[Elecraft] Analog vs. Digital Front Ends

[Eric Swartz - WA6HHQ, Elecraft]

I think what everyone is missing here is that Adam has clearly stated that his 
tests were not designed to be tests to directly compare analog and direct 
sampling radios, and certainly they were not intended by Adam for direct 
comparisons to the tests run by the ARRL and Sherwood. They were originally 
intended as -additional- lab tools for technical people and those like us here 
at Elecraft and Flex, ICOM, Yaesu etc to use for evaluation of our designs going 
forward. We can certainly take additional data in the lab to make these tests 
even more useful as comparisons between different technologies etc.

Your comment about including the mds of the rig and the A/D limit level at the 
settings used for each of his tests will help a lot though in comparing the 
results. I'd also like to see that.

One other note - the 2.7 kHz SSB filter he used on his test of the K3 (without 
new synth) does not have as good of a stop band and shape factor as our 2.8 kHz 
and narrower 8-pole SSB filters. Thereis a big difference between thge two for 
this type of testing.  I'd like to see the noise power test data for a K3 (or 
K3S) with new synth and an 8 pole 2.8 kHz or narrower filter. We did 
surprisingly well with the 2.7 kHz 5-pole in his tests. I know what he will see 
with the 8-pole filter and new low noise Synth. :-)

73,
Eric
/elecraft.com/

On 9/15/2015 1:36 PM, Joe Subich, W4TV wrote:
>
>
>
> 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
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to eric.swartz at elecraft.com
>