Hi Duane,
40 years ago when all of this begin, we did focus on IP3.
We didn't know better.
Some time in the 90s the focus switched to DR3 which, as you say, is far
more meaningful because we can shove IP3 up or down with attenuation or
amplification.
The JA companies continue to promote IP3 figures.
However on SDR radios, DR3 is not a meaningful number either.
I guess if it were low, it would be meaningful, but just because it's high,
that does not mean the receiver is a great receiver.
The list ranked by DR3 as it currently is, is causing a lot of confusion.
Too many people believe that Rob intended this to be a ranking of ultimate
performance for each radio, which it is not.
73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Duane -
N9DG via TenTec
Sent: Sunday, September 11, 2016 5:37 PM
To: tentec@contesting.com
Cc: Duane - N9DG
Subject: [TenTec] IP3 vs. DR3 (IMD) confusion
There seems to be some confusion and conflation of these two RX performance
measurement parameters in recent threads. They are NOT the same thing at
all. I'm seeing people write "IP3" (Third Order Intercept Point) when I
think they mean IMD DR3 (3rd order Intermodulation Distortion Dynamic Range
- particularly "close-spaced"), or they are not understanding the difference
between between them. For example saying that Rob's chart is sorted by
"IP3", it is not, actually Rob doesn't even list IP3!
IP3 is a USELESS measurement metric for direct sampling RX designs, and I'd
argue not a particularly useful performance metric for any RX design. Though
it is a number that the JA brands make a big deal about in their
advertising, and have for years now.
IMD DR3 - is arguably the most important RX performance metric, if not the
most important, that is why Rob sorts his chart based on that number. But
also notice that you will never see a radio with poor phase noise ("LO Noise
dBc/Hz") near the top of that ranking, because in those poor phase noise
radios, the phase noise is the key limiting factor for its performance.
Direct sampling designs excel at having low phase noise - as long as the
designers didn't botch the A to D's clock oscillator design.
Duane
N9DG
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