App-28
IM 760301-01E
• When the Waveform of the Synchronization Source Is Distorted
Change the synchronization source to a signal that allows more stable detection of
the period (change from voltage to current or from current to voltage). Also turn ON
the frequency filter. For the setup procedure of the frequency filter, see section 4.8.
The WT3000 applies hysteresis to the zero crossing detection to reduce the effects of
noise. If the synchronization source is distorted or harmonic or noise is superposed on
the signal to a level exceeding this hysteresis, zero crossing detection due to high
frequency components occur frequently, and the zero crossing of the fundamental
frequency cannot be detected stably. Consequently, this may cause the measured
values of voltage and current to be unstable. Also, 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. The validity of
setting the filter can be verified by a more accurate and stable frequency
measurement result. In this way, the frequency filter also functions as a filter for
detecting the zero crossing of the synchronization source. Therefore, the frequency
filter is sometimes called the synchronization source filter or zero-crossing filter.
Frequency
filter ON
• When Measuring a Signal with No Zero Crossing due to 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 more stable
detection of the period (change from voltage to current or from current to voltage). The
frequency detection circuit is AC coupled. The period of AC signals with no zero crossing
due to an offset can be detected if the AC amplitude is greater than or equal to the
detection level of the frequency measurement circuit (see section 12.5).
With this feature, the measurement period is set to an integer multiple of the period of
the AC signal.
Signal
Data update interval
Measurement period
Data update interval
AC coupling
• When Measuring a DC Signal
If ripple exists on the DC signal and its level is greater than or equal to the detection
level of the frequency measurement circuit (see section 12.5) and the period can be
detected accurately and stably, a more accurate DC measurement becomes possible.
If a large AC signal is superposed on the DC signal, a more stable measurement can
be achieved by detecting its period and taking the average.
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 the measured values such as voltage and current
may be unstable. If the synchronization source is set to None, these types of
erroneous detection 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 measurement signal and the measurement objective.
Appendix 6 Setting the Measurement Period