Noise floor

    Hi, I have submitted a peper, and the reviewer said that many of the turbulence measurements will be significantly contaminated by instrument noise in slack tide because of the turbulence are very low,  he asked us to give the noise floor values of  the ADV. How could we get this values? What is the different between SNR and noise floor? Thanks!

Hi Liu Han,

The easiest method to determine the noise floor of the instrument is to examine a velocity spectrum. At some point the spectrum will flatten out in the higher frequency region (or wavenumber depending on what you are plotting). You can use this value to determine a variance due to noise for your measurements. For a velocimeter this will largely be a function of the selected nominal velocity range. This is the method used here (http://www.nortek-as.com/lib/bibliography/internal-wave-generation-in-lakes-with-very-slow) to determine the variance for a HR Profiler.

SNR is the Signal-to-Noise ratio and is a data quality indicator. For good measurements we typically want this to be above 15-20 dB for the Nortek Vector (I'm assuming this is what you used since you were measuring in a tidal flow). You can find quite a bit about SNR in Nortek's instruments by searching the forums (http://www.nortek-as.com/en/knowledge-center/forum/search?q=snr). This will give you a start on understanding SNR (quoting from one of Atle's posts found from the search link above):

"The SNR is calculated by subtracting the noise level in counts from the amplitude levels measured in counts during velocity measurement. The noise level is measured at the beginning of each burst (and at the beginning of continuous measurements). This non-dimensional SNR value in count is then multiplied by the scaling factor of 0.43 dB/count, to arrive at the SNR in dB."

Hope that helps.

P.J.