[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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