NI-DAQmx: Equivalent Time Sampling (ETS) in DAQmx Using a Pulse-Train

This example demonstrates using DAQmx to generate a Equivalent Time Sample pulse train for use in sampling a periodic signal.

Equivalent Time Sampling (ETS) is a method that allows you to acquire a periodic signal at acquisition rates below the Nyquist Rate. This is achieved by sampling at a divide down of the signal frequency plus a time varying offset. For example, if you wanted to sample a 100 hz signal and you wanted 20 points per period, you would normally have to acquire at 2kHz. However, using ETS you sample at every (N*signal period) + X*.5ms, where N is an integer and X increments on each sample. 

To generate an ETS pulse train, a gate signal is generated at the frequency of the signal to be acquired. A divide down of the frequency may also be used. The Auto Increment feature of DAQmx allows you to add a constant offset on each subsequent pulse to be generated. The offset is defined in terms of number of ticks of a timebase. This allows you to effectively sample a signal at Fs = 1/offset, where the offset is a number of ticks of an onboard timebase. Figure 1 below illustrates the timing.   

For more information on ETS Pulse Generation, see "Pulse Generation for ETS" in the M-Series User Manual.

For this example, a counter generates the gate signal. The counter must generate a divide down of the acquired signal frequency so the signal frequency must be known beforehand. Also, the generated gate signal cannot be faster than the ADC max sample rate becuase the output is used as the sample clock for an analog input.  Signals higher than the bandwidth of the device will still be filtered before being acquired.

Requirements

Filename: etspulsetrain.vi

Software Requirements

Application Software: LabVIEW Professional Development System 7.1
Language(s): LabVIEW

Hardware Requirements

Hardware Group: Multifunction DAQ (MIO)
Driver: NI-DAQmx 8.0

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