High-Speed Event and Defect Detection with Real-Time Response

Publish Date: Apr 07, 2009 | 12 Ratings | 4.17 out of 5 | Print


Event and defect detection (transient signal capture) is by nature an unpredictable practice requiring fast, accurate detection. A wide array of applications, including semiconductor reliability testing, disk drive manufacture, neurology, physics, meteorology, seismology, nondestructive testing (NDT), material characterization, and many others, require measurements of rapidly occurring events and/or transient periodic signals. A few high-frequency transient signals from these disciplines that require fast, well-resolved measurements are:
  • Electrostatic discharge (ESD)
  • Neurological action potentials
  • LIDAR and RADAR signals
  • NDT ultrasonic echo signals and eddy currents
  • Reflected laser signals from disk drive surfaces for defect detection
  • Breakdown (quasi and full) events in semiconductor oxides

Table of Contents

  1. NI PCI-5112, NI PXI-5112
  2. Event Detection from Seconds to Months
  3. Multirecord Capability and Deep Memory with Real-Time Response to Events
  4. Timestamping over Months with Nanosecond Accuracy
  5. Illustration of Multirecord Acquisition
  6. Conclusions
  7. Appendix

1. NI PCI-5112, NI PXI-5112

The National Instruments 5112 two-channel high-speed digitizers, for either the PCI or PXI platforms, come with unique event detection capabilities that can detect events or transient signals continuously over months and timestamp these events with nanosecond accuracy. With a maximum sampling rate of 100 MS/s, an analog input bandwidth of 100 MHz and deep memory of up to 32 MS/channel, an NI 5112 can digitize thousands of transients with resolution high enough to extract detailed information from the signal.

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2. Event Detection from Seconds to Months

Suppose you need to capture the following signal output from a transducer (Figure 1). Note that the signal spans an hour or so. During this period, a total of six ‘spikes’ occurred, indicating significant change in the application or experiment. The main objectives are to determine the times these spikes occurred relative to each other and to look at the spikes in detail. The signal between the spikes is not relevant.

The measurement device, in this case a digitizer, should be capable of three tasks:

  • Continuously monitor the signal for spikes during the duration of the application or experiment
  • Determine the times the spikes occur at relative to each other
  • Digitize at a high rate to resolve the details of the spikes

For example, in NDT applications a microsecond impulse stimulus is applied at a 15 kHz rate, typically, and the material response is measured. Depending on the application, you might be interested in events such as threshold violations or the whole response (so-called A, B, C, and D-images or scans) to the impulse stimulus. Essentially there are two measurements that can be made.

  • To determine stimulus propagation in the material, the whole response to the impulse stimulus is acquired and images of the propagation are constructed.
  • To detect structure flaws, threshold violations can be detected and measured.

Figure 1. Transient Signal Illustrating Spikes Occurring at Random Intervals

A common approach to the task is to digitize the entire signal from start to finish. Although this approach yields more complete information, it has two major drawbacks:

1. In acquiring the entire signal, the measurement system cannot react to event detection in real-time. In numerous applications, the detection of events and defects has to trigger something else and the response time has to be extremely fast.

2. You need to supply a large high-speed memory buffer to store the digitized signal. If it is not critical to resolve the finer details of the signal, you can digitize the signal at a slower rate and thus capture the signal for a longer period. In most applications, however, both the times at which the spikes occur and the details of the spikes are important. In NDT applications, for example, shapes of responses plays an important role. You could have many spikes (or hills and valleys) in a signal. These indicate reflections, which can be translated into the geometry of objects or flaws. A quick calculation of sampling at 100 MS/s for one minute indicates the need of 100 MB of data storage at 8-bit resolution. Sampling at MHz rates for minutes imposes demands for memory buffer sizes similar to common desktop PC Zip drive memory sizes, which escalates the cost of measurement systems.

So how do you resolve these issues?

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3. Multirecord Capability and Deep Memory with Real-Time Response to Events

An intelligent approach is to make the digitizer smarter by making it selective in what it digitizes. Because an NI 5112 performs multirecord acquisitions, one can capture multiple triggered waveforms without software intervention. Multirecord acquisition is a practical and efficient way of capturing the relevant portions of the signal. Each record is stored in a separate buffer in the onboard memory of the digitizer. Thus, with accurate analog triggering circuitry, one can capture the signal at the correct times. Furthermore, when each record is triggered, the NI 5112 can output a TTL-level signal to trigger other devices in real time as well for real-time response to event detection.

For example, in semiconductor reliability testing, gate oxide breakdown events are of interest. When a full breakdown event occurs, the voltage stress applied to the failing part needs to be shutdown soon thereafter. In most reliability testing systems, many parts are tested either in parallel or in series. If they are tested in parallel, a failing part must be switched out of the circuit to prevent it from drawing a current large enough to alter the voltage stress on the other parts. If they are tested in series, then a special shorting circuit must be switched in to continue testing the rest of the parts. A real-time response to a full breakdown event is critical in either parallel or series reliability testing.

