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05-11-2015 03:43 AM

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I have a PXI-5402 card sat in a PXIe chassis. I am only interested in sine wave output at frequencies up to approx 10kHz. I know that it is posible to request an output frequency and then query the acutal output frequency but I would rather be able to calculate it before hand. All I can find in the literature is a figure of 0.355uHz for frequency resolution.

Is there a better description of the frequency resolution? If not, is the resolution exactly 0.355uHz or is this an approximation (to 3 significant figures)?

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Accepted by topic author Dave.M

05-11-2015 10:18 AM

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This webcast is an excellent way to learn the process of which the NI 5402 and NI 5406 operate to generate their periodic functions: http://www.ni.com/webcast/75/en/

The 0.355uHz value is a theoretical value of achievable frequency rates based on the Clock Rate and Phase Accumulator size. This is the closest thing I can find on ni.com for you to use to calculate the value: http://zone.ni.com/reference/en-XX/help/370524R-01/siggenhelp/ni_5401_11_31_frequency_resolution_and...

I believe Fc for the NI 5402/5406 should be 100M and the accumulator size is 48-bit. Therefore frequency resolution = Fc / 2N = (100 × 10^6) / 2^48 = 3.55271368e-7

Keep in mind that the device has a VCXO frequency accuracy spec of +/- 25ppm, if you do not PLL lock it to a better source.

Product Support Engineer

National Instruments

National Instruments

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Accepted by topic author Dave.M

05-11-2015 11:35 AM

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Fc / 2^N = (100 × 10^6) / 2^48 = 3.55271368e-7

There was a typo above.

Product Support Engineer

National Instruments

National Instruments

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05-12-2015 03:16 AM

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Thanks!