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THD measurement. How many harmonics included.

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hojongch

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If we calculate Total harmonic distortion(THD), we need to obtain the spectrum data which has 2nd,3rd,4th, n th harmonic data. I am wondering how many nth harmonic data we need to include to obtain THD value(% or dB).

Original signal from function generator is 100MHz 0.2V and I obtained 5th harmonic in the oscilloscope. 5th harmonic one is around -50dB(amplitude).

Chris
 

zorro

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Hi hojongch,

This is another method:
If you can measure the total power (usually this is done in time domain) and the dc and 1st (funtamental) components, you have

Harmonics_power = Total_power - DC_power - Fundamental_power

It this way, in theory all harmonics are included. The problem is that in the case THD is low, as this is calculated as a difference, it can be subject to a big error. It depends of how you perform measurements.
Regards

Z
 

chuckey

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With audio measurements, usually the fundamental is filtered out, and its called total harmonic distortion. Your ears will hear all the distortion products. With RF its the actual components that are quoted i.e. second harmonic is lower then 50 dB or what ever. THD is not relevant to RF because there will only be a certain bandwidth available and any non-fundemental components must then be below some figure. If you were building a transmitter, the spec could be all spurious outputs are -70db or less with a maximum power of 2mW (if it was a 10kW transmitter).
Frank
 

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