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Intermodulation distortion (IMD) is another measure of distortion due to nonlinearity in the device under test (DUT). IMD measurements use a dual-tone test signal composed of a low-frequency tone at f₁ and a high-frequency tone at f₂. When a dual-tone signal is input to a nonlinear device, the device output will contain intermodulation components of f₁ and f₂ given by m( f₂) ± n(f₁) where m and n are integers.

You often use IMD to measure the distortion of a DUT near the high-frequency limit of the DUT or the measurement system. You also can arrange the test so that many IMD components occur within the measurement bandwidth.

The IMD measurement is dictated by the IMD standard used for the measurement. Several standard configurations for IMD measurements exist, and these configurations use one of two types of IMD test signals.

In the first type of IMD test signal, with a low-frequency modulation tone and a high-frequency carrier tone, the intermodulation components appear as sidebands around the high-frequency carrier tone, as calculated in the following equation:

f₂ ± n * f₁

The following front panel shows the sidebands around the 8 kHz carrier tone.

In the second type of IMD test signal, with two closely spaced tones near the high-frequency limit of the measurement bandwidth, the intermodulation components appear at multiples of the difference frequency calculated by the following equation:

m * (f₂ – f₁)

The following front panel shows a typical IMD test spectrum.

The intermodulation components are found at multiples of 1,000 Hz, the difference frequency between the twin 11 kHz and 12 kHz tones.

Several standards define IMD measurements. Each of these standards has particular advantages and disadvantages when trying to reveal distortion. The following table lists the standards, the associated test signals, and the typical use cases.

IMD Standard	Test Signal (f1, f2, ratio)	Typical Applications
SMPTE/DIN	(60, 7000, 4) (250, 8000, 4) others	Excite low-frequency distortion mechanisms such as thermal distortion in power amplifiers; disk recording and film
ITU-R (CCIF)	(11000, 12000, 1) (14000, 15000, 1) (19000, 20000, 1) others	ADC and DAC slope-induced distortion

The frequencies f₁ and f₂ are the frequencies, in hertz, of the low- and high-frequency tones, respectively. The ratio is the ratio of the amplitude of the low-frequency tone to the amplitude of the high-frequency tone.

Note  Do not expect to get the same measured IMD when performing the measurement with different standards. You must specify the test signal used to perform the measurement when you report IMD measurement results.