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Why most circuit have I/O impedance of 50ohms?

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wylee

wylee

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Why 50ohms?

I noticed that most circuits designed for output/input impedance of 50Ω...why is that?

Why it must be 50Ω (or 75Ω), why can't we use something else like 100Ω, 1kΩ...what is so special about '50Ω'? Who set the rules?

Can anyone explain it to me?
 

Re: Why 50ohms?

this because low loss is at 75ohm and high power is 37ohm for transmission line.
so 50 ohm is trade off between low loss and high power.
 

Re: Why 50ohms?

I think one have to go back some time ago when pioniers in RF transmission tried to figure ou the best shape of antennas. For example: folded diapole 2-wire anntena has the input impedance close to 300 ohm and therefore in many today's TV instalation we have 300ohm flat cable. Another example: the radiation resistance of 1/2-λ antenna in free air, with the antenna remote from everything else, and antenna made of infinitely thin conductor, is approximately 73ohm. Therefore cable that connected this antenna with transmitter/receiver has impedance of 75ohm.
We are talking here about resonant values, but in fact they are "nearly resonant" or "close to resonant".
 

Re: Why 50ohms?

You can refer "The Design of cmos radio frequency integrated circuits"
which wrote by Thomas H. Lee. The page 141
 

Re: Why 50ohms?

Hi,

I think this due to standard coaxial-cable impedances of 50 and 75 . Today, commonly available impedance for coaxial cable is 50 W and it is a good low-loss . Most test equipment work best with 50W. Ethernet went with 50 rather than 75W coaxial, because 50 W is more tolerant of the capacitive loading effects of the transceiver taps.
 

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