App-19
IM WT310E-01EN
Appendix
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Index
Because of the reasons described above, the measured voltage and current values may be unstable.
If this happens, lower the data update interval so that more periods of the input signal fit within the
data update interval.
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.
This instrument reduces the effects of noise by using hysteresis when it detects zero crossings. 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 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 zero crossings. Use of the filter is appropriate if it makes the measured frequency
accurate and more stable. Because the frequency filter can be used to facilitate the detection of the
synchronization source’s zero crossings, it is sometimes called the synchronization source filter or the
zero-crossing filter.
Frequency filter
ON
When Measuring a Signal That Has No Zero Crossings Because of
a 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. Even
with AC signals in which there are no zero crossings because of an offset, the period can be detected
if the AC amplitude is greater than or equal to the detection level of the frequency measurement circuit.
With this feature, the measurement period is set to an integer multiple of the period of the AC signal.
Data update interval
Measurement period
Data update interval
AC coupling
Signals
When Measuring a DC Signal
When there are ripples in the DC signal, if the level of the ripples is greater than or equal to the
detection level of the frequency measurement circuit 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. You can prevent these kinds of
erroneous detections by setting the synchronization source to OFF. All of the sampled data in the data
update interval is used to determine measured values. Set the synchronization source according to the
signal under measurement and the measurement objective.
* See the conditions listed under “Accuracy” under “Frequency Measurement” in section 7.4,
“Features,” in the Getting Started Guide, IM WT310E-02EN.
Appendix 4 Setting the Measurement Period
