[SI-LIST] Re: impedance determination of a perticular interface.....

  • From: <colin_warwick@xxxxxxxxxxxx>
  • To: <malli.1729@xxxxxxxxx>
  • Date: Sun, 21 Sep 2014 14:12:22 +0000

Hi Mallikarjun,

Steve Weir pointed out to me off list that I might have misunderstood your 
question "What drives these characteristic impedance values? "

At first blush I thought you meant "What physical quantities drive 
characteristic impedance values?" but on mature reflection you may have meant 
"What design goals drive us to seek a particular impedance target?"

It turns out that there's a lot of fuzzy urban myth about how the 50 ohms (or 
75 ohms or 300 ohms) standards came about. There's a good essay with some 
plausible arguments here: 


According to the essay, the historical design goals that led to the 50/75/300 
ohm standards aren't relevant to SI because (for example) we don't use co-ax 
t-lines much (especially air-spaced co-ax t-lines) and maximal power handling 
isn't generally a goal in SI.  However 50 ohms lives on through "installed 
base" inertia. Let's say connector manufacturers have historically made 50 ohm 
connectors. Then, all else equal, you might design everything else to match 
that just to leverage the high volume/low cost connector. 

Best regards,

-- Colin
Colin Warwick
Product Manager for Signal Integrity, Power Integrity, and EMI/EMC tools for 
High-Speed Digital Design
Keysight EEsof EDA 
Phone +1 978 681 2406
Blog updates: http://signal-integrity.blogs.keysight.com/newsletter/ 

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