> These kinds of shortcomings are not specific to DSP
> designs. Any RX that has a ton of mixers will share
> these issues. This is precisely why I'm partial to a
The point is DSP filters are so far back in the system, they might
as well be at the headphone jack. While I can find many radios that
have narrow selectivity after just second mixer, virtually all DSP
filters are after three or more mixers and a half dozen other stages.
> direct conversions scheme along the lines of the
> Collins 95S-1A. As anyone will tell you a DC RX is one
> of the cleanest sounding designs there is, just don't
> expect any selectivity. But that is where the
> wonderful new world of ADC and DSP come in.
What is the IM and blocking dynamic range of this "wonderful new
world"?
> <snip>
> > The last thing I want is a receiver that moves
> > selectivity even further
> > back in the system running at frequencies my dog can
> > hear so
> > marketing departments can say it is "true IF DSP".
>
> I'm still puzzled why low frequency DSP IF designs are
> so frequently derided. With a direct conversion/DSP
> scheme the selectivity is moved to being just after
> the very first (and only distortion producing) mixer.
I admit any audio filter will be a nice addition to a direct conversion
receiver, but my concern is working weak signals near very strong
signals. My point is the typical DSP-based radio runs the DSP
system at a few dozen kilohertz well to the tail end of a complex
system.
Moving something from the audio line to an additional special last
IF near a few dozen kilohertz is not actually much different that just
sticking the DSP on the audio line.
This all goes back to my earlier point marketing departments are
selling people on systems that are another step down in
performance, and making a backwards step in performance a
"desirable feature".
While an additional DSP filter can be useful in some cases,
depending on the over-hyped currently available DSP systems for
selectivity simply results in a system that does not handle nearby
strong signals nearly as well as conventional filtered radios.
Bottom line is I don't want or need 2000 filters, especially if the
price is decreased close-spaced (none of those silly useless
measurements outside the passband of the roofing filters please)
performance. Five selectivity settings that can actually handle
strong closeby signals are enough, thank you very much!
73, Tom W8JI
W8JI@contesting.com
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