Impedance - which is origin of this magic word?

umashankar.s

Joined Jul 1, 2008
9
Can you tell me why we are using 50 ohm impedance for single ended?
Is it because of cable having 50 ohm only?

also the reason for using 75, 90, 100 ohm impedances ..

y not we use 100 ohm for USB?

What is the exact difference between resistance and impedance?

If impedance mean for the restriction of AC current, why we have to consider it for DC.?

Salgat

Joined Dec 23, 2006
221
Impedence is a combination of DC resistance and AC reactance (aka AC resistance and phase difference of current and voltage). Wiki seems to cover almost any inquiry about the physics side of electronics.

umashankar.s

Joined Jul 1, 2008
9
could u pls elaborately tell me the ans for my qns asked?

umashankar.s

Joined Jul 1, 2008
9
the wiki link just covers the basics of impedance....

I want the reason & justification for my qns

umashankar.s

Joined Jul 1, 2008
9
Basically the impedance is depending on the thickness of the conductor. If the thickness decreases then the impedance increase. We can not reduce the thickness further after the certain limit. Since if we do then the conductor strength become weak. It can be cut at any point. So in general all the manufacture makes the conductor at constant thickness. For that constant thickness the impedance will be 90 ohms so that only we are asked to maintain 90 ohms on our PCBs to avoid impedance mismatches between on our PCBs and on the Cable.

one more

The arithmetic mean between 30 ohms (best power handling) and 77 ohms (lowest loss) is 53.5; the geometric mean is 48 ohms. Thus the choice of 50 ohms is a compromise between power handling capability and signal loss per unit length, for air dielectric.