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Electronics => Projects, Designs, and Technical Stuff => Topic started by: cncjerry on December 28, 2015, 01:20:54 am

Title: THD Measurement
Post by: cncjerry on December 28, 2015, 01:20:54 am
What is the generally accepted way to calculate THD?

I've seen it using the fundamental only as the denominator and also the summation of the fundamental plus the harmonics as the denominator.

I'm currently using THD = (sqrt((P2+P3+P4+P5+P6+P7+P8+P9)/P1) x 100) Where the number equals the harmonic with 1 being the fundamental.  Another way to state it is the square root of the summation of the powers of the harmonics divided by the power of the fundamental.  I have a program the measures the fundamental and up to 9th harmonic on a spectrum analyzer in dBm then I convert it to power in watts.  I've been comparing some signal generators I have for spectral purity and alignment if needed.

The alternate way I've seen it is the square root of the summation of the harmonics divided by the summation of the harmonics and fundamental.  This calculation would result in a much lower number. In all cases the resulting fraction is multiplied by 100 to get percent.

Thanks
Title: Re: THD Measurement
Post by: dom0 on December 28, 2015, 01:29:37 am
Almost everyone uses the first definition. Although it doesn't really matter below maybe -40 dB THD.
Title: Re: THD Measurement
Post by: DanielS on December 28, 2015, 04:55:03 pm
I agree with the first definition too.

If you want THD+N instead, since THD+N is everything that isn't the fundamental, you can calculate it by subtracting P1's RMS value from P's RMS value: THD+N = sqrt(PRMS^2-P1RMS^2)/P1RMS