Time Histogram Measurements with NI High-Speed Digitizers

This example demonstrates how to make time histogram measurements with a high-speed digitizer. You need a signal to analyze, so a program is included to generate a waveform using a NI 5411 Arbitrary Waveform Generator (AWG). If you do not have a NI 5411, any device that can produce waveforms can be used.

Time histograms bin data samples based on their time from the trigger. Only data falling in a defined voltage range and time window is included in the histogram. Data is added to a time histogram over multiple acquisitions, which makes time histograms useful for analyzing frequency or pulse-width jitter.

Open and run the VI named 'Generate Time Histogram Waveform.vi'. The NI 5411 AWG generates a square wave with a period that fluctuates in time with a Gaussian distribution.

Open and run the VI named 'Time Histogram Exercise.vi'. This VI programs the digitizer to analyze signal jitter and to trigger on the rising edge of a signal. This VI creates a time histogram based on points that fall in a user-defined window marked with cursors on the signal graph. Each point in the window is counted in the appropriate time bin in the histogram.

As the program runs, change the histogram parameters and then press the clear stats button to clear the histogram’s history (and reset the parameters). Here are a few interesting things to notice:

· Note the shape of the histogram distribution. The pulse width should be a Gaussian distribution.
· Note that two scalar measurements are being computed. A pulse-width measurement and its result as well as a running average are displayed. This is a simple method to analyze jitter.
· The second scalar measurement is Time Histogram Mean. Compare the Time Histogram Mean with the mean value of the pulse-width measurement. Try other Time Histogram scalar measurements. For a description of these measurements, see the NI-Scope VI Reference Help file available in the Start Menu.

The limits of the time histogram are set by the attributes NISCOPE_ATTR_MEAS_TIME_HISTOGRAM_LOW_TIME and NISCOPE_ATTR_MEAS_TIME_HISTOGRAM_HIGH_TIME.
These limits are set during the first time histogram measurement.
The size of the histogram is set with NISCOPE_ATTR_MEAS_TIME_HISTOGRAM_SIZE. The default is 256 bins.
The time histogram also has voltage limits that specify the range of voltages included (it is necessary to exclude points, or each bin in the histogram will be equal).
The voltage limits are set with the attributes NISCOPE_ATTR_MEAS_TIME_HISTOGRAM_LOW_VOLTS and NISCOPE_ATTR_MEAS_TIME_HISTOGRAM_HIGH_TIME. These values can change with every acquisition.

The following scalar measurements can be performed using a multi-acquisition histogram:
NISCOPE_VAL_TIME_HISTOGRAM_HITS
NISCOPE_VAL_TIME_HISTOGRAM_NEW_HITS
NISCOPE_VAL_TIME_HISTOGRAM_MEAN
NISCOPE_VAL_TIME_HISTOGRAM_STDEV
NISCOPE_VAL_TIME_HISTOGRAM_MODE
NISCOPE_VAL_TIME_HISTOGRAM_MIN
NISCOPE_VAL_TIME_HISTOGRAM_MAX
NISCOPE_VAL_TIME_HISTOGRAM_PEAK_TO_PEAK
NISCOPE_VAL_TIME_HISTOGRAM_MEDIAN
NISCOPE_VAL_TIME_HISTOGRAM_MEAN_PLUS_STDEV
NISCOPE_VAL_TIME_HISTOGRAM_MEAN_PLUS_2_STDEV
NISCOPE_VAL_TIME_HISTOGRAM_MEAN_PLUS_3_STDEV

Requirements

Filename: 202631.zip

Software Requirements

Application Software: LabVIEW Full Development System 6.0
Language(s): LabVIEW

Hardware Requirements

Hardware Group: High-Speed Digitizers (Scopes), Signal Source (AWG/FG/AO)
Driver: NI-SCOPE 1.6, NI-FGEN 1.0.1

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