I'de just like to know where Tom Finds all that QRM. I would think the
160m cw test is as bad as it gets. Maybe not. I spent about 4 hours in
that test and just set my filter on the TS-870 for 50hz to 100hz
bandwidth. I had 40-50db over 9 sigs on both sides of me and no QRM
problems. My old Omni C's front end has a fit in this test and the TS930
with the 400hz Inrad is worthless. The Omni V was usable but with only
250hz filters, it was pretty broad in comparison.
-----Original Message-----
From: Robert & Linda McGraw K4TAX <RMcGraw@Blomand.Net>
To: W8JI@contesting.com, George W5YR <w5yr@att.net>,
cyr999@extremezone.com, Duane Grotophorst <n9dg@yahoo.com>
To: <tentec@contesting.com>
Date: Thu, 14 Mar 2002 07:43:44 -0600
Subject: Re: [TenTec] KB7OEX: a big plus favoring ORION
> 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
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
>
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