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

[Joe Subich, W4TV]

> You are right 
> that if you exceed the maximum voltage level into the ADC no 
> amount of processing gain will change this fact. 

Thank you.  Now, since you have set a hard limit on the signal 
to the ADC, any architecture must either allow sufficient 
margin to prevent overload or must install some form of hardware 
AGC to prevent overload from multiple, simultaneous, strong 
signals.  Any headroom comes from the practical dynamic range 
(maximum signal level minus absolute MDS (even with processing 
gain).  Choose the AGC route and you're back into AGC blocking 
where strong unwanted signals can decrease the gain to a point 
that a weak signal drops below detection level even with all 
the processing gain in the world.  

All SDRs, even the Perseus, attempt to minimize the effects 
of strong "unwanted" signals by using some form of front end 
filtering.

It is the strong multiple signal condition that is the problem 
for contesters/DXers on the lowbands where the goal is to copy 
a -140 dBm signal in the middle of 20 or S9+40 dB "locals" all 
within 20 or 30 KHz.  This is the same problem on 7 MHz in 
Europe where a user might be trying to copy a -140 dBm signal 
with three or four 0 to +10 dBm mega-power broadcasters only 
100 KHz away.  In either case, ADC overload (overflow) and/or 
ACG induced blocking are almost guaranteed with ANY SDR design 
that does not utilize very tight front end ("roofing") filters. 

> The weakest signal 
> that can be resolved plays just as important a part in the 
> calculation of the blocking dynamic range of a receiver as 
> its strong signal handling capability - and this is true 
> whether we are talking about ADCs or analog receivers.

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 
 
  


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Philip 
> Covington
> Sent: Monday, December 01, 2008 4:40 PM
> 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 3:56 PM, Joe Subich, W4TV 
> <lists at microham-usa.com> wrote:
> >
> >> This is not correct.  You can't just compare the number of bits 
> >> without taking into consideration the ADC sampling rate and signal 
> >> bandwidth.  Please see the concept of "signal processing gain" or 
> >> "process gain".
> >
> > The concept of processing gain is completely different than 
> blocking 
> > dynamic range.  The maximum large signal capability of any 
> DAC has an 
> > absolute limit based on the largest integer the DAC can resolve and 
> > the DAC reference voltage. When more than one signal is 
> present within 
> > the "window" (DC to maximum
> > frequency) of the DAC the DAC needs to be able to handle the sum of 
> > the instantaneous peak (vector maximum) voltages not their average 
> > levels.
> >
> > No amount of decimation and processing will reduce the 
> instantaneous 
> > peak voltage that the ADC must handle to prevent overload.  The 
> > clipping (overload) level results in distortion (IMD) or imposes an 
> > operating condition that must be prevented by the application of 
> > hardware AGC (gain
> > reduction) ahead of the ADC (blocking).  Decimation and processing 
> > gain are only of value as long as he hardware is operating 
> within its 
> > linear range.
> >
> > 73,
> >
> >   ... Joe, W4TV
> 
> I assume you mean ADC above and not DAC...  You are right 
> that if you exceed the maximum voltage level into the ADC no 
> amount of processing gain will change this fact.  What the 
> processing gain does give you is the ability to hear weak 
> signals that are much lower than the 14 or 16 bit ADC without 
> decimation would be capable of hearing.  The weakest signal 
> that can be resolved plays just as important a part in the 
> calculation of the blocking dynamic range of a receiver as 
> its strong signal handling capability - and this is true 
> whether we are talking about ADCs or analog receivers.
> 
> -- 
> Phil Covington
> Software Radio Laboratory LLC
> Columbus, Ohio
> http://www.srl-llc.com _______________________________________________
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