Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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Spectrum Analyzer

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Spectrum Limits Dialog

Controls: Spectrum Dialog >> Limits
Macro: SpectLimits

This button opens a dialog that allows Pro-license or trial users to apply limit curves to the spectrum data displayed in Y-log (power spectrum) mode, to allow Pass/Fail testing of transducers or other frequency-sensitive devices or circuits.

Separate Max and Min limit curves can be used, superimposed on the trace in any desired color or line style. If the spectrum of the test system remains within the limits, a bright green 'PASS' message is shown at the upper right of the trace area. When any spectral peak rises above the Max curve or falls below the Min, the message changes to a bright red 'FAIL'.

The test results can be determined via macros that can detect separate fail states for Max and Min of each channel. The macro can then take the appropriate action, such as halting further tests until a new test device number is entered, and showing a custom message of arbitrary size, color, and position.

The Limits test option is normally used with a frequency response measurement. The nature of the driving signal must be considered when applying a Limits test. A noise signal (white, pink, or arbitrary) gives a full spectrum on every trace update, but due to the random nature of the noise any individual trace will not give a representative result... spectrum averaging is needed. Because each spectrum contributing to the average may have large peaks and dips at arbitrary frequencies, the initial frames of the average may produce spurious 'FAIL' messages until the average accumulates enough frames.

Swept or stepped sweep frequency responses test only one frequency at a time, so to see a full spectrum you must use the spectrum averager in Peak mode. (Not to be confused with the Peak cursor readout option in the Spectrum Dialog.) The response typically appears as a "moving cliff", with the leading edge being the current test frequency. Untested frequencies typically have a response that is below the Min threshold, so the 'FAIL' is shown until the last frequency is tested.

The default 'PASS/FAIL' message can be toggled off, if desired, so there are no spurious messages during the test average. Then at the end of the average the final Pass/Fail result can be shown via macro. (The macro can produce a custom message, or simply toggle the default message option back on.)

Noise-based frequency responses have an advantage over sweeps for rapid production Pass/Fail testing. A representative spectrum will appear fairly quickly; waiting for more frames to accumulate just smooths out finer irregularities in the response, which may be well within the test limits anyway. If you test with long averages to allow tight limits, it is easy to spot seriously defective parts early and abort the test.

A full swept response may be best at producing a "publication quality" spectrum, but the quality comes at a speed penalty because only one frequency is tested per frame. For a full 512-line spectrum, 512 frames must be averaged, and you don't get any early indication for untested parts of the spectrum.

You may wish to experiment with multiple-component sweeps to boost the speed: For example, you can use 4 Generator streams to produce 4 continuous or stepped sweeps simultaneously, each covering one-quarter of the spectrum. (Note that each must have its Level set at 25% or less.) This would allow a full spectrum in 128 frames.

Note that Spectrum Limits can be applied to any spectrum, not just a conventional "driven" frequency response. For example, you could test the output from an automobile horn, smoke detector alert, wind chime, musical instrument, or even a human vocal sound like a sustained vowel.

Macro Notes:

If the Spectrum Control Dialog is open, SpectLimits=1 opens the Spectrum Limits dialog, SpectLimits=0 closes it, and SpectLimits=x toggles between open and closed.

You do not need to open the Spectrum Limits dialog to operate any of its controls via macros.

See also Spectrum Control Dialog


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