I won't speak to the technical points under discussion here, because
I am not qualified to do so.
However - I will say that this discussion is some of the best stuff I
have ever seen posted. Informative, based on facts and data, and no
flames. Excellent !
73 de Gary, AA2IZ
----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: "Jim Reid" <jimr.reid@verizon.net>; "Hare,Ed, W1RFI" <w1rfi@arrl.org>;
"George, W5YR" <w5yr@att.net>; "Smith, Douglas" <DSmith@arrl.org>;
<tentec@contesting.com>; <kh7t@arrl.net>
Cc: "Tracy, Michael, KC1SX" <mtracy@arrl.org>
Sent: Wednesday, March 05, 2003 8:09 PM
Subject: [TenTec] Re: ARRL Rcvr 3rd and 2nd IPs Test Methods
> Now here again, Ed has responded to my last post. He
> has also requested that I repost to the reflector, so here
> goes (this one came here about only 10 minutes ago):
>
> "
> > I am especially pleased that you plan to review, and
> > probably do some updates to the procedures, Ed. I like the
> > idea of a standard input signal level for the pair of tones;
> > the Collins Radio idea of using -109 dBm plus the rcvr noise
> > figure will be somewhere near a particular radios S5 meter reading I
> > would bet, hi.
>
> Actually, by "Collins" standard, I was referring to the Collins S-meter
> standard, where S9 is 50 microvolts (-73 dBm at 50 ohms) and each S unit
is
> 6 dB. I don't know what levels Collins used/uses for their IP2 and IP3
> measurements. My guess is that they would have derived their IP3 from the
> noise floor and dynamic range measurements. Many of the older radio
> references *define* IP3 as a relationship between sensitivity and dynamic
> range. This is a reasonable assumption for most receivers, although
subject
> to all the factors in the sidebar I wrote for Doug's article. Other texts,
> especially dealing with high level mixers, where there may NOT be a noise
> floor to measure (a passive mixer, for example) use a more generic
> definition of IP3. Considering the way that the "real" IP3 varies with
> signal levels in some receivers, a signal level for IP3 measurements above
> the noise floor is quite justified -- it just becomes a matter of
selecting
> an appropriate signal level. I won't repeat the material in my earlier
> post, but if you repost it as I had asked, folks can read all about my
views
> there. :-) As a quickie here, I will point out that any refinements ARRL
> makes will NOT make the difference between night and day on the results;
the
> measurements as made are reasonable and accurate and are measuring the
> equipment under test, NOT measuring the test instrumentation. I have been
in
> test engineering for 25 years now, so I know where to draw the lines!
>
> > Or maybe just settle on the -97 dBm level for all radios;
> > haven't quite gotten all the subtleties you have outlined
> > digested just yet!
>
> That could work for me, except that there can be a 20 dB difference in
> sensitivity between different radio designs. An 80-meter receiver with
> a -120 dBm noise floor would be QUITE usable on 80 meters, but -97 dBm
would
> really be a different level inside the receiver than it would for a
receiver
> with a -144 dBm noise floor. And I could imagine an insensitive receiver
> with a high AGC threshold that might not be showing any AGC action at all
> at -97 dBm. It bears more thought and some investigation.
>
> W1RFI wrote:
>
> >> 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-linearity's 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.
>
> I will clarify that in the above paragraph, I am discussing making IP3
> measurements at at S5 receiver output level (approximately -109 dBm, if
> memory serves) and making dynamic range measurements at the noise
> floor, the very definition of dynamic range.
>
> >> 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 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 receiver
> >> 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."
>
> > Somewhere, Ed, in your above two paragraphs must lie the
> > reason for the disparate numbers that Ten Tec is listing for the
> > IP2 and IP3 numbers of their new Argo V, vs. the numbers your
> > lab has reported in the April 2003 QST. I went to the
> > Ten Tec web site this morning, and yes, they do list IP2
> > of +66 dBm, and they also add that the ARRL Lab test method
> >was used to determine both it and their listed IP3 number of
> >+4 dBm, both tested using 20 kHz signal pair spacing.
> > However, they make no claims about 5 kHz signal spacing.
> > I thought it was especially interesting that they specifically
> > mention and list use of your ARRL Lab procedure,
> > that is they used a power level for the two input signals such
> > that the S meter on the Argo V read S = 5. And who knows how
> > accurately that was read by them, or, as you suggest how
> > accurately the S 5 setting was read by you folks during the test.
> > The S meter does not have a mirror behind the needle as the old
> > time Triplett meters did to aid in the elimination of parallax, hi.
>
> Parallax is not going to make that much difference. If the reading were
> S4, S5 or S6, if those lines of first- and higher-order responses were
> straight, the calculated IP3 would be the same, as long as the parallax
> error were the same in both cases. If the lines are not ideal, and they
> probably aren't, the IP3 calculation difference would be different only by
> the amount that the first- and higher-order responses deviated from ideal
> over that small portion of the curves.
>
> To my knowledge, the rig never did make claims wrt 5 kHz spacing. What you
> saw on the TT Web site was exactly what we were told the specs were when
we
> bought the rig. I think that some may have confused this rig with the
Orion,
> which DOES make claims for 5 kHz spacing.
