And, Ed also asked that his second reply to me be posted
to the reflector. That is the post to which I most recently
responded which contains much of his remarks, but with
a lot of my stuff tossed in. Here is Ed's post without all
my jibberish tossed in:
"
Hi, Jim,
Thanks for the followup. I am still waiting for my tentec@contesting.com
subscription to be authorized by the moderator, so my post is actually in
limbo. I imagine it will show up soon, or I can repost it once I am
official. If this one shows up but not the original, I will repost, so
everybody can figure out more about what we are talking about. :-)
I am glad we are moving towards a meeting of the minds. And do feel free to
disagree with my conclusions. I have given this matter a LOT of thought,
over my entire time at ARRL HQ and think I have the right balance and
methods to test what is sometimes a moving target -- real-world receivers.
The only sticking point appears to be dynamic range. Most agree that
testing IP3 at a level somewhere in the AGC range of the receiver is the
best test, although I don't think that most hams understand how and why IP3
varies in real receivers based on signal levels. If nothing else comes of
this, understanding that may help hams to stop quibbling over a dB or two.
Those who work with high-level mixers and/or visualize receivers whose
non-linearities follow a classic response tend to think in terms that noise
floor, dynamic range and IP3 are all precisely related. Ulrich, for
example, whose opinion I greatly respect, has stated that dynamic range can
be calculated from the noise floor and an IP3 measurement made at a high
level. When you consider that the formula ends up assuming that the
1st-order and 3rd-order responses are 1:1 and 3:1 sloped and makes a
calculation for the levels at the noise floor of the receiver, it is, IMHO,
more accurate to make a measurement at the noise floor because it can be
done. No one has yet convinced me that making a measurement at a higher
level and assuming ideal slopes of lines that are probably not ideal is a
better way to determine dynamic range than actually making a measurement
that can be made accurately.
Of course, one must always remember that Product Review is dealing with a
sample of one. IP is really a very nebulous number for a number of reasons,
and I have just scratched the surface in my explanations. When we are
dealing wtih receiver intermod with receivers that are linear over 90-110
dB, look at how small a deviation from perfection a linear range of 100 dB
would be. Differences of fractions of fractions of a percent in the non
linearity of a mixer can make tens of dB of difference in the dynamic range.
And if a manufacturer derives his IP3 from the measured linearity and the
specified recevier sensitivity and makes a receiver more sensitive than the
spec, but whose linearity is as specified, the IP3 calculated from the
actual measurements is lower than the spec. So if they make a better
receiver, it doesn't "meet spec."
So what DOES it all mean? It means that one needs to evaluate the overall
performance of a receiver, not necessarily stack them up against each other
in a number by number comparison over dBs that are a moving target at best.
And I do recall you now at dinner; I hadn't associated your call with the
pleasant chat we had. It is always nice to put that face to the name,
because having met someone is a much better indicator of who they are than
words on a screen. :-)
Did you get a Certifi-Cat for working the TT2?
73,
Ed Hare, W1RFI
-----Original Message-----
From: Jim Reid [mailto:jimr.reid@verizon.net]
Sent: Tue 3/4/2003 8:14 PM
To: Hare,Ed, W1RFI; George, W5YR; Smith, Douglas; tentec@contesting.com
Cc: Tracy, Michael, KC1SX
Subject: Re: ARRL Rcvr 3rd and 2nd IPs Test Methods
Aloha to all following this topic,
Ed Hare has written a very detailed and well presented
reply to my posts and concerns, thank you Ed. And I
accept your criticism of my "low blow"; was not intended
as such, but on re-reading can certainly see your point.
I apologize for over stepping in my comment.
Comments on only a couple of Ed's comments. He has
made an excellent presentation of the facts about the
ARRL Lab test methods and use, well worth reading
by all.
> The article in question is available for download at:
http://www.arrl.org/tis/info/pdf/020708qex046.pdf
Thank you , that will be helpful to others.
I am now going to cut out a bunch of Ed's great material,
and skip on down near to the end.
Ed's concluding paragraph:
> "In this post, I have outlined some of the reasons that ARRL is
> making the testing choices it is using. I believe them to be the
> correct choices, offering a reasonable level of standardization
> in testing and reporting on receivers with a wide range of
> capabilities and "real-world" receiver performance. There are
> improvements in the works, but they are not going to make
> a night and day difference in results, because the test methods
> used give good results for the test conditions employed, and
> most improvements I can think of will serve only to tighten up
> a bit on the test conditions."
Again, I greatly appreciate the time Ed has taken to prepare this
thourouh discussion of the ARRL lab methods and the reasons for
their use.
Now, I would like to understand why the great difference in IP's
reported by the Lab for the Ten Tec Argo V from the "claimed"
IP's for that radio by Ten Tec.
The ARRL lab came out with IP's well below the
Ten Tec specs. TT spec is +4 dBm for
the third order IM intercept point, at least that is what
the ARRL lab test reports TT has specified.
At 3.5 MHz, 20 kHz signal spacing, the ARRL Lab
measured only -4.5 dBm; and at 14.2 MHz, only -3.4 dBm.
Those are really significantly lower numbers than the
Ten Tec claimed spec. And, for 5 kHz signal spacing, the
deltas are even greater: 3.5 MHz, ARRL says -30 dBm;
and for 14.2 MHz, -29 dBm. Those are huge differences IF
TT really specs +4 dBm IP3 for 5 kHz two signal spacing;
I do not know if Ten Tec specs this spacing for the Argo V.
Also, TT specs the second order IP to be +66 dBm.
But the ARRL lab reports only +47 dBm. Again, a very large
difference.
Certainly nothing in the QEX piece, downloadable as above, is
there any reason given to explain such large deltas between
what the engineers at Ten Tec measured, probably several
times to come up with "typical" IP2 and IP3 specs for the new
Argo V and the alarmingly lower numbers reported by ARRL.
These differences between TT claims and the ARRL Lab report
are what initiated my posts about this topic. I still do not
understand why the great deltas, and worry about the future
tests on the new Ten Tec Orion! It seems to me it is imperative
to identify why the differences in numbers, and to provide some
assurance about what we will read next.
Thanks again, Ed. And also, thank you again for your visit
out here to Hawaii a few years ago. And yes, I did have an
opportunity to work the Tuna Tin II you left with the Hilo QRP
gang. Had a very good signal up here to Kauai, some 350
miles away and through the Mauna Kea Volcano to boot!
And I enjoyed our across the table visit over dinner at the
concluding convention event. You will have to visit us again!
73, Jim KH7M
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