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
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