I have the following problem.

enter image description here


I found the formula for the total harmonic distortion to be this:

\$THD = \frac{\sqrt{V_2^2+V_3^2+V_4^2+V_5^2+...}}{V_1} \$, where the subscript denotes the harmonic.

Looking at the spectrum above, it seems like I'm supposed to make the following conclusions: V1 = 18.9241, V2 = -67.363, V3 = -8.5635, V4 = -61.8046, V5 = -13.3634.

Inserting these values in the formula gives me a result of \$THD = 4.90 \$ which doesn't seem reasonable at all, I must be doing something wrong. I'm also unsure of where I'm supposed to use the information about the forward voltage of the diode.

I hope someone can help me or clarify things for me.


1 Answer 1


The amplitude distortion plot you showed is in dB, dBV specifically (dB's relative to 1 V). So you need to take those dB values and covert them into volts before putting them into your formula.

For example, -67.363 dBV is 0.000428 V (RMS) (using an on-line calculator).

You should be able to take it from here.

  • \$\begingroup\$ Also worth noting that the fundamental frequency (= the desired output) should not be included when calculating distortion. \$\endgroup\$
    – jpa
    May 18, 2020 at 6:00
  • \$\begingroup\$ @SteveSh - Okay, I am now converting all the voltages from dBV to V. V1 = 8.835V, V2 = 0.000428V, V3 = 0.3731, V4 = 0,000812 V, V5 = 0,2147 V. So far so good, but where do I go from here? Do I just plug these number into the equation, or do I need to take the diode into consideration as well? \$\endgroup\$
    – Carl
    May 18, 2020 at 8:05
  • 1
    \$\begingroup\$ I would have to see the schematic. But if the specan levels are after the diode, then you don't need to do anything else to account for the diode. \$\endgroup\$
    – SteveSh
    May 18, 2020 at 12:21
  • \$\begingroup\$ @SteveSh - Sadly, there were no schematic included for this problem but I still think I have to take into account the diode. I just have to figure out how. \$\endgroup\$
    – Carl
    May 18, 2020 at 17:13

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