[Elecraft] Analog vs. Digital Front Ends

[Joe Subich, W4TV]

On 9/15/2015 3:39 PM, Lynn W. Taylor, WB6UUT wrote:
> Maybe I'm wrong, but if the thing we're testing is supposed to be a
> radio, and we want to compare how radios work under conditions we'd
> encounter in actual use, it just seems intuitively obvious to the most
> casual observer that the tests should be the same.
>
> 73 -- Lynn


Therein lies the rub ... direct digital conversion SDR devices have
their best performance (best sensitivity) at (composite) signal levels
just below the clip (overflow) point of the ADC in use.  However, at
that point they can not withstand any overload.  If one decreases the 
input signal (e.g., turns off the preamplifier) to provide headroom or
avoid ADC overflow, the sensitivity (MDS) suffers in direct proportion
to the decrease in signal level.

ARRL's review of the Flex-6700 and Flex-6300 shows this inescapable
fact of nature (law of physics) very clearly.  AB4OJ's noise power
ratio testing hides the fact by not providing MDS values under each
test condition *and* fails to indicate that even with *no preamplifier*
the total noise signal is more than 10 dB *less* that that used for
testing with traditional superhetrodyne or downcoversion DSP designs.

73,

     ... Joe, W4TV