Wolf

Many thanks for all your hard work in writing Spectrum Lab.

I have a question about the noise function

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noise(freq1, freq2)

returns the noise floor in the specified frequency range. Uses an algorithm suggested by G4JNT (which is also used in SpecLab's ALERT module).

The definition of noise levels is not easy. Here is the basic algorithm of the 'noise' function:

An array of amplitudes (usually dB values) from the last FFT calculation is sorted into order of increasing amplitude.

The amplitude of the lower quartile value (for example bin number 256 in a sorted set of 1024 points) PLUS 3dB is then returned as an estimate of the mean noise level.

This technique automatically throws away very high values (strong signals) that would otherwise affect the result.

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In your code for this function, is the complete FFT result contained in the 1024 points or do they represent only one side of the spectrum, plus the DC value?

Is the 3dB a typo? In a book written by G4JNT ("Command", published by RSGB)he mentions 5dB not 3 dB.

I have attempted to check this by computer simulation using voltage data generated by a Gaussian random number generator from the book "Numerical Recipes". For large data sets (say > 32K samples), the results suggest that the factor should be 5dB rather than 3dB.

My reason for seeking clarification of this is that I am using Spectrum Lab to measure S/N of nanowave receivers.

73

Barry, G8AGN