Pretty technical explanation.
I try to intrprete.
Tom/Will please correct me if I get this wrong:
The core of this is all receivers today have a wide
(> 10 kHz bandwidth) first intermediate frequency filter.
And that largely ruins the receiver no matter
how good the second or third intermediate frequency
stages are.
The current second and third intermediate frequency
filters can have better ability to filter than the
current DSPs we see in Amateur radio receivers for the
second or third IF.
The pricing estimate for a good-enough DSP unit has
maybe fallen to a half of Tom's year 2001 estimate.
And in two-three years is again a half of the current
price.
This is what we now wait for:
When comes the time we can have a DSP or two DSPs in
a box without 500Hz-3kHz analog filters limiting the
performance of the receiver or two receivers and the
transmitter in the same box.
And taking into account the box must be below 4000 USD
for the consumer equivalent to year 2003 USD pricing.
Until then:
We are waiting for the first company to offer us a
decent upgrade how to filter the first intermediate
frequency stage better than the curreny factory version
radios that all use wide, poor shape factor filters.
We already have been:
Waiting to see factory version radios using good
filters at the first IF.
Now with Orion we see it means having lower frequency
for the first IF.
My addition to this:
We must remember why the first IF was set well above
30MHz and hope the manufacturers take measures to
overcome those effects already in the factory versions
of the radios.
And like VR2BrettGraham pointed out, the radios must
be such we can operate with a linear (= some RF) in
the schack.
73,
Jukka, OH6LI (in contests OH4A or OH0V)
> -----Original Message-----
> From: ext Bill Tippett [mailto:btippett@alum.mit.edu]
> Sent: 11 May, 2003 17:34
> To: cq-contest@contesting.com
> Subject: [CQ-Contest] Orion
>
>
> KE9V wrote:
> >Is one Orion worth two PRO2's in a contest?
>
> W8JI wrote:
> >I'd pay twice the amount for good RF performance than
> I would bells and whistles and 9 thousand filters.
>
> I was researching some info posted by KC1SX (ARRL's
> Product Tester) and came across W8JI's post which addresses
> some of the issues being discussed here. This was written
> on the Topband reflector over 2 years ago. At least one
> manufacturer may have been listening...
>
> 73, Bill W4ZV
>
> P.S. If you want a good education on strong signal handling
> issues, read all the messages in the MDS thread beginning
> with the post by Tony AB2CJ on January 9, 2001.
>
>
> Topband: Minimum discernible signal ?
>
> Tom Rauch <mailto:W8JI%40contesting.com>W8JI@contesting.com
> Wed, 10 Jan 2001 05:36:15 -0500
>
> Hi Tony,
>
> > There is much talk about the ICOM 756 Pro's low noise
> floor and weak
> > signal capabilities when compared to analog or AF DSP
> receivers. The shape
> > factor of the DSP filters are on the order of 1.2:1 and
> can be narrowed
> > down to 50hz ! The multiple AGC loops are said to do a good job and
> > blocking dynamic range is in the 125~130db class. Third
> order IMD dynamic
> > range is around 90 ~95db. With that said, those who test
> this rig say
> > that there is more to the story than what these numbers
> reflect. In terms
> > of weak signal performance, the IF DSP in combination with
> an extreamly
> > low noise floor is said to be a cut above analog or
> crystal filtered rigs.
>
> That's fine Tony, but you have to consider the spacing of the test
> signals used in the ARRL tests. Virtually all receivers today have a
> roofing filter that follows the first mixer. This filter is
> generally 10 or
> more kHz wide, and most often has a somewhat poor shape factor.
>
> It's a "cheap" filter, and a wide one, but it makes a significant
> difference in test results. Testing outside the BW of the
> roofing filter
> takes all later stages, including mixers and amplifiers, out of the
> picture.
>
> While wide spacing IMD and blocking DR tests are useful if you
> have a ham "down-the-road" operating 30 kHz or more away, there
> should *always* be a test well within the roofing filter BW of
> receivers. As far as I know, the ARRL does not do this.
>
> If the testing party does not test or publish narrow-spaced tests
> (spacing much less that the BW of the roofing filter) we have no
> idea at all how the receiver will work in normal CW or SSB
> conditions. Any blocking or IMD numbers, if taken at more than a
> few thousand Hz spacing, are almost meaningless in the
> conditions we typically face on amateur bands below 50 MHz.
>
> For example, my FT1000D looks very good in ARRL tests, yet the
> receiver was almost useless in the very first CW contest I operated
> with it. I heard all kinds of bloops and bleeps that weren't really
> there, because of design flaw in the noise-blanker that allows the
> noise blanker to mix signals ****even when the noise blanker is
> NOT turned on*****!!!!!
>
> I looked at that rig, and wondered why on earth I bought it.
> Fortunately the fix for that only took a few cents worth of parts and
> a few hours of my time, and now it is a decent receiver.
>
> However, if the modified FT-1000 and the stock FT-1000D were
> compared in the ARRL test...they would look the same.
>
> I look at the roofing filter BW, and the test signal spacing, and if
> test signals are NOT both placed within the roofing filter BW of the
> receiver I ignore the results (unless they are bad, in which case I
> know closer results will be even worse).
>
> > Question - Forgeting about desense from QRM for a momet,
> if two rigs are
> > said to have identical minimum discernable signal figures
> by ARRL Lab
> > standards, how is it possible that a receiver with DSP
> IF's is said to
> > hear better near the noise floor than conventional crystal filtered
> > receivers?
