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[TenTec] Orion Roofing Filters and Front End Design

To: <tentec@contesting.com>
Subject: [TenTec] Orion Roofing Filters and Front End Design
From: ghoffman@spacetech.com (Gary Hoffman)
Date: Thu Apr 17 21:21:29 2003
Now this is a really informative and useful write-up.

Thanks Jim !!

73 de Gary, AA2IZ

----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: <tentec@contesting.com>; "Orion Discussion Group"
Sent: Thursday, April 17, 2003 7:25 PM
Subject: [TenTec] Orion Roofing Filters and Front End Design

> Hi,
> Have been engaged with others discussing this topic
> off the reflectors,  just private give and take idea bouncing.
> Has been suggested that I share some points on this
> topic as some criticism has been levied against the Orion
> design.  Disguising all names/conversant participants
> to protect their privacy,  here is my summary of the
> situation:
> I was sent the following,  in part:
> > The attached excerpt from the Rockwell-Collins textbook
> > "HF Radio Systems and Circuits" aptly illustrates the receiver
> > architecture favored not only by Icom, but by all the
> > "heavies" in the mil/gov and commercial HF radio
> > communications field.
> And another wrote,  again only in part:
> > Extending the same argument or viewpoint, if a 1000 Hz roofing
> > filter is used instead of a 15 KHz roofing filter, what does
> > it accomplish?
> In response to the first comment, even a "super strong" front end
> cannot prevent IMD in the IF stages unless special AGC is
> employed,  as Ulrich Rohde describes in his article in QEX,
> Jan./Feb. 2003, page 28, his Fig. 43 and as in the text ref.
> above.  These techniques both apply AGC
> action at the front end,  in Rohde's example by increasing
> attenuation ahead of both front end LP and BP filters and
> the first RF amp; and in the Rockwell-Collins text case,  the AGC
> actually reduces the gain of the first RF amp and/or
> increases attenuation, not actually clear from the diagram.
> But now in both these schemes,  dynamic range is sacrificed
> because the gain is reduced in the presence of strong QRM.
> The Orion scheme does not reduce gain unless analog
> AGC is forced to do it because of what's passed by the
> roofing filters.  In fact,  in the Orion design,  analog AGC
> action does not begin necessarily until signals of some
> S9 + 30 dB are present in the pass band,  see Doug
> Smith's discussion of the design,  see,  for example,
> http://www.doug-smith.net/digitalagc.htm   .
> The RX-340 won't do what the Orion will do with close-spaced
> very strong signals, and neither will any receiver not having
> narrow roofing filters.
> And now,  to the question about what does a 1 kHz
> roofing filter do that a 15 kHz filter cannot do,  you only
> have to look at alleged test measurements which have
> been reported (granted,  the only test data you fellows
> are going to accept will be that eventually posted from
> the ARRL labs;  however,  I presume that Doug Smith
> has seen to it that the guys at Ten Tec did,  in fact,
> do their claimed test exactly as Ed Hare and Mike Tracy
> say they do it,  and as described in the Lab's posted
> test procedures).  In fact,  today Ten Tec posted a
> comparative table of such test data to illustrate the
> efficacy of narrow roofing filters, see:
> http://www.tentec.com/TT565.htm
> just scroll down to the large print rcvr comparison chart heading.
> Note that the Icom IC756 Pro ll is 25 dB below the Orion in
> a test of 5 kHz spaced two tones  3rd order dynamic range
> (both outside the passband but one of the tones within/closer
> than 5 kHz  of the passband),  and at the bottom of the list of
> five rigs in  the  5 kHz 2 tone IP3 test.  Per Doug Smith,  this
> would only be possible with the use of the narrow roofing filters
> OR, giving up weak signal sensitivity,  thus moving the entire
> dynamic range span higher up in the power spectrum using
> the Rhode and Schwartz/Collins feed AGC to a front end
> attenuator approach.  However,  NOT giving up rcvr sensitivity
> while contending with very strong near by signals is the
> entire point of instantaneous high dynamic range and high IP3
> for the active DX'ng and contest type radio amateur!  And,
> yes,  that is the market to whom the Orion is directed,  and
> for whom it was engineered.
> The Orion was not designed  for the user interested in
> rcv'g up to four simultaneous 3 kHz wide ISB channels to be
> processed by the DSP,  as described as one purpose of
> the Rockwell-Collins example presented.
> Per Doug Smith,  the Orion design has accomplished
> exactly what amateurs want:  a minimum detectable
> signal level that is NOT limited by IMD's,  but by phase
> noise (again see the phase noise plot about the Orion
> at the above web site).  And we don't want to give up that
> MDS level just to reduce the impact of nearby strong
> signals -- the R&S, R-C solution previously discussed.
> Finally,  I personally would rather have the 4-pole roofing
> filter designs because they present less of the ripple and
> group-delay distortions in the passband you have previously
> described,  thereby preserving the characteristics of the
> DSP filters as much as possible.  I assume digital phone and
> data operators don't want 2 dB of ripple and 1 ms of group
> delay variation,  or whatever they might be,  from multiple
> pole crystal filters.  However,  I am very pleased by the
> dynamic range,  preserving instantaneous MDS levels,
> and high 3rd order,  close spaced strong signal IP3
> performance,  as realized by the Orion design.
> But,  as I mentioned earlier,  you fellows will remain skeptical
> at least until the ARRL Lab results are posted,  and maybe
> even beyond,  hi.  And,  I probably will not be able to really
> post any convincing evidence either,  as I just don't seem
> to experience any S9 + very many dB signals out here at
> my rather removed QTH from others operating in the amateur
> bands.  But,  I will see what I can do in comparing the Orion
> to the RX-340 when an Orion at last gets here.  Maybe
> I can get John,  KH7T down on the Big Island of Hawaii,
> some 320 miles SE from here,  to use his big Force 12
> beam and kW to send a good strong signal up my way;
> but will also have to find someone else also to have
> two strong,  closely spaced signals to experiment with,  hi.
> Somebody on Oahu,  maybe could go up to KH7R and
> squirt a kW to me,  that is only about 110 miles SE,  and
> they have stacks of monobanders there!  See,
> http://home.hawaii.rr.com/kh7r/
> check out the list of antennas on each of six towers!  Lots
> of photos also.  BTW,  the entire QTH is for sale,  interested?
> Long enough for now,  and always fun to bounce ideas
> back and forth with you guys!!
> 73 and Aloha,  Jim  KH7M
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