THD+N stands for
You can think of THD+N as everything coming out of your device other than the signal you put in.
Harmonic distortion and noise come from different sources. An ideal sine wave has only one frequency. In the real world, though, imperfections (or "nonlinearities") in real devices modify or "distort" the wave, most often by adding harmonics.
Harmonic distortion is easy to identify because the distortion products always occur at integer multiples of the original signal's frequency. With a distorted 1 kilohertz fundamental signal, you'll see energy at 2k, which is the second harmonic, 3k (the third harmonic), 4k (the fourth harmonic) and so on. If the fundamental is at 4k, the second harmonic is at 8k, the third at 12k, and so on, following the same pattern.
A THD measurement sums the total of all the distortion products. It adds up all that extraneous harmonic energy into a single value, "total harmonic distortion."
The "noise" of a device is all the energy coming out of the device that's not related to the input signal. Noise sources can include power supply hum, radio frequency interference, switching noise, even the thermal noise of the circuit components themselves.
THD+N Level is expressed in volts rms, or in a related absolute unit. THD+N Ratio is the more common measurement because what you usually want to look at is not the absolute distortion level, but the relative level of the distortion products compared to the total signal. As a ratio, it's usually expressed in either percent or in decibels. For percent, we take the THD+N level and divide by the total signal level. 0.01 % distortion is a typical THD+N ratio for a power amplifier. Distortion ratios express a wide range, and it's often convenient to use a logarithmic scale and state THD+N in decibels. Since the sum of the distortion products will always be less than the total signal, the THD+N Ratio will always be a negative decibel value, or a percent value less than 100%. -1 dB would be 89% distortion; -40 dB is 1% distortion.
Hopefully your device is more like -80 dB or .01% distortion or even -100 dB, which is .001%. A good rule of thumb is every additional 20 dB down moves the decimal one place to the left. Of course, APx calculates this for you automatically.
Keep in mind when making measurements, the analyzer has to have a significantly lower THD+N than the device under test. The THD+N of an APx525 is -112 dB (or 0.0002%), when measured with a 1 kilohertz signal at 2.5 volts rms across a 20 Hz to 20 KHz bandwidth. The 2700 Series has the lowest THD+N of any audio analyzer in the world: at the same amplitude and range, it has better than -118 dB THD+N, or 0.0001%.
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