Calculating the true output frequency of a PXI-5402

[Dave.M]

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)?

[A_Donfigulation_Error_Has · Accepted solution]

The 0.355uHz value is a theoretical value of achievable frequency rates based on the Clock Rate and Phase Accumulator size. 

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.

[A_Donfigulation_Error_Has · Accepted solution]

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

There was a typo above.

[Dave.M]

Thanks!

[kirsch]

NI-FGEN allows you to configure the frequency and then read it back without actually calling Initiate(). This will returned the coerced frequency as well. No calculations needed. No need to start generation.

Keep in mind that at this levels of precision, you start getting into small imprecisions with regards to what a double can represent.

Good luck!