PXIe-6738 Sine Wave generation clock

[JohnW242424]

Trying to determine what kind of accuracy to expect from a PXIe-6738 analog output channel using the Voltage Sinewave Generation mode on the test panel. I.e. just need to output a simple sinewave at a known frequency and be assured it is accurate to some ppm.

I'm assuming one of the internal base clocks is used? Then some divider logic is used to get me the desired frequency. Can anyone confirm if this is indeed how this is accomplished? 

[santo_13 · Accepted solution]

In the SFP, you specify the sampling rate and in turn, determines the Sine signal frequency (1/1000 of the sampling rate).

In terms of your question on the accuracy, there is only one 100MHz timebase onboard with 50ppm accuracy and the other 20MHz and 100kHz timebase are derived from this 100MHz.

The actual frequency will depend on the coercing of the sample clock to the available divisor with the base clock. Unfortunately, you don't have a direct way to read back the coerced sampling rate. If you're using LabVIEW/any ADE for automation, you can read back actual sampling rate using property node and thereby you know the actual frequency.

In the below snippet you can see that, though you request 17356 as sampling rate, the generated frequency is not 1/1000 (coerced due to available sampling rate) and turned out to be 17.3551Hz.

Ideally, the 50ppm base clock accuracy directly translate to sampling clock accuracy and if you're generating a complete cycle, then 50ppm directly applies to the frequency generated.

[JohnW242424]

Got it.

Knowing that all timing is derived from 100MHz clock is what I needed to know.

Thank you