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75
First, press the knob and then spin it to change the harmonic order. For example, the order is set to 3 in Fig.67.
Fig.67 Cubic harmonic measurement of square wave
How to get the amplitude of the n
th
harmonics
How to calculate the harmonic theoretical value of the square wave: suppose that the peak-peak value of square
value is E and the angular frequency is
ω
. After Fourier expand, the result is:
))
sin(
1
)
5
sin(
5
1
)
3
sin(
3
1
)
(sin(
2
)
(
t
n
t
t
t
E
t
f
Where the n
th
harmonic is a sine wave:
)
sin(
2
)
(
t
n
n
E
t
f
Therefore, the valid value of the n
th
harmonic is :
n
E
R
2
We can verify that whether the measured result is close to the theoretical result. In this example, the peak-peak
value E is 160mV, and the calculated value of first harmonic is:
mV
mV
R
025
.
72
1
160
2
The calculated value of third harmonic is :
mV
mV
R
008
.
24
3
160
2
Compare the measured result with theoretical result according the above method.
