While it is nice to experiment with software-based radios or discover a
cheap analog switch makes a reasonably good mixer, we have to be careful to
not get carried away thinking these systems are going to do something to
help us work DX.
The largest single problem we have on 160 is noise, much of which is
unavoidable except through directivity of receiving antenna. The curable
stuff is largely caused by defects in transmitter designs.
Its true there are some bad receivers that need help, but ironic that MOST
of the really bad radios are DSP based. My inclination is that DSP systems,
unless extremely powerful (and expensive), bring more problems to the table
than they cure.
Some radios have spurious response problems (the 775DSP and JRC radios I
have seen), some have design flaws causing IM issues (Yaesu NB systems for
one), some have noise in systhesizers. Many have filter shortfalls. The cure
really is just fixing the defects, it isn't a matter of changing technology.
If you don't tightly "beam" your receiver through very strong signals to get
to the DX (and live in a location with very low noise) you probably don't
need anything beyond a normal "cleaned up" FT1000 or any other reasonably
good radio. A regular good properly working radio would put most systems to
the point where the other guy's transmitters are the problem even when
living in a congested RF area like the USA or Europe.
Those of us without large receiving arrays in lower amateur population
regions could almost get away with a Knight-Kit Star Roamer technology
receiver with a Time-Wave DSP and do 99.9% of what the best possible system
would do.
By the way, none of the data or measurements I have been able to find
indicate these software-based radios are as good overall as what we have
now. 40dB TOI is absolutely meaningless unless we know a whole lot more. An
absolute piece of receiving-junk could have 40dB TOI.
As for BW. Ringing is a direct function of bandwidth, the shape of the
bandwidth slope, and varying group delay times through the system as
frequency is changed. As W4ZV pointed out, we can't possibly use 20Hz or
even 100Hz BW with "brick wall" skirts for normal CW. The filter MUST pass
all sidebands generated by the rise and fall time, or it extends the rise
and fall!!!
We have to pass the significant sidebands, or the signal "rings". The
spacing of the sidebands is dictated by the rise time and fall time. This
would also apply to any noise pulses going through the filter. Narrower
selectivity lengthens the duration of noise pulses...even if it is a
software filter....and makes noise "ring". The more it rings, the more noise
sounds like CW.
73 Tom
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