>
> I went into the screen room today and set up our Rohde and Schwarz ESH-2
> EMC receiver and made some IP2 measurement. I measured the IP2 of
> the R&S at +86 dBm or so at a number of different levels, indicating
> that the test-fixture intermod is at least that good. In the past, with
our
> older
> HP8540Bs, we had determined that the IP3 setup was good to at least +33
dBm
> and that IP2 measurements could be made to at least +75 dBm. Our newer
> Marconi generators are even better, and using a separate test fixture
> donated by Ulrich Rohde, we had verified the IP3 capabilty to at least +40
> dBm, and possibly higher. I presume that the IP2 capability is similarly
> increased between the HPs and the Marconi, and I believe by the way that
the
> step attenuators were behaving that I was measuring the ESH-2 directly
> (every dB of attenuator changed the product by 2 dB for IP2).
>
> I may have a bit more to add after I do some testing of the rig again,
but
> I want to coordinate this with Ten Tec before adding anything to what was
> said in the review. The review process contains checks, balances, retests
of
> anything at all out of line, consultations with multiple engineers in the
> ARRL Lab and full communication with manufacturers before it all goes to
> press. I want to give what I say here about any specific product the same
> full consideration. We have, btw, made mistakes in the lab testing, but
> this system of checks and balances has, to the best of my knowledge,
caught
> them all, though a few have been caught by the manufacturers at the 11th
> hour. :-) And I thank them for it, because manufacturers know their
products
> better than the ARRL Lab dudes could.
>
> I will also point out that MOST of the time, though, when a manufacturer
> questions a result, the original result has held up. A few manufacturers
> have even sent their engineers to HQ to work with us on a problem and when
> all was said and done, a Feedback in QST was not necessary. :-) I believe
> we had to run one Feedback because one of the Lab guys made a calculation
> error from raw data. That one broke a 15+ year record of no Feedbacks due
to
> a Lab error, thanks in part to careful cross checking and good
manufacturer
> communication. :-)
>
> > On the other hand, Ten Tec has listed some outstanding numbers for IP2,
> > IP3, and phase noise performance of their soon to be released Orion
> > rig. BUT, their beautiful IP3 curve of +25 dBm typical for all signals
> > spacings down to about 5 kHz spacing, followed by a gradual roll
> > down to about +21 dBm at 1 kHz spacing was performed using two
> > input signals of 0 dBm, 1 mW, 0.22volts, or S = 9 plus 73 dB, if
> > I did my arithmetic correctly, hi. See their Fig. 1 on down this
> > page about the Orion from the TT web site:
>
> > http://www.tentec.com/TT565.htm
> >
> > They have certainly performed their IP3 test to produce that curve well
> > up into "the AGC range of the receiver as the best test" to quote
> > from one of your paragraphs above, hi. But that is way above where
> > the Orion S meter would read S = 5 !! Bet there will be a difference,
> > based upon your past results, between the IP3 as measured with
> > 0 dBm signals and when with something like - 97 dBm signals.
>
> That is quite a bit higher than the S5ish levels we would use. I don't
> want to cross that bridge until we come to it, but that is a lot higher a
> level than I would use to make the test. I have not measured a lot of rigs
> at an S9 level, but one of the rigs we measured for my sidebar showed
quite
> an increase in IP3 for S9 level recevier output. The Orion is, however, a
> quite robust SDR type rig, where much of the performance is in the DSP
used
> for signal generation and detection, so some differences in the way it is
> tested -- or even should be tested -- may be in order. Naturally, Mike
will
> work closely with Ten Tec. If nothing else, it will be a very interesting
> expanded test-result report. :-)
>
> For most receivers with final selectivity done at IF with filters, one
> could possibly not really measure IP3 with 0 dBm signals. Those signals
> would pin the S meter, so it couldn't be used as an indication of received
> signal level. The filtering would not allow the test signals at 5 kHz
> spacing and the resultant IMD products to all pass through to the receiver
> output amplified exactly the same. But with DSP filtering, it may be
quite
> possible to gain access to the IF before the final DSP and look at the
test
> tones and resultant IMD products. If so, one can use those levels to
> calculate IP3, at least before the DSP. To know how ARRL would want to
> apply all this, I really would want to look at the radio, do some
> preliminary testing and talk to the designers (and possibly a few
Technical
> Advisors!).
>
> But this DSP/SDR stuff that Ten Tec is doing is personally very exciting
> to me, so I can't wait to see this radio show up in the Lab. I may kick
> Mike Tracy out of the screen room, or at least spend a lot of time hanging
> over his shoulder. :-)
>
> >> Did you get a Certifi-Cat for working the TT2?
>
> > No I did not, but probably my fault, as I did not send Dean, KH6B a
> > QSL for the contact, so he didn't bother to send something, hi.
Anyway,
> > I have too many QSL's in drawers here already as well as a few hundred
> > hanging on my shack wall!
>
> Send me the contact info, including both RSTs, and I will send you a
> Certif-Cat! Trust me, it is WELL worth asking for it. It is a
one-of-a-kind
> sorta' thing. :-)
>
> > Enjoy these discussions, Ed
>
> As am I. I hope that when all is said and done, folks will realize that
> the ARRL Lab *is* a world-class test facility. I saw another post on that
> subject that is a better place to address that, so I am waiting for my sub
> to be approved and my posts to show up.
>
> Please post this one to the reflector, too, and we can let people catch up
> on where it started later. :-)
>
> 73,
> Ed Hare, W1RFI
> ARRL Lab "
>
>
>
>
>
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