Andy,
         thanks for your suggestion. As a matter of fact I am already using
your algorithm ! I use it to compute the brightness levels for the spectrogram,
but only when AGC is on. So, with AGC on, the brightness is a (non-linear)
function of the ratio between the magnitude and the base noise level
computed with the CFAR method. I learnt it from your previous post of many
months ago, I liked it, and I put it into immediate use.
I will use it when implementing the S/N display, together with the choice of
selecting first a frequency range, whose mean level will be the noise reference
level for the computation.
For the present problem, displaying a relative strenght level, I think I will
allow the choice between the two main methods that have been suggested,
i.e. referring it either to the saturation value of the ADC, or to a previously
choosen reference level.

73  Alberto  I2PHD

Talbot Andrew wrote:

One method for determining S/N ration in an environment where signals
and noise are constantly changing  - used a lot in Radar and Other
techniques - is an algorithm called Constant False Alarm Rate.   I
covered this in detail in postings to this reflector several months ago,
but basically the technique is as follows :

1)      Perform an FFT on a block of data
2)      Sort all the bins into increasing order of amplitude.
3)      Take the amplitude of the lower quartile bin.
4)      Add 3dB to this figure and this gives a very good approximation
to the noise level
5)      For new signal alarm and signal detection add a threshold - 10dB
is a good starting point
6)      Check for successive hits above this threshold N out of M times
before indicating valid.

You will only be interested in satges 1) to 4).  This assumes the real
noise level is flat across the FFT width, which I hope is true for all
narrowband work, but not necesssarily so for full SSB bandwidth
sampling.

Andy  G4JNT