Tom does have some good points, factually. However, as I understand today's
technology, while practical to do so, not economical to do so. What Tom
really wants is to be able to create the selectivity and noise reduction
between the antenna and the RF amp and certainly prior to any mixers and IF
amps. That's ideal! Imagine a 250 Hz wide filter for CW prior to the RX RF
amp and any mixers. WOW! But not in economical practice today!
So manufacturers do the next best thing, put the DSP in the system,
operating at a frequency to which we can afford to by the silicon. That's
in the lower IF's
Just wait, one day the ideal of today will be here but not today.
And yes if you insist on selectivity like that, selectivity which is prior
to any mixing, it is available today. Single frequency, narrow bandwidth,
crystal filters will provide amazing selectivity.........for a price. And
remember they are single frequency. So if you have your favorite CW
frequency, a filter that 250 Hz wide at the 3 dB points and down 60 dB some
300 Hz from center frequency can be placed between the bandpass filter and
the 1st mixer. Keep in mind that if the sending station has moved frequency
100 Hz then they most likely won't be heard. Oh I forgot to mention that
these filters are custom and available for some ~$3000 per frequency. :) So
we divide the 20M CW band into 250 Hz slices, buy a filter for each one (600
of them or 1200 if we overlap) then the radio front end will cost $1,800,000
or $3,600.000 for overlap frequencies. Now I know some serious DX'er will
drool at that possibility but in truth it makes the price of the ORION
reasonable and with in reach of every one.
:)
73
Bob K4TAX
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <W8JI@contesting.com>; "George W5YR" <w5yr@att.net>;
<cyr999@extremezone.com>; "Duane Grotophorst" <n9dg@yahoo.com>
Cc: "tentec" <tentec@contesting.com>
Sent: Thursday, March 14, 2002 4:21 AM
Subject: Re: [TenTec] KB7OEX: a big plus favoring ORION
> > 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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