Monday, April 2, 2012

Mask Testing Feature in Oscilloscopes

In this post we will look at the Mask testing feature found in today's oscilloscopes. Mask testing is a valuable feature that adds pass/fail testing to a scope’s traditional functions. Mask testing lets you capture a ”golden” waveform and define tolerance limits to create a test envelope. Incoming signals are compared to the allowable tolerance range and flagged as pass or fail. You can then select the action the oscilloscope performs if it detects a violation of the mask. Examples of some of these actions include stopping the test after one failure, saving screen images of waveform failures, and making measurements on failure waveforms.

As an example, on an Agilent 3000-X series scope I captured my golden waveform, navigated to the Mask test menu, and selected Automask. The Mask shown below in the figure was setup around my golden waveform. 

In the above figure the failure area for the Mask test is the gray around the waveform. The black area represent the pass area. By adjusting the Y and X parameters (bottom of figure) you can easily tune the size of the pass and fail area of the mask. You can also directly edit the Mask test file for more detailed editing of the Mask pass and failure areas. The below figure shows the example Mask test after being run a number of times.

Notice in the above figure the Mask test statistics. You can see the Mask test was run over 1.7 million times and in that time span 14 failures occurred. You can see in red where the failures occurred. 

The easiest way to perform a Mask test is to capture your golden waveform and use the Automask feature and then adjust the mask as needed. You can also create a custom mask file if you would like. This is done using X and Y parameters and regions. Regions are the separated failure areas in the Mask test. For instance, the above example there are two regions, the top one above the waveform and the bottom one below the waveform. There is typically a limit to the number of Mask regions you can create. For example on Agilent's 3000-X series up to 8 regions can be created for a Mask test. The below figure shows an 8 region Mask test setup.

In this post we looked at the Mask testing feature found on modern scopes. We looked at some Mask test examples using an Agilent 3000-X Series scope. As always if you have any questions feel free to email me and if you have any comments please leave them in the comments section below.