About Limit Testing
Limit testing is a math function where the oscilloscope captures a waveform, makes a measurement of a waveform parameter, and then compares the measured parameter value to user-defined maximum and minimum values for that parameter. A failure is indicated if the measured parameter falls outside the set max and min values.
Limit Testing Applications
Limit testing can be used in any testing environment; however, it is commonly used in environments when continuous and/or long-term monitoring is required. This type of testing is sometimes also referred to as pass/fail or go/no-go testing.
For example, an oscilloscope user could set up the instrument to monitor a device as it runs overnight. In the morning, the user can easily see how many times the device exceeded the set limits, out of the total number of waveforms that were captured during that time.
ZTEC Limit Testing
ZTEC’s M-Class and C-Class modular oscilloscopes provide more than just an indication of a failure when limit testing. They also display a number of statistics, providing additional insight into the behavior of the acquired waveforms. Statistics include the total waveforms acquired (passes + failures), the absolute maximum and minimum values of all acquired waveforms, a running average value for the parameter that is being tested, a running value for standard deviation of the parameter (only on M-Class instruments), and the parameter value for the last waveform acquired. The statistics value can be reset at any time.
Additionally, for M-Class instruments, the external output can be set to output a pulse when a limit failure occurs. This external output can be used to signal a problem – sound an alarm, trigger the device-under-test to shut down, etc.
Limit Testing Examples
The screen image below shows the Calc1 channel being setup for limit testing. The source of the calculation is the waveform input on channel 1 of the oscilloscope. The operation is set as “Limit”.
Next, the user selects which measurement (waveform parameter) is to be tested, and what the allowed limits of the parameter are. In this example, we will measure the Peak-to-Peak voltage of each acquired waveform. Below is the list of waveform parameters that can be selected for limit testing.
AC RMS, Amplitude, Average, DC RMS, Duty Cycle High, Duty Cycle Low, ENOB, Fall Crossing Time, Fall Edges, Fall Overshoot, Fall Preshoot, Fall Time, Frequency, High, Low, Maximum, Mid, Minimum, Peak-to-Peak, Period, Phase, Pulse Width Positive, Pulse Width Negative, Rise Crossing Time, Rise Edges, Rise Overshoot, Rise Preshoot, Rise Time, SFDR, SINAD, SNR, Standard Deviation, THD, Time of Maximum, Time of Minimum
The allowed limits have been set as 4.1V for a maximum allowable peak-to-peak voltage, and 3.9V for a minimum allowed value.
The following screen image shows the results of a limit test. The most recently acquired waveform is shown in yellow and the most recent waveform that failed the limit test is shown in orange. At the bottom of the display in orange, you see the statistics that we referred to above. In this example, we have acquired a total of 116 waveforms, 5 of which were failures. The minimum peak-to-peak value of the 116 waveforms was 4.043 V (pass), and the maximum value was 4.4922 V (fail). The average peak-to-peak value of the 116 waveforms is 4.0827 V, the standard deviation is 84.9175 mV, and the peak-to-peak value of the most recently acquired waveform is 4.0625V (a pass).
Additional Resources
