed. This differs from time-domain av-
eraging in that averaging can be performed without trigger synchronization. However, if
the characteristics of the input waveform allow triggering, using the trigger for synchro-
nization is recommended.
Spectrum peak hold:
After performing FFT calculations on the acquired waveform, peak values are retained
(held) and displayed within the frequency range.
: Settable, ×: Cannot be set,
O
: Partially settable
Analysis Mode
Averaging
Waveform Averaging Spectrum Averaging
Simple
Exponen-
tial
Simple Exponential
Peak
Hold
OFF
×××××
Storage Waveform
×××
Histogram
×××
Linear Spectrum

O
*2
O
*2
O
*2
RMS Spectrum

O
*2
O
*2
O
*2
Power Spectrum

Power Spectrum Density *1

LPC analysis(Power Spectrum Density) *1
×××
Transfer Function

O
*2
O
*2
O
*2
Cross Power Spectrum

O
*2
O
*2
O
*2
Impulse Response

Coherence Function
×××
Phase Spectrum
×××
Auto-correlation Function

Cross-correlation Function

1/1 Octave Analysis *1

1/3 Octave Analysis *1

*1. Not available for external sampling
*2. Not available when the y axis is real (linear) or imaginary (linear), or for Nyquist plots
12.3 Setting FFT Analysis Conditions
258
Trigger Modes and Averaging
If the trigger mode is [Single] or the calculation setting is [Once]
Measurements continue until the specified number of averaging points is acquired.
If the trigger mode is [Continue] or the calculation setting is [Repeat]
Measurement continues after the specified averaging count has been acquired.
When the specified averaging count is exceeded, averaging is repeated and measurement continues until
[STOP] is
clicked.
When the trigger mode is [Auto]
•For time-domain waveforms:
Data is acquired when [START] is clicked, even if trigger criteria are not met after a certain interval. So
if averaging is applied to an asynchronous signal, the resulting data is meaningless.
•For spectrum values:
When [START] is clicked, measurement starts. Even if the trigger criteria are not met, the specified
amount of data is acquired, and after FFT analysis, the results are averaged.
When the specified averaging count is exceeded, averaging is repeated and measurement continues
until [STOP] is clicked.
When the acquired count < the specified count *
1
When the acquired
count = the speci-
fied count
Start Measurement
[START]
End *
2
Trigger
criteria met
Input
Signal
Awaiting
Trigger
Waveform acquisi-
tion (specified no.
of points)
Averaging
Display*
3
FFT
analysis
(Spectrum averaging)
(Waveform averaging)
*1. Awaiting trigger continues until the specified count is reached.
*2. Measurement stops automatically when the specified count is
reached. If measurement was interrupted by clicking [STOP], the
averaging result up to that point is displayed.
*3 If [Reference] is [New Data] and the automatic saving setting is
ON, data is saved when the specified number of times is
reached.
FFT
analysis
Repeats until [STOP] is clicked
Start Measurement
[START]
End*
1
Trigger
criteria met
Input
Signal
Awaiting
Trigger
Waveform acquisi-
tion (specified no.
of points)
Averaging
Display*
2
FFT
analysis
(Spectrum averaging)
(Waveform averaging)
*1 When stopped before the specified count, the average up to that point is displayed.
*2 If [Reference] is
[New Data] and the automatic saving setting is On, data is saved
when the specified number of times is reached.
FFT
analysis
[STOP]
Repeats until [STOP] is clicked
Start Measurement
[START]
End*
After a certain interval,
acquisition starts even if
trigger criteria are not met
Input
Signal
Awaiting
Trigger
Waveform acquisi-
tion (specified no.
of points)
Averaging
Display
FFT
analysis
(Spectrum averaging)
(Waveform averaging)
* When stopped before the specified count, the average up to that point is clicked.
FFT
analysis
[STOP]
12.3 Setting FFT Analysis Conditions
259
11
Chapter 12 FFT Function
12
By specifying a setting factor (rate) to be applied to the input signal, the display of data exceeding the result-
ing threshold can be emphasized. This feature is useful for viewing waveforms that may otherwise be
obscured by noise.
The reliability of phase spectrum values is poor when discrete Fourier transform values are extremely small.
For example, in the case of a pure sine wave, almost all phase values at frequencies other than the input fre-
quency result from calculation errors. By treating the maximum value of the power (or cross-power) spectrum
of the input signal,
P
max
, as a reference value, data that exceeds that value multiplied by rate R can be dis-
played with emphasis.
12.3.8 Emphasizing Analysis Results (phase spectra
only)
Power Spectrum
Phase Spectrum
Emphasis
Display
10logP
max
[dB]
10logRP
max
[dB]
Attenuation Rate and Value
Attenuation value: A [dB]
Attenuation rate: R
-A = 10log
10
R
1 x 10
-6
R 1
0
A 60
1
Enable the highlighting function.
Move the flashing cursor to the [Highlight(phase)] item.
Select
2
Set the attenuation rate or attenuation value.
To set an attenuation rate
Move the flashing cursor to the [Attenuation rate] item.
Enter the attenuation rate.
See: "7.1.3 Alphanumeric Input" (p.141)
To set an attenuation value [dB]
Move the flashing cursor to the [(db)] item.
Enter the attenuation value.
See: "7.1.3 Alphanumeric Input" (p.141)
Off Emphasis display disabled. (default setting)
On
Emphasis display enabled.
Procedure
To open the screen: Right-click and select [STATUS] [Status] sheet
1
2
3