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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: [TECH] A question
Date: Wed, 28 Mar 2001 08:48:26 +0100
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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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