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4. Timestamping over Months with Nanosecond Accuracy

Users doing transient capture over any period of time also need to know exactly when the transient occurred. You can use multirecord acquisition to easily capture all six spikes shown in Figure 1 by setting up the digitizer to acquire six records and to trigger on a rising slope and signal level of +0.3 V. To determine when these spikes occurred with respect to each other, you use the "timestamping on events" feature of an NI 5112 digitizer. NI 5112 digitizers use a clock to accurately timestamp the trigger event to within 2 ns. On an NI 5112, this clock is a high-precision 48-bit counter clock. Using timestamps, you can correlate multiple records or even multiple acquisitions. You can, for instance, determine the time between acquisitions, or between multiple records for up to 130 days and have each acquisition temporally correlated to within 2 ns accuracy.

In meteorology studies, for instance, lightning strike detection is important. Here the randomness of the lightning strike makes it imperative that your measurement system be capable of monitoring the thunderstorm for all lightning strikes that occur. The storm may span minutes or hours and the lightning strikes can occur seconds or minutes apart. Using the timestamping abilities of an NI 5112, you can monitor the entire storm, detect the lightning strikes, and know when they occurred.

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5. Illustration of Multirecord Acquisition

Consider the signal output in Figure 2. The signal reflects events occurring at 7 kHz intervals. If we sample at 100 MS/s and acquire a single record of 500 kS, we capture 31 events for a period of 5 ms.

Figure 2. Single-Record Capture of High-Speed Phenomena

Figure 3. Multirecord Acquisition of the Signal in Figure 2.

Figure 3 shows the signal from Figure 2 digitized in multirecord acquisition mode. With a memory buffer of a total of 200 kS (4 kS/record), you can capture 50 events spanning a period of more than 8 ms with each event timestamped. In multirecord acquisition mode, one can capture a larger number of events with optimal use of the onboard memory. As pointed out earlier, when each event is detected, the NI 5112 can also output a TTL signal as a real-time response.

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6. Conclusions

Using the following features of NI 5112 digitizers, you can address applications calling for high-speed event and defect detection:

  • Maximum sampling rate of 100 MS/s,
  • Input analog bandwidth of 100 MHz,
  • Deep memory with multirecord acquisition mode of up to 65,536 records
  • Timestamping of records to nanosecond accuracy
  • Real-time response output to event or defect detection

For information on how two NI 5112 high-speed digitizers have been used by Seagate Technology in this type of application, click here.

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7. Appendix

The following section is an excerpt from the NI High-Speed Digitizers Help File which details the available NI 5112 triggering modes available for event, defect, and transient signal detection.

Triggering Modes of the NI 5112
There are several triggering methods for the NI 5112. The trigger source can be one of three modes.

  1. Analog trigger – the trigger can be an analog level that is compared to the input signal or an auxiliary analog signal.
  2. Digital trigger – the digital triggers are TTL-level signals with a minimum pulse-width requirement of 10 ns.
  3. Software trigger – you can call a software function to trigger the device.

The main trigger modes useful for event, defect, and transient signal detection are analog and digital triggering, with analog triggering being the prevalent trigger mode for this class of applications.

Figure A-1 shows the different trigger sources.

Figure A-1. Different Trigger Sources

For further information on RTSI and PFI lines refer to the NI 5112 User Manual.

Analog Triggering Modes

Analog Trigger Circuit
The analog trigger on the NI 5112 operates by comparing the analog input to an onboard threshold voltage. This threshold voltage is the trigger value, and can be set to any voltage within the input range. A hysteresis value associated with the trigger is used to create a trigger window the signal must pass through before the trigger is accepted. Triggers can be generated on a rising-edge or falling-edge condition as illustrated in Figures A-2 and A-3.

Positive-Slope Hysteresis Analog Triggering Mode
A positive-slope hysteresis trigger is generated when the signal crosses below the voltage specified by the trigger level parameter minus the hysteresis parameter and then crosses the trigger level.

Figure A-2. Positive-Slope Hysteresis Analog Triggering Mode

Negative-Slope Hysteresis Analog Triggering Mode
A negative slope hysteresis trigger is generated when a signal crosses above the voltage specified by the trigger level parameter plus the hysteresis value and then crosses the trigger level.

Figure A-3. Negative-Slope Hysteresis Analog Triggering Mode

Edge Triggering
An edge trigger occurs when a signal crosses a trigger threshold you specify. The slope can be specified as either positive (on the rising edge) or negative (on the falling edge) to the trigger. Figure A-4 shows edge triggers.

Figure A-4. Positive and Negative and Edge Trigger Modes

Window Triggering
A window trigger occurs when a signal either enters or leaves a window you specify.

Figure A-5. Entering Window Trigger Mode

Figure A-6. Leaving Window Trigger Mode

Digital Triggering
Digital triggering is useful for applications where another device initiates operation of the application and the digitizer is required to commence sampling. NDT applications where ultrasound impulse stimuli are applied to materials at high repetition rates require digital triggering.

A digital trigger occurs on either a rising edge or falling edge of a digital signal. Digital triggering is available on the RTSI lines, PFI lines, and the PXI Star Trigger line. For more information on these lines refer to the NI High-Speed Digitizers Help File.

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