>
> Maybe, but it would have little to do with the DSP system vs
> analog filters. Digging into noise requires all stages are quiet, and
> the bandwidth is as narrow as possible. A 50Hz filter will ALWAYS
> make the MDS look good, even if you add it after the receiver.
> Noise (white noise) level is directly related to the BW of the
> receiver.
>
> The question is how narrow a BW can you really use in normal
> conditions, and how "smooth" is the noise. In practice, I find a 250
> Hz filter is about as narrow as I can ever use and not have major
> problems tuning the station. If there is "rough" noise, like QRN or
> power leaks, a narrow filter can make things worse. My ears do a
> much better job than any filter (including DSP) will in that case,
> and during the summer I often use a 1.5 kHz filter or wider on CW.
>
> Narrow filters ALWAYS tend to ring out the noise, and blur it over
> the signal. That's true no matter what the type of filter is, although
> smooth flat filters that are properly designed can be better, for a
> given low attenuation BW, than a crummy poor design.
>
> I always thought that if two rigs show a MDS of -140dbm for
> > instance, that they would both hear the same? Or is there
> more to IF DSP
> then the MDS figure represents?
>
> It sounds to me like the 50 Hz filter "inflates" the MDS. If you tack
> a 50 Hz wide audio filter on a regular receiver, the MDS normally
> goes down.
>
> > Question 2 - Obviously atmospheric and man made noise
> limits weak signal
> > performance on HF but, what difference can one expect on a
> quiet near
> > noise free band ( 15 meters in mid winter ) from a
> receiver that's said to
> > ave -135dbm and one with -141dbm? Under these conditions,
> and in terms of
> > weak signal performance, is 6dbm a substantial gain?
> Again, forgetting
> > about desense from QRM for a moment.
>
> This is a 160 meter reflector. My 160 meter noise floor, after pre-
> amplifiers, on a quiet winter night when the band is open to Europe
> is about -117 dBm measured at 3.5 kHz bandwidth. With a 350 Hz
> bandwidth, the "off-the-air noise floor" of my system is -127 dBm.
> With a 35 Hz filter it is -137 dBm.
>
> To give you an idea of signal levels, W4ZV (the loudest station I
> normally hear when beaming Europe) is normally -30 dBm here.
> DF2PY is normally (average night) -80 dBm. Of course the weakest
> Europeans are down at or below noise floor.
>
> I need a blocking DR of more than 100 dB at CLOSE spacing if I'm
> going to work a weak signal within six kilohertz of Bill. If I don't
> have that DR, it's my problem. I give a "hoot" about the blocking
> DR at a wider test spacing that the entire DX area of the band!
>
> Contests are even worse, as we have multiple -50 to -20 dBm
> signals just a few hundred Hz from noise-floor signals. IMD and
> blocking are important at test spacings of a few hundred Hz!
>
> That's why my primary receivers are now Drake R4C's with 600 Hz
> roofing filters and 125/250 Hz eight-pole IF filters, diode mixers,
> and MMIC IF amplifiers. I have a blocking DR with 300 Hz spacing
> (both signals in the passband of the roofing filter) of almost 150 dB
> now. The system is now totally limited by the spurs of closeby
> transmitters, rather than internal defects.
>
> > I enjoy working CW-QRP and digital mode QRP ( PSK31) and
> if these claims
> > about the IC756 Pro are true, then it should be a real
> winner for weak
> > signal work. Since I'm not a contester, I'm not worried
> about the less
> > than perfect blocking dynamic range of the Pro.
>
> Contesters and DXers should be worried. And if digital modes get
> more crowded, you might be too. Manufacturers pay far too little
> attention to transmitter waveshape and bandwidth on CW,
> harmonic distortion and IMD in transmitters when the audio system
> is used, and close spaced performance in receivers. I'd pay twice
> the amount for good RF performance than I would bells and
> whistles and 9 thousand filters. Harmonic distortion in SSB
> transmitter audio and modulator systems will be particularly
> important with new digital modes piped into the mic plugs on
> radios, and it is NEVER measured.
>
> > Lab stated that they were able to pull out individual CW
> siganls in the
> > middle of pile-ups using the 50hz filter setting. They
> also said that they
>
> So what?. Pulling an S-9 signal out of a pile up is nothing. Pulling
> an S1 signal out of a bunch of 40 dB over signals is great. They
> didn't say, or you didn't quote, how weak the signal was.
>
> > were able to pull QSO's 200 and 300hz apart during the CQ
> World Wide 160
> > Meter CW Contest. How bad can the dynamic range be?
>
> Bad.
>
> he fact stations spaced 250 Hz apart can be "pulled-out" is
> absolutely meaningless. I can do that with a FT101E or a *stock*
> R4C, and both are horrid receivers that I would never consider
> using. What I want to do, and what most of us want to do, is be
> able to hear a noise floor signal a few hundred Hz from strong
> stations when multiple strong signals are present.
>
> It also would be nice if someone tapped an engineer on the
> shoulder on occasion, and reminded him that key clicks are not a
> necessary part of sending CW.
>
> DSP filtered radios, without a narrow IF filter upstream of the DSP,
> simply won't do that. The last time I looked A/D conversion with
> enough speed and bits would cost a thousand bucks. That would
> buy a lot of crystal filters, which when properly designed are just as
> good in BW and don't suffer overload!
>
> Hopefully someday manufacturers will someday look at what is
> important and improve operation on crowded bands.
>
> 73, Tom W8JI
> W8JI@contesting.com
>
> http://dayton.akorn.net/pipermail/topband/2001-January/000993.html
>
>
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