This page will discusses ways of measuring very low levels of THD and IM distortions. These techniques go beyond the straightforward use of a THD or IM analyzer.

For THD measurement, the distortion residual from a THD analyzer is fed to a spectrum analyzer for spectral analysis. The spectrum analyzer separates out the noise and any remaining fundamental from the various distortion products, yielding a much more sensitive arrangement. Even greater sensitivity can be had with the use of a Distortion Magnifier (DM) circuit. The DM is fed the source sinusoid and the output of the amplifier under test. These input and output signals of the amplifier under test are scaled to be of equal amplitude, adjusted for exact opposite phase, and subtracted. A selectable amount of the fundamental is then added back to the result so that there is some known value of the fundamental for the subsequent THD analyzer to lock onto. Typically this process results in a relative magnification of the distortion by a factor of ten or one-hundred.

Consider a power amplifier that is producing 20 volts rms at its output with a THD of 0.001%. The distortion component will be on the order of 200 microvolts (100 dB down from the fundamental). The amplifier has a gain of 20, so the input is 1V rms. The output is scaled down by a factor of 20 to 1V rms, and the input is then subtracted from it. That leaves a distortion component of 10 microvolts. If a distortion magnification of ten is desired, an amount of the fundamental equal to 0.1V is then added to this signal, and the result is fed through a gain of ten. The result is a 1V rms fundamental and a 100 microvolt distortion signal to be fed to the THD analyzer. It can be seen that a 100 microvolt distortion signal on a 1-volt fundamental amounts to a signal with distortion down 80 dB, or 0.01 percent. The THD analyzer thus displays a THD value that is ten times the actual value. For a magnification of 100X, a 0.01V amount of fundamental would have been added back to the subtracted signal, and the result would have been multiplied by a 100X gain, resulting in a 1-Volt signal containing 0.1 percent THD.

Note that distortion and noise in the sinusoidal source are also reduced in the same relative proportion. As long as the DM is implemented with operational amplifiers with very low distortion, the distortion measurement floor of the resulting system is reduced by 20-40 dB.

Twin tone IM distortion is measured by summing two high-frequency tones, such as 19 kHz and 20 kHz, applying the result to the amplifier under test, and viewing the output of the amplifier with a spectrum analyzer. This measurement can also benefit in the same way from the use of the Distortion Magnifier. The DM effectively increases the useable dynamic range of the spectrum analyzer by 20 or 40 dB. Indeed, most types of distortion measurement can benefit in the same way.