I think perhaps this is what Tom was attempting to say or
SHOULD HAVE
SAID and if so, we agree that the 100 dB BDR versus 150 dB
BDR for the
FT1000D is a thing. My disagreement with the published
numbers is that I
have never measured one of these radios with numbers that
were as poor
as given here EVER. He attributed things to mixer noise,
etc. and we
disagree with the mechanism. The measurements done by me or
witnessed by
me were done in professional laboratories with state of the
art
equipment and I am biased to believe what I am absolutely
certain have
been repeated by world class professionals.>>
First, there is little doubt about what I measured. My data
is always within reasonable limits of what anyone else with
a good lab and good techniques measures. I'm not going to
waste bandwidth arguing a few dB when the major problem is
caused by a 20-30dB difference in performance.
Second, so far as the display. A claim was made a SDR
display could, in effect, substitute for packet and allow a
person running single op in a crowded contest to appear at a
new station's frequency in an instant while working people
down the band. That claim is nonsense for several very
obvious reasons. The display, when looking at a large window
that is loaded with signals every few hundred Hz, can't
isolate a pileup. There isn't an operator in the world that
can memorize the position of 500 dancing pips and instantly
spot a new one in the crowd. The receiver also goes "nuts"
when the transmitter comes on...even with 3000 feet of
antenna separation and a 20-30dB polarization null. That's a
blocking or overload problem caused by limits of the
system, and it is not just a few dB problem. It is a 30-40dB
problem. If something new one pops up on a packet spot and a
single op appears instantly again and again, it isn't
because he has a SDR radio and is scanning 50 kHz of a
crowded band and can instantly see it! So the radio just
couldn't do what we wanted it to do. Nothing can, except
packet.
Those were two of the main points.
The third was mixer noise. This isn't EME work. We all know
or should know ANY real mixer has a noise figure, it also
has IM distortion. On 160m, if the mixer is out of
saturation and of any reasonable design, the propagated
noise is all we ever hear. A system can go from a 15dB noise
figure to a 1 dB noise figure and with any normal antenna
(outside of a screwdriver blade vertical) it won't make any
difference at all in what we can copy. What we often mistake
is the fact that reducing system gain can make things appear
better when the receiver is saturated with noise. We often
confuse excessive or unnecessary gain in one system with
proper gain distribution in another.
The final was selectivity. Every other amateur radio in my
house uses 6dB points to define selectivity. My Yaesu,
Kenwood, Drake, and ICOM manuals use 6dB. Because a radio
might say the selectivity is xx Hz, it doesn't mean it is xx
Hz by the standards other radios use. Some software radios
define selectivity by 3db points, and this confuses users
who then compare filters at the common method of 6dB with
filters defined at 3dB points. As an example of the
confusion this causes, my FT1000 measures 230Hz bandwidth
with the 500Hz filter if I use 3dB instead of 6dB. Everyone
should try to use the same measurement point.
The bottom line is a certain speed CW requires a certain
bandwidth or the filter softens the rise and fall. It also
mushes up any noise, making it sound like "pinging" or
ringing. The skirt shape also greatly affects ringing, so
broad skirts (poor shape factor) will improve keying sound.
It true we can optimize things better with digital filters,
but it can't make magic.
My opinion as a low band CW DX and contest station owner is
the best combination to date is a narrow roofing filter
followed by some form of digital processing. Omit the
roofing filter in the superhet and we suffer with the fact
the accumulated signal levels from hundreds of signals (or
even one or two strong transmitters way up the band) can
limit performance. That's just how the system works and we
all know it.
Certainly there are many good things about software defined
radios, but like any system in the world there are also
shortfalls. There also is no magic, and that includes
wasting hardware driving a four square with four
transmitters. It's always good to dream, but at some point
the dream has to merge with real life physics or it just
isn't productive. The real world is what separates good
ideas from bad ideas, and that's where the rubber finally
hits the racetrack.
73 Tom
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