[TenTec] Re : [Elecraft] Trying to improve the K2's IIP3; mission (almost...) impossible!

[Bill Tippett]

Hi Dick,

PA2DW wrote:
 >By the way, talking about IIP3 values. The New Icom radio IC7800 claims +40
dBm IIP3, but I can not find the spacing with which they measured that. 5,
20 maybe 100 kHz!?!?! Does anyone know that?


         Trying to improve the K2's IP3 may be nearly as impossible
as finding Icom specifications!  There is no qualification anywhere in
their brochure or specifications as to signal spacing.  The closest
I have been able to find is the following which is a copy of the Test
Report included with each 7800.  It implies IP3 is measured at 100 kHz
(which is stated for the dynamic range measurement).

http://www.qsl.net/ab4oj/icom/ic7800/7800test.html

The first couple of SM5BSZ's measurements in Table 6 below imply IP3
of +40 dBm at 20 kHz, although the measurement technique is flawed
for measurements at normal levels for interfering signals.  However,
if IP3 is met for interfering signals of  -1 dBm (equivalent to an S9+72 dB),
it should certainly be at least as good at normal levels (e.g. ARRL uses
-17 dBm which is equivalent to S9+56 dB).

http://antennspecialisten.se/~sm5bsz/dynrange/ssa2004/ssa2004.htm

         I believe a more worthwhile mod to the K2 would be a better
filter.  I've heard rumors that someone may be looking into this.  My
reason for saying so is the serious -24 dB degradation of the K2's
IMD for spacings under 5 kHz.  Here's the data:

IMD @ 5 kHz     91 dB (ARRL)
IMD @ 2 kHz  70 dB (Sherwood)
IMD @ 1 kHz  67 dB (ARRL)

Sources:
http://www2.arrl.org/members-only/prodrev/pdf/lab/k2.pdf (graph page 20)
http://www.sherweng.com/table.html

By comparison, Orion's performance at close spacings using the 1000 Hz
roofing filter degrades only about -8 dB from 5 kHz to 1 kHz:

IMD @ 5 kHz  92 dB (ARRL)
IMD @ 5 kHz  94 dB (Sherwood...measured 95 dB using 1800 Hz filter)
IMD @ 2 kHz  93 dB (Sherwood)
IMD @ 1 kHz  84 dB (ARRL)
IMD @ 1 kHz  85 dB (Sherwood)

BTW, these measurements give me confidence in both ARRL and
Sherwood since they only differ slightly at 1 kHz and 5 kHz.

Sources:
http://www2.arrl.org/members-only/prodrev/pdf/lab/orion_etrr.pdf (bottom 
left graph p. 20)
http://dayton.akorn.net/pipermail/orion/2004-March/000309.html
http://dayton.akorn.net/pipermail/orion/2004-March/000310.html

         Please don't shoot the messenger...I own both a K2 and
Orion and am very happy with both.  And I also predict both will
probably beat the IC-7800's IMD at 5 kHz spacings whenever ARRL
or others publish test reports.  A hint of this can be found in Table
5 of SM5BSZ's data which I belive is a convoluted measurement
of BDR.  Although it is impossible to make direct comparisons, it
appears BDR performance degrades seriously at close spacings:

Relative signal strength to cause 3 dB blocking (referenced to the
+ 1.2 dBm signal needed to cause the same blocking at 100 kHz):

50 kHz  -6.2 dB
20 kHz  -14.1 dB
5 kHz    -36.7 dB
2 kHz     -38.7 dB
1 kHz   -51.8 dB

These results are SO BAD it makes me wonder if either the 7800 or
measurement technique was defective.  If I've misinterpreted the data,
please do comment!  For comparison, both the K2 and Orion BDR
degrade in the order of -10 dB over the same range of interfering signal
frequencies, which is why I have trouble believing the above.

Sources:  BDR graph on page 19 of K2 ETR cited previously.
BDR graph on page 19 of Orion ETR cited previously.
SM5BSZ webpage cited previously.

         Please do not share this data with any Icom 7800 zealots...
they simply do NOT want to see any performance comparisons
with the $599 K2.  ;-)

                                 73,  Bill  W4ZV