DAC - IMD vs Amplitude Measurements

[Tranquility Bass]

Introduction:

The IMD vs Amplitude measurements are good for testing a devices linearity. In this test, usually two or more tones are mixed together and fed to the EUT (equipment under test) to see how it responds. In an ideal test what is fed into the EUT should also be present at its output at the same relative amplitudes and without any other frequency components or artifacts present if the EUT is perfectly linear. However in the real world some non linearity is always present and we expect to see some frequency artifacts in the form of difference or summation of the two frequencies which show up on a frequency spectrum as side bands around the fundamental test frequencies.

We also use this test to show up a flaw in some Esstech Sabre DAC implementations in the form of a hump in IMD measurements at a certain band of signal amplitudes.

Test Methodology

From Amir's excellent website:-

Intermodulation Distortion

When an ideal linear system is fed two tones, it produces two tones. But when fed to a system with linearity errors, we get modulation frequencies above and below our two tones. This is called intermodulation distortion. There are many dual tone tests. For this, I have picked the SMPTE test which combines a low frequency (60 hz) with a high frequency (7 kHz) in a 4:1 ratio. Here is the explanation from Audio Precision:

The stimulus is a strong low-frequency interfering signal (f1) combined with a weaker high frequency signal of interest (f2). f1 is usually 60 Hz and f2 is usually 7 kHz, at a ratio of f1_f2=4:1. The stimulus signal is the sum of the two sine waves. In a distorting DUT, this stimulus results in an AM (amplitude modulated) waveform, with f2 as the “carrier” and f1 as the modulation.

In analysis, f1 is removed, and the residual is bandpass filtered and then demodulated to reveal the AM modulation products. The rms level of the modulation products is measured and expressed as a ratio to the rms level of f2. The SMPTE IMD measurement includes noise within the passband, and is insensitive to FM (frequency modulation) distortion.​

[Tranquility Bass]

No IMD hump present !