[SI-LIST] : Zo Choices & Why

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From: Chris Padilla (cpad@cisco.com)
Date: Tue Jan 02 2001 - 11:08:16 PST


Hello Group and a Happy New Year to all!

My questions revolves around why, for instance:

(1) 50-ohms is the choice for instruments?

(2) 75-ohms is the choice for CATV/Satellite?

(3) 100-ohms is the choice for Ethernet?

(4) 28-ohms is the choice for Rambus?

(5) Any other "standards" I've left out? (300-ohms for VHF, UHF?)

I've read various things in the past explaining this but I have lost the
link nor can find original posts describing this.

I recall it being related to "What is the goal of the particular signal I
wish to transmit?" For CATV, it is picture quality over distance and I
think 75-ohms had the least amount of attenuation for the given
cable. 50-ohms for intruments was deemed "quietest" or something like that.

Anyone have the reasons?

Thanks----->Chris

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