Why Can't I Set The Output Sample Rate of My PXIe-54x3 Waveform Generator to 800MHz?

Updated May 3, 2018

Reported In

Hardware

  • PXIe-5413
  • PXIe-5433
  • PXIe-5423

Driver

  • NI-FGEN

Issue Details

On the block diagram of my PXIe-54x3 series waveform generator, I see that there is a 800MHz clock on the output of the device. Why am I not able to set my update rate to 800MHz while in the arbitrary waveform mode of operation?

Solution

The reason that the user is not able to set the output rate of samples, while in arbitrary waveform mode, is due to limitation in physical voltage rise and fall times. There is a discrete amount of time that it takes for output voltages to change from one value to another. This is called rise time and fall time. Farther apart in magnitude the voltage values are, the longer it takes for the output to transition between values.

In arbitrary waveform mode, the user is able to define what the output voltages are at will. This means that the user can potentially set one sample at the absolute minimum output, and the next sample at the absolute maximum output. Because these two voltages are so far apart in magnitude, the time required for the output to transition from one sample to the next is greater than the time between output updates at 800MHz (1.25ns).

Therefore, to account for the possibility of large voltage transitions, the maximum output rate of the device in arbitrary waveform mode is capped at a fixed value.

Additional Information

The internal hardware of the PXIe-54x3 series performs interpolation on the output buffer before it outputs samples. This means that the voltage on the output of the card will actually change every 1.25ns. However, these changes are calculated by the internal hardware. 

Example:
  • The first two samples of the output array are 0 and 1 [0, 1, ....] Volts
  • Output rate is set to 200MHz (5ns between samples)
At t = 0, the out put of the device will be 0V
At t = 1.25ns, the output of the device will be some value between 0V and 1V (determined by the internal hardware)
At t = 2.5ns, the output of the device will be some value between the value at =1.25ns and 1V 
...
At t = 5ns, the output of the device will be 1V

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