[600MRG] WSJT/WSPR S/N numbers

[Larry Molitor]

The following is a clip from a message posted by Joe Taylor to the Yahoo WSJTGROUP. I believe everyone trying to use the S/N numbers generated by software should read this.


Perhaps this is a good time to remind everyone that estimates of 
signal-to-noise ratios for very weak signals *always* have an associated 
(if unexpressed) uncertainty.  When S/N is quoted in dB, this can have 
important consequences.

In a stochastic (noise-like) process, experimentally measured values 
will be scattered around the unknown "true" value; some will be pretty 
accurate, some too high, and some too low.

Suppose a signal's true S/N (in the detection bandwidth) is equal to 
1.0.  In a series of measurements of that signal, some of the estimated 
("measured") values will be close to 1; some will be higher, maybe as 
high as 2 (or even more), and some will be as low as 0 (or even less). 
These are *linear* values of S/N: estimated ratios of signal power to 
noise power.

When the S/N is expressed in dB -- a logarithmic scale -- you quickly 
see the problem.  A perfectly plausible estimated S/N=0.1 means that in 
dB, S/N_dB = -10 dB.  At S/N=0 we get "minus infinity dB".

Some people think it's a big deal when they see a JT65 decode displayed 
with an estimated S/N_dB of, say, "-30 dB".  This is nothing more than a 
statistical fluctuation of noise and an illustration of the nature of 
logarithms.

	-- 73, Joe, K1JT