[TenTec] IP3 vs. DR3 (IMD) confusion

[Duane - N9DG]

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