YOKOGAWA WT500 User Manual page 179

Explanation
IM 760201-01E
You can select a PLL (Phase Locked Loop) source to use to determine the fundamental
period, which acts as the reference in the analysis of the harmonic orders.
Selecting the PLL Source
Select the PLL source from the choices below. The selectable items vary depending on
the number of elements installed in the WT500.
U1, I1, U2, I2, U3, I3, and Ext Clk (external clock)
* If you select "Ext Clk," harmonic measurement is performed using the frequency of the signal
applied to the external clock input connector as the fundamental frequency.
When Setting the PLL Source to Ext Clk
Apply a clock signal that meets the following specifications to the external clock input
connector (EXT CLK) on the rear panel.
EXT. CLK
Item Specification
Connector type BNC connector
Frequency 10 Hz to 1.2 kHz
Input level TTL
Input waveform 50% duty ratio rectangular wave
Applying a voltage outside of the range of 0 to 5 V to the external clock input
connector (EXT CLK) can damage the WT500.
Note
• The synchronized measurement slave setting and the PLL source Ext Clk setting cannot
be made at the same time because they both use the same connector (see section 12.6 for
information about synchronized measurement).
• To connect a BNC cable to the external clock input/external start signal output connector
(EXT CLK) when a GP-IB cable is connected to the GP-IB connector, first remove the GP-IB
cable. Then connect the BNC cable. Finally, reconnect the GP-IB cable.
• Select a signal that has the same period as the target signal for harmonic measurement.
For stable harmonic measurement, choose an input signal for the PLL source that has as
little distortion and fluctuation as possible. If the fundamental frequency of the PLL source
fluctuates or if the fundamental frequency cannot be measured due to waveform distortion,
correct measurements will not be obtained. When the measured item is a switching power
supply and in other cases where the distortion of the voltage signal is smaller than that of
the current signal, we recommend that the PLL source be set to the voltage.
If all of the input signals are distorted or the amplitude is small compared to the
measurement range, the specifications may not be met. To achieve accurate measurements
on high harmonics, set the PLL source to an external clock signal and apply a signal with
the same period as the input signal to the external clock input connector.
• If the fundamental frequency is less than or equal to 440 Hz and the signal contains high
frequency components, we recommend that you turn the frequency filter on. This filter is
only effective on the frequency measurement circuit.
• If the amplitude level of the signal applied to the element that is specified as the PLL source
is small compared to the range, PLL synchronization may not be achieved. If the crest factor
is set to 3, set the measurement range so that the amplitude level of the PLL source is at
least 50%. If the crest factor is set to 6, set the measurement range so that the amplitude
level of the PLL source is at least 100%.
• If the frequency of the PLL source changes, correct measured values are displayed a few
data updates after the change. Correct measured values may not be obtained immediately
after the PLL source or its frequency changes, because the PLL circuit inside the WT500
redetects the frequency.
• The synchronized measurement slave setting and the PLL source Ext Clk setting cannot be
made at the same time because they both use the same connector.
6.3 Selecting the PLL Source
*
CAUTION
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App
Index
6-9