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

[Bill W4ZV]

Joe Subich, W4TV-3 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. 
> 

...which is why the hybrid approach of a narrow roofing filter before the
ADC works so well in Orion and the K3.  The narrow roofing filters limit
extremely strong signals outside the passband which would otherwise kick in
ADC protection AGC.  I don't think there's a way around the fundamental
limitation of ADC resolution in the direct conversion designs.  

Phil, on the QS-1R specifications webpage, I see BDR is specified at 125
dB...very similar to the Flex 5000's 123 dB.  I haven't seen any independent
measurements of the QS-1R yet, but hope one will be forthcoming by ARRL. 
Until SDR receivers are in the same >140 dB league of the K3, I'm not sure
the contest community will bite this apple.  When higher resolution ADCs
eventually become available at a reasonable price, I'm sure the situation
will change.

73,  Bill

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