[Icom] IC-7800 intell.

[Bill Tippett]

Hi David!

N1EA wrote:

 >The Orion seems to compare favorably with the projected specifications of 
the IC-7800 especially considering the price.

         Not even considering price I believe the 7800 will suffer the
same close-in (<5 kHz) IMDDR3, IP3 and BDR problems that every other
general coverage design using wide roofing filters suffers.  Even
though the 7800 has added a 6 kHz roofing filter to the typical 15
kHz, even a 6 kHz filter will yield poor close-in strong signal
handling results at <5 kHz interfering signal spacing.  I don't
recall many contests or pileup with 5 kHz between signals!  Orion's
ham-band only front end with a choice of narrow roofing filters at
9 MHz in the main receiver, will not have these inherent problems.
As can be seen in Figure 1 below, IP3 does not deteriorate much
until 1 kHz spacings when the 1 kHz roofing filter begins to stop
doing its job.

http://www.tentec.com/TT565.htm

         Here are a few references (if you can read only one, read the
first one):

SP7HT in "The DX Prowess of HF Receivers", Sept/Oct 2002 QEX:
http://www.arrl.org/tis/info/pdf/020910qex036.pdf

Peter Hart G3SJX (RSGB's Radcomm) IP3 summary at 20 kHz, 10 kHz and
3 kHz 
spacings:<http://perso.wanadoo.fr/f6crp/ba/rx.htm>http://perso.wanadoo.fr/f6crp/ba/rx.htm 


ARRL Test Reports Summary (click second item in the top box
"5 kHz Spacing Receiver Test Data":
http://www.elecraft.com/K2_perf.htm

Full ARRL Test Reports are here but for members only:
http://www.arrl.org/members-only/prodrev/bymfg.html

Sherwood Engineering "Receiver Test Data" (an old list but it does
include the IC-781.  Note the far right columns [wide and narrow]
how badly (20 dB) the 781 dynamic range deteriorates (20 dB) as
interfering signal spacings are decreased from 20 kHz to 2 kHz):
http://www.sherweng.com/table.html

Tom W8JI has written some good receiver comparison comments here:
http://www.w8ji.com/receivers.htm and measurements here:
http://www.w8ji.com/receiver_tests.htm

         We need to wait for independent 3rd party evaluations before
getting too excited by unqualified claims like "IP3 of +40 dBm".
Rhode & Schwarz, from which ICOM has apparently licensed some
technology, achieves this by automatically switching in an
attenuator.  This increases IP3 (third order) measurements by 3X
the attenuation selected.  If I want an IP3 measurement to be
better by +30 dBm, I simply switch in a 10 dB attenuator!  Of course
I could do that myself by selecting more attenuation manually,
but this comes at the expense of sensitivity and dynamic range.
The problem is when that S3 signal I'm trying to copy disappears
as I switch in 10 dB of attenuation to reduce IMD products.

         What we need from ICOM is the interfering signal spacing, the
sensitivity/attenuation and the dynamic range for which their IP3
measurement applies.  ARRL and others will help sort out issues like
this and make sure we are comparing apples to apples.  You wrote:

"I think I'd hold off to see what happens.  It is going to be an
interesting horse race."

...I could not agree more.  Competition is good for everyone!

                                                 73,  Bill  W4ZV