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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