Created on 2009-05-11 23:32:00
DIM stands for Dynamic Intermodulation distortion. It is atechnique used to measure the nonlinearity of a device, and it’s designed to beparticularly sensitive to distortions produced during transient conditionstypical of audio program material. In DIM measurements, a square wave at afrequency of 3.15 kHz is low-pass filtered and then linearly combined with asine wave at a frequency of 15 kHz. DIM 30 and DIM 100 use single-pole low passfilters with cutoff frequencies of 30 kHz and 100 kHz, respectively.
Fig 1 DIM 30 waveform.
If nonlinearities are present, the DIM signal inducesintermodulation distortion products at 9 difference frequencies ranging from0.75 kHz to 13.35 kHz. DIM is then calculated as the ratio of the root meansquare (RMS) sum of the levels of the 9 intermodulation components to the levelof 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 analyzerequipped with the IMD option has the ability to generate DIM 30 and DIM 100waveforms. But the DIM measurement provided by the system’s Analog Analyzer isjust an approximation of the DIM level specified by IEC60268-3, because it onlyconsiders two of the nine intermodulation components specified in the standard.A DIM measurement that conforms exactlyto IEC-60268-3 can be conducted using the DIM generator in conjunction with theFFT analyzer available in DSP equipped systems (2712 and 2722). The attachedzip archive contains an AP2700 Basic macro that will conduct DIM 30 and DIM 100measurements per IEC 60268-3.
To use the macro, set up the AP2700 software just as youwould to do an analog DIM measurement: In the Analog Generator panel, set theWfm: control to IMD – DIM 30 or DIM 100, in the Analog Analyzer panel, set theFunction Reading meter to DIM, and in the Sweep Panel, set Data 1 to Anlr.DIMand Source 1 to Gen.Ampl A (Figure 3). Now, start the macro to make themeasurements and plot the graph. You can also make a single point measurementby checking “Single Point” on the Sweep panel.
Figure 3 AP2700 panel setup for macro.
The macro works as follows: First, it checks to see if thepanels are set up as outlined above. If they are not, the macro displays anerror message and then exits. Next, the macro reads the sweep parameters fromthe sweep panel, conducts an FFT-based DIM measurement for each step in thesweep, calculates the DIM levels according to IEC 60268-3, inserts the DIMresults 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 hadbefore the macro was run.
Fig 4 Data editor with results.
Fig 5 Graphed DIM 30 results.
Accompanies KB article "DIM 30 and DIM 100 Measurements per IEC 60268-3 with AP2700"