6.2.2.2
Considerations for selection of PLL source
Please select the signal with the same cycle as that of the harmonic measurement object signal. In order to stably
measure harmonics, select the input signal with little distortion as the PLL source. If the fundamental wave
frequency of the PLL source changes or the fundamental wave frequency cannot be measured as a result of
waveform distortion, accurate measurement results may not be obtained. When the test object is a switch type
power supply and the voltage signal is subject to less distortion than the current signal, it is recommended to use
the voltage as the PLL source.
6.2.2.3
Signal level of PLL source
If the amplitude level of the signal input into the unit as the PLL source is small relative to the range,
synchronization with PLL will fail. If the crest factor is set as 3, the amplitude level of the PLL source should exceed
50% of the range at least during setting of the range. If the crest factor is set as 6, the amplitude level of the PLL
source should exceed 100% of the range at least during setting of the range.
1
When the fundamental wave frequency is lower than 440Hz and contains high-frequency components, it is
recommended to turn on the frequency filter. This filter only applies to the frequency measurement circuit.
If the frequency of the PLL source changes, the correct measured value will be displayed after data are updated
several times. The frequency of the PLL source needs to be tested again via the PLL circuit in the instrument in case
of changes of the PLL source or its frequency, the correct measured value cannot be obtained at once.
6.2.3
Harmonic analysis order
The harmonic analysis order k is an integer number ranging from 0 to the upper limit of analysis orders. Zero
indicates the DC component. The upper limit of analysis orders is automatically determined by the frequency of
the PLL source. The upper limit of harmonic analysis orders can be determined automatically or set manually. Take
the smaller value. 50 orders at most The relationship between the upper limit of harmonic analysis orders and the
fundamental wave frequency of the PLL source is shown in the table below.
PLL fundamental frequency
Sampling rate
Window width
Harmonic order upper limit
10Hz
–
75Hz
f*1024
1
50
75 Hz
–
150 Hz
f*512
2
32
150 Hz
–
300 Hz
f*256
4
16
300 Hz
–
600 Hz
f*128
8
8
600 Hz
–
1200 Hz
f*64
16
4
NOTE The upper limit of analysis orders may be reduced.
NOTE If the analysis window determined by the fundamental wave frequency of the PLL source is longer than the
data updating cycle, no harmonic data will be detected. In this case, the data updating cycle should be reduced.
For example, when the fundamental wave frequency of the PLL source is 10Hz (cycle: 100ms), the analysis window
CHAPTER 6. HARMONIC MEASUREMENT FUNCTION
34
is one-wave long, and the data measurement interval is 100ms. In this case, the harmonic measurement time is
approximately morethan or equal to 150ms (data measurement interval and data processing time). Therefore,
please select the data updating rate of 250ms or more for measurement and display of harmonic data.
1
When the fundamental wave frequency is no more than 200Hz, the frequency filter must be turned on.
