Created on 2009-05-11 23:32:00

DIM stands for Dynamic InterModulation distortion. It is a technique used to measure the non-linearity of a device, and it’s designed to be particularly sensitive to distortions produced during transient conditions typical of audio program material. In DIM measurements, a square wave at a frequency of 3.15 kHz is low-pass filtered and then linearly combined with a sine wave at a frequency of 15 kHz. DIM 30 and DIM 100 use single-pole low-pass filters with cutoff frequencies of 30 kHz and 100 kHz, respectively.

Fig 1: DIM 30 waveform

If non-linearities are present, the DIM signal induces intermodulation distortion products at nine difference frequencies ranging from 0.75 kHz to 13.35 kHz. DIM is then calculated as the ratio of the root mean square (RMS) sum of the levels of the nine intermodulation components to the level of the 15 kHz sine wave. It is typically expressed as a percentage or in dB.

Fig 2: Nine DIM intermodulation components

An analog domain Audio Precision 2700 series analyzer equipped with the IMD option has the ability to generate DIM 30 and DIM 100 waveforms. But the DIM measurement provided by the system’s Analog Analyzer is just an approximation of the DIM level specified by IEC 60268-3, because it only considers two of the nine intermodulation components specified in the standard. A  DIM measurement that conforms exactly to IEC 60268-3 can be conducted using the DIM generator in conjunction with the FFT analyzer available in DSP-equipped systems (2712 and 2722). This zip archive contains an AP2700 Basic macro that will conduct DIM 30 and DIM 100 measurements per IEC 60268-3.

To use the macro, set up the AP2700 software just as you would to do an analog DIM measurement: In the Analog Generator panel, set the Wfm: control to IMD – DIM 30 or DIM 100; in the Analog Analyzer panel, set the Function Reading meter to DIM; and in the Sweep Panel, set Data 1 to Anlr.DIM and Source 1 to Gen.Ampl A (Figure 3). Now, start the macro to make the measurements and plot the graph. You can also make a single-point measurement by checking “Single Point” on the Sweep panel.

Fig 3: AP2700 panel setup for macro

The macro works as follows: First, it checks to see if the panels are set up as outlined above. If they are not, the macro displays an error message and then exits. Next, the macro reads the sweep parameters from the sweep panel, conducts an FFT-based DIM measurement for each step in the sweep, calculates the DIM levels according to IEC 60268-3, inserts the DIM results into the Data Editor panel (Figure 4) and plots them in the Graph panel (Figure 5). Finally, the macro reverts the Sweep Panel to the setup it had before the macro was run.

Fig 4: Data editor with results

Fig 5: Graphed DIM 30 results

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