Topband: Sherwood data
btippett at alum.mit.edu
Fri Feb 16 09:35:28 EST 2007
>I have looked at Sherwood's data and I am not savvy enough to
understand what is being measured. I am sure there are many others
like me on the Topband reflector and we need to be educated in what
to look for as we evaluate what is the best "bang for your buck."
Read the following:
"QST Product Reviews-In Depth, In English", p. 32-36, August QST:
There are 3 key issues involved:
1. IMDDR3 (two-tone dynamic range)
2. BDR (single tone dynamic range)
3. Phase noise (which can override both)
In a situation such as receiving while transmitting
at the same location (such as W8JI described),
2 and 3 above are more important. In a contest
situation such as W5UN describes, 1 and 3 are
The SDR-1000 is excellent at 1, but only
mediocre at 2, according to ARRL measurements.
Dan Tayloe N7VE (who popularized but did not
actually invent the QSD core of SDRs) generally
agrees that BDR is the weak point of SDRs as
currently implemented. That could change when
and if better sound cards become available.
Although the SDR-1000's IMDDR3 dynamic range of
~100 dB is quite good versus superhets, its BDR
is ~110 dB which is about 20-25 dB worse than
a good superhet. Due to QSD direct conversion,
the dynamic range of ~100 dB for both IMDDR3 and
BDR is limited by the dynamic range inherent in
currently available 24-bit ADC sound cards. Until
better sound cards are available, I doubt that BDR
in SDRs will approach the ~130-135 dB of superhets.
Since ARRL has not yet done a full Expanded Test
Report of the SDR-1000, I am not sure exactly how
it fares with phase noise, which can actually override
both BDR and IMDDR3 resuts at close spacings.
73, Bill W4ZV
P.S. QSD above = Quadrature Sampling Detector
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