Pass/fail testing using an oscilloscope’s signal processing capabilities

Article By : Arthur Pini

Pass/fail testing is an analysis function of an oscilloscope that shows whether waveforms meet a set of defined criteria.

Pass/fail testing is an analysis function of an oscilloscope that shows whether waveforms meet a set of defined criteria. The criteria are defined by a series of one or more qualifying conditions that judge whether the waveform passes or fails to meet the criteria.

There are two broad categories of qualifying situations mask testing or parameter comparison. Mask testing compares the sampled values of an acquired waveform to a pre-defined template or mask to see if the waveform sample fall inside or outside the mask. Parameter comparison offer two alternatives, one compares a measurement, based upon the waveform being tested, to a pre-defined value (Param compare) or to another measurement parameter (Dual param compare). Multiple qualifying conditions (termed Qn) each using either method, which are then enabled by selecting them on the pass/fail dialog and defining what results constitute a passing or failing conditions and then what Actions can be taken.

Figure 1 is an example of a simple mask test. This type of test is available on most mid- to high-end oscilloscopes.

mask test setFigure 1 A simple mask test set to stop the acquisition when any sample falls outside the mask. The red exception marker denotes the sample outside of the mask. A test summary shows the number of test completed and the number past as well as the failure rate.

For this test the qualifier Q1 is set to be a mask test of waveform C3. The number of qualifiers varies with the oscilloscope model. This oscilloscope has twelve test qualifiers. Mask testing can be done, as in this case, using a mask based on the waveform or using an industry standard mask. Industry compliance tests generally supply test masks with the test software. Mask test can be gated so as to confine testing to just a portion of the trace.

Pass/fail qualifiers can be combined in a number of ways to determine if the test is passed or failed. Logical combinations of subgroups are also supported offering a good deal of flexibility in the testing.

Pass/fail testing allows designers and technicians to test devices based on the waveforms they produce. The testing can also be applied to mathematically derived functions such as the Fast Fourier Transform (FFT), parameter tracks or the signal envelope.

Consider testing the radar pulse. Shown in Figure 2. This is a radar chirp signal with a 1 GHz carrier.

measurement gatesFigure 2 Testing based on measurement parameter values can be restricted to the active portion of a transient waveform using gating. In this example the measurement gates indicated by the dashed vertical lines contain the measured region of the waveform.

Modulated transient waveforms, such as this radar pulse, are tricky to measure because they consist of active elements and inactive deadtime. Measurements need to be made only while the waveform is in an active state. Oscilloscopes support measurements of transient waveforms using gating. The user places measurement gate cursors about the dynamic portions of the waveforms and measurements are restricted to the area between the cursors. In this example the frequency is being measured between gate markers shown as dashed vertical lines on either side of the pulse.

The frequency of the pulse has a mean frequency of 1.000013 GHz but the minimum and maximum values span a range of from 985.3 MHz to 1013.5 MHz as might be expected for a frequency modulated chirp. Measurement gating has been used to restrict the measurement to the confines of the pulse, ignoring the noisy baseline signal where the pulse amplitude is zero. A second measurement parameter, the peak to peak amplitude of the pulse, is also measured. Pass/fail testing can be enabled testing the state these parameters. The waveform is, however, very complex and there are other measurements that may be considered. Think about a test to assure that signal was symmetrical in the sense that the rise time and fall time of the pulse envelope were identical. Basic measurement parameters wouldn’t work because there are no direct reading parameters to measure that characteristic.

It is possible to extract the signal envelope by amplitude demodulating the pulse. Parameters could then be applied to the demodulated envelope as in Figure 3. This a viewing the signal in the modulation domain.

radar pulse amplitude envelopeFigure 3 Determining the 20-80% rise and fall time of the radar pulse amplitude envelope using the demodulated function and testing that they are within 1 ns of each other.

The amplitude envelope of the radar pulse is extracted using the demodulate math function in math trace F3 displayed in the lower grid. The 20%-80% rise time and fall time parameters are applied to the envelope and the measurements displayed as parameters P5 and P6. Pass/fail testing has been set up, in test condition Q3, based on the comparison of these two parameters. The test criterium is that the 20%-80% rise time and fall time should be equal to within a delta of 1 ns. Parameter comparison is a relatively new test condition that is not based on the absolute value of the parameters but on their relative values, i.e. more than, less than, equal, equal within a tolerance.

For this test only one of the twelve available test qualifiers, Q1 through Q12, is being used but other tests can be setup in which all twelve and be used to determine the passing of failing state of the test.

Tests that can be structured based on other modulation domain information. The chirp is a linear frequency modulation of the radar carrier. Tracking the variation of the frequency sweep can be accomplished by again demodulating the pulse, this time using frequency demodulation. The result of this analysis is shown in Figure 4. The lower grid contains the frequency demodulated waveform. The frequency changes linearly over a range of 985.3 MHz to 1013.5 MHz as was measured previously. The demodulation function is only valid in the region where the pulse is non-zero. A mask (the blue overlay) has been created about the linear portion of the sweep this is controlled by employing gating again, this time applied to the mask test. The vertical range of the mask is 50 kHz about each point in the trace. The equivalent time increment is 100 ns about each sample point. Testing to make sure that all points are within the mask assures that the frequency sweep is linear.

frequency demodulationFigure 4 Using frequency demodulation in order to test the linearity of the radar chirp by creating a mask about the demodulated waveform.

Another way to examine the waveform is to look at it in the frequency domain via the oscilloscope’s FFT. This will show the waveform as a function of frequency as seen in Figure 5.

The FFT of the radar chirp signal is shown in the lower right grid of the figure. It shows the signal’s spectrum centered at 1 GHz. The spectral shape is characteristic of an angle modulated signal, in this case frequency modulation. Note that due to the transient nature of the radar pulse, the FFT weighting function uses rectangular weighting. The broad, flat top indicates the variation in frequency. The FFT response can be tested using mask testing as was done with the modulation domain views of the radar signal. Gates are used to include only the frequencies containing significant power levels. The mask is shown as a gray overlay.

All of these tests can be combined so that the final test consist of the following qualifying conditions:

  1. The FFT trace is all within the overlaid mask.
  2. The frequency demodulated signal has a linear frequency sweep fully contained in the associated mask.
  3. The 20 percent to 80 percent rise and fall times of the modulation envelope are equal to within one nanosecond.
  4. The peak to peak amplitude of the pulse is equal to 147 mV ±10 mV.

The summary view of the testing at the bottom of the figure indicates that all to tests were passed for this acquisition. This testing was done on a Teledyne LeCroy WaveMaster class oscilloscope. Other oscilloscopes will have differing capabilities. Users should consult their oscilloscope’s supplier to determine the test capability of specific models.

spectral viewFigure 5 Adding the spectral view using the FFT completes the elements of the test using the time, modulation, and frequency domains: Four individual tests were combined using a logical combination of the four test results. The summary view shows that all test elements were passed.

Pass/fail testing using the oscilloscope signal processing functions provides a very flexible range of tests for even the most complex signals. In the example data from the time, frequency and modulation domain is being used.

Pass/fail testing is helpful for testing a small run of devices during development, but it can also be used in the automated test environment (ATE) during production. ATE testing in the oscilloscope is generally faster as only the test results have to be transferred rather than bringing all the signals over to a test controller and processing the data offline.

Arthur Pini is a technical support specialist and electrical engineer with over 50 years experience in electronics test and measurement.

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