PXIe-5840 Internal Frequency Reference Specification

[chriswj88]

The PXIe-5840 Specification for its Internal Frequency Reference specifies accuracy in a table of unitless numbers.  Is this meant to be interpreted as fractional uncertainty, or something else?

Most NI guides I have found on frequency accuracy suggest it will be specified as PPM but that does not seem to be the case here.

[Spex]

1e-6 is an alternate notation for 1ppm.

Source and more info: https://www.rapidtables.com/math/number/PPM.html

[chriswj88]

Spex, Thanks for the quick response.  Sure, I agree that’s the obvious interpretation absent of anything in the spec saying that is how the data should be interpreted, but I was hoping to get confirmation this is in fact what NI intended.

[Spex · Accepted solution]

Hi Chris,

You can consider it confirmed the intention was to represent the frequency stability spec as a fractional uncertainty (ppm).  I'm not sure why some NI manuals use the ppm nomenclature and others use N * 10^-6.  The initial adjustment accuracy is relative to the frequency of interest, e.g. for a frequency of 1GHz, the uncertainty is +/- 200ppb * 1GHz or 200 Hz.

So I can better understand your concern and provide feedback to the spec process, how else would you consider interpreting that spec?

• If the representation was absolute uncertainty, I would expect to see a unit (+/- n Hz).
• Fractional uncertainty (ppm, ppb, etc.) is inherently unitless. ppm is numerically equivalent to 1/1,000,000 or 10^-6 (ppm is not a unit)
• I think it would have been irresponsible if NI published a percentage uncertainty without the % symbol or representing as 0.0001 x 10^-2
• Any other way to interpret?

[chriswj88]

Spex, Thanks for the detailed response.  This certainly clears things up.  Sorry, I missed the that you were responding for NI in the first reply.

I guess it would be good if they were consistent across all product specifications in how that type of information is presented.  Thanks again for your reply!

[Spex]

Hi Chris,

Glad I could clear things up.  I agree with your feedback about consistency.  I'm not sure if different domains prefer to publish that type of spec slightly differently, or why we have inconsistencies.  I tried to do some background work, and I did see that that there is some debate in the engineering community about ppm and ppb as acronyms because they are not part of any formal SI unit standardization.  The m and b are not consistent with "nano" nor "pico" nor "Giga" nor "Mega" prefixes that are standardized in SI, and million and billion are language specific.  So in that sense, the N * 10^-6 notation is more universal, even if it is less well known.  

Regards,