WA6HHQ:
>I discussed the issue of the missing IMDDR3 numbers (Third Order Dynamic
Range) with Michael Tracy at the ARRL a couple of weeks ago. I expressed
my strong concern that they had dropped these from the review data and
were only posting a footnote showing how to calculate them from the IMD
level and MDS. I pointed out that we, and many others, use the IMDDR3
numbers as the primary IMD Dynamic range comparison between rigs and
that the IP3 numbers were not as useful for receiver comparisons, since
they can artificially be inflated by turning on the attenuator, or
making a receiver have low sensitivity (deaf).
I agree completely. It was very surprising they omitted the IMD
numbers from the tables yet included them in the bar graphs at the
very front of the article.
>Also, the multi-level IP3 numbers were in direct response to requests to
the ARRL from several members of the advisory group that Rob mentions
below.
I know W8JI who is on the group believes IP3 is simply
a subject of much confusion and abuse. My guess is that
one of the European VHF guys in the group lobbied for this.
But I agree it's mostly a meaningless theoretical number
better represented by measuring MDS and IMDDR3 separately.
>Michael Tracey and the ARRL test lab are above reproach in my opinion.
I agree completely but I wish more of his objectivity were
shared by some of the folks writing the QST reviews. I feel
some of Sherwood's criticisms are completely justified, but
this is of course not Michael's responsibility. Rob also has
his own axe to grind at times so you have to take some
of what he says with a grain of salt. I had an Orion for 4
years and *never* heard the AGC hang artifact even though I
spend 90% of my time on 160 meters where there are plenty
of BIG QRN crashes. Rob can also go a bit overboard into
conspiracy theories about QST advertisers. I don't buy that.
It sounds like ARRL responded appropriately. I also
found it humorous that W1ZR in the very same issue was
describing IMDDR3 and its importance yet it was omitted
in the FT-2000 tabes! :-) None of us is perfect.
On the plus side, I commend ARRL for gradually
moving toward more realistic testing...such as posting
2 kHz spaced measurements in the QST articles rather
than simply burying the data in an obscure graph in
the Expanded Test Reports. Same for TX signal purity
(keyclicks and phase noise). They're getting better but
IMHO they need to tone down the fluffiness in the QST
articles. My personal favorite is G3SJX in RSGB's Radcom.
It would almost be worth the cost of joining RSGB just to get
his reviews.
BTW Sherwood measured the SDR-5000's IMDDR3 at
5 kHz as 96 dB. It's going to be a real horse race to see if
the K3 may top that but if not it should be very close.
73, Bill W4ZV
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