[Elecraft] K3 & ADAT ADT-200A by HB9CBU

[Joe Subich, W4TV]

> You are making the incorrect assumption that we have no 
> control over the upper or lower limits.  If band or sky noise 
> is the limiting factor on the low end, then adding 
> attenuation in front of the ADC to adjust for this has the 
> benefit of increasing the high end limit also.  This ideally 
> has not changed the dynamic range, but only shifted the upper 
> and lower limits.

No, if band noise at -110 dBm is the limiting factor as it 
often is below 5 MHz, no amount of attenuation is going to 
allow you to resolve a - 125 dBm signal.  Attenuation may 
help you handle blocking signals, particularly those 20 to 
100 KHz away that are eliminated by the narrow filtering 
in other receiver designs and thus deal with the "window 
overload" issues but that's all that moving the "IMD range" 
can accomplish. 

> The Perseus or QS1R will easily outperform the K3 in the case 
> of multiple strong signals in discerning a weak signal as you 
> have described.  This is WITHOUT AGC in front of the ADC.

I do not believe that any wideband SDR can cope with the 
instantaneous peaks greater than the ADC rail in the 
limiting condition (a large number of extremely strong, 
say greater than -10 dBm,  within the bandpass filter 
"window.").  While the chance of the instantaneous maximum 
decreases as the number of signals increases, the average 
signal level continues to increase at an exponential rate.  

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Philip 
> Covington
> Sent: Tuesday, December 02, 2008 6:12 AM
> To: microham at microham-usa.com
> Cc: elecraft at mailman.qth.net; Bill W4ZV
> Subject: Re: [Elecraft] K3 & ADAT ADT-200A by HB9CBU
> 
> 
> On Mon, Dec 1, 2008 at 8:38 PM, Joe Subich, W4TV 
> <lists at microham-usa.com> wrote:
> 
> > Of course, but there is still a finite range between the
> > two limits.  The maximum instantaneous peak is driven by
> > the number and strength of incoming signals and the minimum 
> > discernable signal level is defined not by processing gain 
> but by "sky 
> > noise."  All the processing gain goes only so far ... the limit is 
> > still well above the theoretical ability to detect one 
> signal in the 
> > absence of noise.  Processing gain may be of value at microwave 
> > frequencies with a quiet sky but it is not going to help 
> below 10 MHz 
> > with extreme signals and high noise levels.
> >
> > 73,
> >
> >   ... Joe, W4TV
> 
> You are making the incorrect assumption that we have no 
> control over the upper or lower limits.  If band or sky noise 
> is the limiting factor on the low end, then adding 
> attenuation in front of the ADC to adjust for this has the 
> benefit of increasing the high end limit also.  This ideally 
> has not changed the dynamic range, but only shifted the upper 
> and lower limits.
> 
> The Perseus or QS1R will easily outperform the K3 in the case 
> of multiple strong signals in discerning a weak signal as you 
> have described.  This is WITHOUT AGC in front of the ADC.
> 
> -- 
> Phil Covington
> Software Radio Laboratory LLC
> Columbus, Ohio
> http://www.srl-llc.com _______________________________________________
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