George, W5YR, sent me the following in a private e-mail:
> Do you recall the QEX article in which Doug Smith discussed
> such measurements and there was a long sidebar by Ed Hare
> enumerating the practical difficulties faced in carrying them out?
I did not, but after looking thru some back issues, found their
work in the July/August 2002 issue (I have a short memory!).
Sure enough, a rather complete presentation by both Doug and
Ed covering the topic and the many measurement issues being
faced by those doing so.
The conclusions which can be drawn from that piece of work are:
1. Referencing measurements to the MDS level is not
best, because establishing the MDS level is a matter
of the perception of the person doing the test! MDS should
be dropped from the process, per Doug. Also, Doug
recommends the use of the Audio Spectrum analyzer as
far superior to locating spurious signals (IMD's) near/in
the noise floor or below it; not using an audio meter
as is now used in the ARRL Lab with who knows what
accuracy.
2. Assuming that a modern rcvr IMD characteristics exhibit
cube law (for 3rds) and square law (for 2nds) throughout
their power handling ranges is wrong.
3. How the introduction of noise (from reciprocal mixing, from
inaccuracies about knowledge of the rcvr's noise figure,
imperfection about "knowing" the MDS of the particular
rcvr, etc.) is accounted for can cause inaccurate measurement
results; how these are accounted for ought to be part of
the report of results for the measurement presented of IMD,
IP's, etc. Also to be included is how the actual BW used
in the measurement was determined and that number.
(Note: ARRL procedure is to just select whatever the
rcvr has stated to be at or close to 500 Hz BW; Doug
notes that in various rcvrs tested the actual BW's
ranged from 300 to 700 Hz for filters with 500 Hz labels).
4. Doug discusses the ARRL IMD and IP measurement
equipment set up and use explicitly. He suggests the use
of a better hybrid combiner for "summing" the outputs of
the two input test signals; he mentions his new design for
same, notes the ARRL is/was evaluating it, but no mention
of whether the ARRL labs will be using it now/in the future.
He points to several isolation/filtering issues which need
more attention within the present set up.
5. Doug points to the fact of non-uniform dynamic power
transfer characteristic among rcvrs to suggest that the
cube and square law slope curves must be tested at several
input signal pair power levels, not just one as is now done
at the ARRL lab, Point 2 above.
ARRL now is using a single test level "relative" power
level: whatever the actual power level is, not really known,
just increase power until the S meter of the rcvr under test
reads S = 5 (somewhere near 3.2 uv, or -97 dBm (50 ohms),
maybe).
6. Ed Hare, ARRL Lab Supervisor, and Doug agree on the
above point; Ed asks the question, "What is the "real" IP?"
7. Ed contributes to the piece with a two page report of exactly
the sort of testing in #5 above on three radios among those
at W1AW at HQ: a Yaesu FT-1000MP; an Icom IC-746Pro;
and an Icom IC-765. IP measurement results vary on each
radio depending upon which of three to five S meter power
levels are run for the test signal pairs and whether the
industry "standard" method of actual curve measurements
or the ARRL MDS level method is used. For the tests on the Icoms,
ARRL labs procedure/method yielded lower/poorer IP
reported numbers. On the Yaesu radio, the MDS method yielded
an IP3 number 1.3 dB higher than the power curve method.
8. Ed's conclusion in his sidebar piece seems a good summary
comment to all of this: " In the case of the receivers (tested here)
what is the "true" intercept point of each receiver? There really
is no true number........ the tests took considerable time. QST
readers want to see Product Reviews as soon as possible,
and the ARRL Lab can't take time to do much extra testing
for radios being reviewed. Measurements made at the noise
floor are difficult to make, and the influence of the measured
noise on an IP3 calculation made from receiver response at
the noise floor is not a very accurate way to make
measurements." (!)
And yet, that is exactly what is done by the ARRL and reported
in their published rig test reports! Not only that, but note that
every rig tested is going to be tested with different power levels
since not every rig's S meter is going to read S = 5 with an
exact -97 dBm input signal power pair. And ARRL Labs take
no account for the differing BW's of the selected filter "closest
to a 500 Hz bandwidth, nor even if the rig has one!
It seems that ARRL has neither the time, the equipment nor
the interest in publishing more accurate test data. Hope folks
will agree that this throws the accuracy responsibility back
to the manufactures and ourselves, their customers and our
own "on the air" judgments about the performance of our
radios.
Finally, Doug also discusses transmitter IMD's briefly. That
is the spurious output signals from our transceivers and how/
whether the added linear amp we often use also contributes
spurs. This is a very current and lively topic over on the
contesting.com Amps reflector where it has been demonstrated
that transceivers seem to exhibit a "sweet spot" output power
at which these spurious output signals will be most low; it
may or may not be at the rated max. transceiver output
power, and it may or may not be at the optimum drive
level for a particular linear amp; as with all else, it is
more complex and an instance by instance situation.
This issue of transceiver spurs/noise splatter output
and the following linear amp impact are another topic
for another time; try the Amp reflector archive to come
up to date.
So, now I will rest from all of this, hi.
73, Jim KH7M
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