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Consider the following DFT plot below, showing the output spectrum of an 9-bit ADC.

enter image description here

I want to calculate the THD (Total Harmonic Distortion). I am wondering how to do it correctly. I am aware of the following formula.

$$ THD=10\log\Bigg(\frac{P_{HD}}{P_S}\Bigg)=10\log (P_{HD}) - 10\log (P_S) $$

So from my understanding $P_{HD}$ is the sum of all harmonics. But the signal power is shown in dBc, so I can only say how far the power of the harmonics are below the carrier, but I do not have the information about the absolute power levels. Is this correct? Then is it actually possible to calculate the THD, if yes, how?

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What is shown in the plot is the PSD (Power Spectral density), which is measured in W/Hz (or dBc). So, in order to get the actual power in a given frequency range, you'd need to multiply with the bandwidth of each bin (which is determined by the length of the DFT window). Let the bandwidth of each bin be $B$.

Then, your formula becomes

$$ THD = 10\log\left(\frac{P_{HD}}{P_S}\right)=10\log\left(\frac{\sum_i B\cdot S_i}{B S_{HD}}\right) $$

where $S_i$ denotes the Density at the $i$th harmonic and $S_HD$ denotes the density at the fundamental. As you see, B cancels in the expression, so you can ignore it. Note, that $S$ are in linear scale, (i.e. you'd need to transform the measurements in the diagram to linear scale before summing them up etc.)

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