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If a specific project specification says "...amplifier must be linear such that the harmonic distortions are maintained below -40dB...", does that mean that a THD of -30dB would meet the requirements or would something like -50dB be correct?

I ask this because I know with gain, \$ A_v \$, we usually take absolute values so it's a little hard to understand which one is correct. Any ideas?

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A temperature of -50C is below -40C. Likewise for dB; -50dB is below, or less than, -40dB.

For power, -40dB is a ratio of \$\frac{1}{10,000}\$ while -50dB is a ratio of \$\frac{1}{100,000}\$.

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The wording of your project specification usually is interpreted to mean that the maximum value of any harmonic must be at least 40 dB below the fundamental. Otherwise, a total harmonic distortion (THD) value, which is the result of all of the harmonics, would have been specified. Assuming that, it is not obvious how much the THD must be to meet the stated specification. A value of -30 dB for THD might be OK if none of the harmonics are greater than -40 dB. This would depend on how many harmonics there are and their values since THD is the vector sum of all of the harmonics. A THD value of -50 dB would be OK since it could not be obtained if one or more harmonics were -40 dB (the THD cannot be less than the highest harmonic level).

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  • \$\begingroup\$ The meaning is perfectly clear. THD is Total Harmonic Distortion, not the magnitude of just one harmonic. \$\endgroup\$
    – user207421
    May 6 '13 at 23:31

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