[TECH] A question]

[Andre' Kesteloot]

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
>
> > -----Original Message-----
> > From: Alberto di Bene [mailto:dibene@usa.net]
> > Sent: 2001-03-27 13:33
> > To: rsgb_lf_group@blacksheep.org
> > Subject: LF: [TECH] A question
> >
> >
> > I have a question for you all.
> >
> > I am implementing both in Argo and Spectran a function that will
> > display the relative level (in dB) of the spectral lines when
> > you hover the
> > mouse cursor over them. So far so good.
> > What I am uncertain about, is that word, 'relative'. Relative
> > to what ?
> >
> > One possibility is to first click on a given point of the
> > display, take this
> > bin amplitude as a reference, and the values subsequently
> > displayed will
> > be relative to this reference level.  But frankly I don't
> > like this solution
> > very much, even if this will be perhaps the correct way to measure
> > the S/N ratio, but this is another topic.
> >
> > Another solution which has been suggested is to take as reference
> > level the saturation level of the ADC. This is quite valid is case of
> > a single signal, but on a real case what will happen with this method
> > is that the measured level will vary depending on the total
> > input to the
> > ADC, even if the signal being measured is constant in amplitude.
> >
> > I gave a thought about using the Parseval equation, but
> > before pursuing
> > this further, I decided to ask the question here, as perhaps
> > this problem
> > has been already encountered and solved by others.
> > I am open to any suggestions and advices.   Thanks.
> >
> > 73   Alberto   I2PHD
> >
> >
> >
> >
> >
>
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