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Electromagnetic Interference (EMI) Pt 2

Stubs
5/20/2013 01:17 PM EDT
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sukv
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Impedance
sukv   2/12/2014 4:17:18 AM
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Why exactly are we using 50 ohm impedance ? How we arrived to 50 ?why not 49, 51 etc ?

2: What is the reason behind using 90 ohm impedance for USB and 120 ohm for CAN ?

krisi
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Re: Impedance
krisi   2/12/2014 10:26:20 AM
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There is nothing magical about 50, just a compromise number that became a standard...we could had ended up with 75 for example

betajet
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Re: Impedance
betajet   2/12/2014 2:33:27 PM
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Here's my understanding, based on decades-old experience so the numbers may have changed over the years:

The impedance of a wire is determined by its dimensions, what other conductors are nearby, the distance to those conductors, and what material is used for insulation.  On PC boards, a wire over a ground plane gives you 50 Ohms with a reasonable thickness of the insulating layer.  You could increase the impedance by using a thicker insulating layer, but then you have a thicker board.

For USB and CAN, the dimensions of the twisted pair and insulation type and thickness give you the 90-120 Ohms.

Coaxial cable typically gives you 50-75 Ohms.

Being off by a few percent doesn't matter.  An impedance mismatch gives you reflections, but if the mismatch is small the reflections are small compared to the signal.

krisi
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Re: Impedance
krisi   2/12/2014 3:03:20 PM
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The trade-off is between dieletric breakdown and power delivered to the load...for a fixed inner conductor diameter one could increase outer diameter to increase breakdown voltage...but the characteristic impedance would increase and that would make power deliverable to the load smaller...so there is a well defined ratio of the of conductor diameters that maximizes power handling capability of the impedance of the coax

zeeglen
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Re: Impedance
zeeglen   2/12/2014 3:09:09 PM
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Why exactly are we using 50 ohm impedance ?

The tale I heard is that a 1/4 wave vertical antenna on a right-angled ground plane is about 36 ohms, and a non-folded dipole above the ground effect is about 72 ohms.  50 ohms is a nice round number in the middle.


A 1/4 wave vertical antenna with a ground counterpoise angled downwards (such as might be found on the rooftop of a ham radio operator) is a very close match to 50 ohms. When properly tuned mine usually measured better than 1.1 to 1 standing wave ratio when fed with 50 ohm coax cable.


David Ashton
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Re: Impedance
David Ashton   2/12/2014 3:26:50 PM
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HI Glen.   "The tale I heard is that a 1/4 wave vertical antenna on a right-angled ground plane is about 36 ohms, and a non-folded dipole above the ground effect is about 72 ohms.  50 ohms is a nice round number in the middle."

That's funny!  SWRs don't worry about round numbers, and with most antennas you can get a really good SWR with 50Ω cable.

I think the natural resonance of a folded dipole is around 50 ohms, so that's why 50 ohms is so commonly used.  But TV antennas seem to use 75Ω, as do satellite dishes.

I remember my comms textbooks had a lot to say on this subject, but they are back in Zimbabwe - possibly.......

sukv
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Re: Impedance
sukv   2/12/2014 11:47:15 PM
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Folded dipole ,its relating to antenna right ? - the device which radiates energy.

How can that be related to PCB environment where radiations are not at all allowed ?

David Ashton
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Re: Impedance
David Ashton   2/13/2014 1:43:51 AM
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@sukeshv - "How can that be related to PCB environment..??"

Good point - we do tend to get off-topic here.   Betajet's explanation is good - PCB traces approximate to 50Ω and it is (happily) a commonly used impedance.  But it will vary with the thickness of the board, track size, etc as Betajet points out.      Also ECL - for years the predominant high-speed logic family - works at 50Ω as well, maybe that has something to do with it?

 

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