Rick,
I think you did answer the question correctly when you said:
"Altogether, this is what has led me to the point that I take all
measured
numbers with a grain of salt and prefer to try all the rigs,
comparing them
to several other rigs that I know well - under all kinds of adverse
conditions. Then I have a feeling about the performance of the
rig."
That sum's it up much better than relying on test data from really
unknown sources. The spec's help sell radios, but it's actually
trying the radios under conditions as you describe that really tell
you how well a receiver is designed and how well it's going to
operate under your own operating conditions.
Great IP specs don't necessiarily make a great radio, it's a
combination of the design goals and implementation of that complete
receiver design that makes a great radio receiver.
Tom - W4BQF
Todd,
It's simple. You need good IP3 figures to enable a good dynamic
range
figure, but they have to be the result of excellent design and
implementation. In order to assure that this is the case, it's
better to
look at both numbers.
It is correct that the IP3 can be improved by placing an
attenuator in the
front end. It's normally linear. If you attenuate the front end
by 10 dB,
the IP3 goes up by 10 dB.
The interesting question to be asked is "how much IP3 or dynamic
range do we
need?"
As far as I know, nobody has ever answered this, except to say "as
much as
you can get." Just as good of an answer would be "it depends."
The problem becomes a diminishing return on your investment. At
some point,
trying to get another 3 to 6 dB can cost more money than the rest
of the
front end.
And that is all just part of the answer.
The IP3 will also depend on how it is measured.
If you want to compare numbers on two different rigs, you have to
assure
both numbers were measured exactly the same way.
Altogether, this is what has led me to the point that I take all
measured
numbers with a grain of salt and prefer to try all the rigs,
comparing them
to several other rigs that I know well - under all kinds of
adverse
conditions. Then I have a feeling about the performance of the
rig.
Being a member of one of Europe's largest contest clubs, I get the
opportunity to eventually try everything that comes on the market
with
excellent antennas at sites which are remote as well as located in
an
industrial city.
I know this wasn't a short, specific answer to your question but I
don't
think there is such an answer.
73
Rick
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com]
On Behalf Of KD7EFQ@aol.com
Sent: Thursday, November 25, 2004 4:46 AM
To: tentec@contesting.com
Subject: [TenTec] IP3 controversy
Hello all, I have been reading a few reviews and asking questions
about this
"Third order intercept" number that is all the rave these days. I
have a
vague
understanding of what this is, but some controversy has arisen as
to how
important this really is.
Rob Sherwood of Sherwood Engineering only lists dynamic range
numbers on his
Dayton 2004 comparison chart. I ran into him on the air and asked
him why he
does not show IP3?
He responded that anybody can raise there IP3 number simply by
turning on
their attenuator on any rig and so it wasn't that important to
him. I just
read
the same statement on the latest review of the IC756PROII where
the VK
poster
said anyone can raise their IP3 by inserting an attenuator in
front of the
receiver.
I vaguely remember reading Doug Smith's Article on receiver
performance,
where he stated that dynamic range numbers alone did not tell the
whole
story on
receiver performance, and you had to consider IP3 numbers to get
an accurate
picture. Since he helped develop Orion, I assume he knows what
he's talking
about.
Bottom line, Who's right, and Just how important is IP3?
73, Todd - KT0DD - kd7efq@aol.com
"The race is over...THE RATS WON !"
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