[Icom] Icom 781 vs Icom 756PRO II

Bill Tippett [email protected]
Sun, 13 Jul 2003 15:16:28 -0400


W0YG wrote:

I have extensively read your information on the 756PRO/PRO2 but don't seem
to be able to find a figure for the TOI (third order intercept).  Orion
boasts +12dB.  The Icom 7800 is supposed to be +40dB.  What is the published
TOI of the PRO2?

The PRO2 seems to be a lot better at strong adjacent signal rejection as
well as weak signal reception than the 781.  I wonder if this also holds for
160M where front ends really collapse?

Hi Charlie!

         A few points to ponder:

         1.  Orion's TOI is specified at +25 dBm typical and is still above 
+20 dBm for ONE kHz signal spacings.  The reason it does not fall apart at 
close signal spacings is due to the 1 kHz roofing filter versus the typical 
15 kHz in today's general coverage radios.  The optional 500/250 Hz roofing 
filters should even improve on this excellent performance.  Icom is adding 
a 6 kHz roofing filter for the IC-7800 which indicates to me that they are 
beginning to understand the problem.

See Orion's IP3 graph in Figure 1 below:

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

Also note the following about 1/3rd of the way down the same page:


Numbers quoted for competitor's transceivers are taken from ARRL Product 
Reviews, 14 MHz, preamp off.   8 to 10 dB generally constitutes a 
meaningful difference in receiver performance ability.   Dynamic range 
alone says nothing about how well the radio will handle strong signals; it 
indicates the ratio between the strongest signals and the weakest signals 
that can be handled without front-end distortion and desense.  Third-order 
intercept point is an excellent indicator of how well the receiver will 
react in the presence of nearby signals of significant strength.

                              5 kHz 2 tone 3rd order dynamic range       5 
kHz spacing IP3

ORION                                      101 
dB                                             +24 dBm

Elecraft K2                                 88 
dB                                               +1 dBm

FT1000MP Mk V                      73 
dB                                               -5.2 dBm

Kenwood TS2000                       69 
dB                                               -15 dBm

Icom IC756 Pro II                      76 
dB                                              -18.8 dBm


         2.  The IC-7800 +40 dBm is NOT specified for signal spacing, 
sensitivity or dynamic range.  The much-touted Rhode & Schwarz "technology" 
is little more than an automatic attenuator that is activated in the 
presence of strong interfering signals.  If (I don't know for sure) this is 
the technique Icom is using, then the + 40 dBm spec is very misleading.  I 
can achieve +43 dBm on an Orion simply by manually switching in a 6 dB 
attenuator (25 dBm spec plus three times attenuation selected due to the 
fact it is 3rd order, or 25 + 18 = 43 dBm).  Be very wary of specs that are 
not qualified!  I'll bet the IC-7800 will be well below Orion for 
interfering signals spaced 5 kHz and less.

         3.  While the IC-781 is a good performer for IMDDR3 at wide signal 
spacing, it falls down at narrow spacings (by 21 dB as measured by Sherwood 
from 20 kHz to 2 kHz - far right columns).

http://www.sherweng.com/table.html

         4.  While anecdotal, K9DX noted similar results today on the 
Topband reflector: "I compared the Orion with one of my IC-781's and it 
blows the 781 away by 20 to 25 db in ability to handle large signals at 
offsets 1 to 20 Khz."  As you know, John knows a thing or two about 
receivers and their performance on topband.

http://dayton.akorn.net/pipermail/topband/2003-July/017171.html

         5.  Since you are interested in IP3 measurements, note the 
following RadComm (RSGB's publication) measurements at 20 kHz, 10 kHz and 3 
kHz, which illustrates the problems current radios have with closely-spaced 
interfering signals:

http://perso.wanadoo.fr/f6crp/ba/rx.htm

                                                 Caveat emptor!

                                                 Bill  W4ZV






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