YOKOGAWA WT500 User Manual page 345

IM 760201-01E
When the Waveform of the Synchronization Source Is Distorted
Change the synchronization source to a signal that allows for more stable detection of
the period (switch from voltage to current or from current to voltage). Also, turn on the
frequency filter. For information about setting the frequency filter, see section 4.7.
The WT500 reduce the effects of noise by using hysteresis when it detects the
zero crossing. If the synchronization source is distorted or harmonics and noise are
superposed on the signal to a level exceeding this hysteresis, harmonic components
will cause zero crossing detection to occur frequently, and the zero crossing of the
fundamental frequency will not be detected stably. Consequently, the measured values of
voltage and current may be unstable. When high frequency components are superposed
on the current waveform such as in the aforementioned inverter example, turn the
frequency filter on to stably detect the zero crossing. Use of the filter is appropriate if it
makes the frequency measurement result more stable. Because frequency filter can be
used to facilitate the detection of the zero crossing of the synchronization source, it is
sometimes called the synchronization source filter or the zero-crossing filter.
When Measuring a Signal That Has No Zero Crossing Because of DC
Offset Superposed on the AC Signal
The measured values may be unstable if the period of the AC signal cannot be detected
accurately. Change the synchronization source to a signal that allows for more stable
detection of the period (switch from voltage to current or from current to voltage). The
frequency detection circuit is AC coupled. The period of AC signals that have no zero
crossing because of an offset can be detected if the AC amplitude is greater than or
equal to the detection level of the frequency measurement circuit (see "Accuracy" under
"Frequency Measurement" in Section 14.6, "Functions" for details).
With this feature, the measurement period is set to an integer multiple of the period of
the AC signal.
Data update interval
Signal
When Measuring a DC Signal
When there are ripples on the DC signal, if the level of the ripples is greater than or
equal to the detection level of the frequency measurement circuit (see "Accuracy" under
"Frequency Measurement" in Section 14.6, "Functions" for details) and the period can be
detected accurately and stably, a more accurate DC measurement is possible. If a large
AC signal is superposed on a DC signal, you can achieve a more stable measurement
by detecting the AC signal period and averaging it.
In addition, if a small fluctuating pulse noise riding on the DC signal crosses level zero,
that point is detected as a zero crossing. As a result, sampled data is averaged over an
unintended period, and measured values such as voltage and current may be unstable.
If the synchronization source is set to None, these types of erroneous detections can
be prevented. All of the sampled data in the data update interval is used to determine
the measured values. Set the synchronization source according to the signal under
measurement and the measurement objective.
Appendix 5 Setting the Measurement Period
With the frequency
filter on
Data update interval
Measurement period
AC coupling
1
2
3
4
5
6
7
8
9
10
11
12
13
14
App
Index
App-25