Design Con 2015
Breaking News
Comments
Newest First | Oldest First | Threaded View
David Ashton
User Rank
Blogger
Re: Impedance
David Ashton   2/13/2014 1:43:51 AM
NO RATINGS
@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?

 

sukv
User Rank
Rookie
Re: Impedance
sukv   2/12/2014 11:47:15 PM
NO RATINGS
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
User Rank
Blogger
Re: Impedance
David Ashton   2/12/2014 3:26:50 PM
NO RATINGS
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.......

zeeglen
User Rank
Blogger
Re: Impedance
zeeglen   2/12/2014 3:09:09 PM
NO RATINGS
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.


krisi
User Rank
CEO
Re: Impedance
krisi   2/12/2014 3:03:20 PM
NO RATINGS
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

betajet
User Rank
CEO
Re: Impedance
betajet   2/12/2014 2:33:27 PM
NO RATINGS
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
User Rank
CEO
Re: Impedance
krisi   2/12/2014 10:26:20 AM
NO RATINGS
There is nothing magical about 50, just a compromise number that became a standard...we could had ended up with 75 for example

sukv
User Rank
Rookie
Impedance
sukv   2/12/2014 4:17:18 AM
NO RATINGS
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 ?



Top Comments of the Week
Flash Poll
Like Us on Facebook

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)
EE Life
Frankenstein's Fix, Teardowns, Sideshows, Design Contests, Reader Content & More
Carlos Bueno

Adventures in Userland
Carlos Bueno
Post a comment
Editor’s Note: Excerpted from Lauren Ipsum: A Story About Computer Science and Other Improbable Things, author Carlos Bueno introduces us to Lauren and her adventures in Userland. ...

Max Maxfield

Tired Old iPad 2 vs. Shiny New iPad Air 2
Max Maxfield
24 comments
I remember when the first iPad came out deep in the mists of time we used to call 2010. Actually, that's only four years ago, but it seems like a lifetime away -- I mean; can you remember ...

Martin Rowe

Make This Engineering Museum a Reality
Martin Rowe
Post a comment
Vincent Valentine is a man on a mission. He wants to make the first house to ever have a telephone into a telephone museum. Without help, it may not happen.

Rich Quinnell

Making the Grade in Industrial Design
Rich Quinnell
16 comments
As every developer knows, there are the paper specifications for a product design, and then there are the real requirements. The paper specs are dry, bland, and rigidly numeric, making ...

Special Video Section
The LT8640 is a 42V, 5A synchronous step-down regulator ...
The LTC2000 high-speed DAC has low noise and excellent ...
How do you protect the load and ensure output continues to ...
General-purpose DACs have applications in instrumentation, ...
Linear Technology demonstrates its latest measurement ...
10:29
Demos from Maxim Integrated at Electronica 2014 show ...
Bosch CEO Stefan Finkbeiner shows off latest combo and ...
STMicroelectronics demoed this simple gesture control ...
Keysight shows you what signals lurk in real-time at 510MHz ...
TE Connectivity's clear-plastic, full-size model car shows ...
Why culture makes Linear Tech a winner.
Recently formed Architects of Modern Power consortium ...
Specially modified Corvette C7 Stingray responds to ex Indy ...
Avago’s ACPL-K30T is the first solid-state driver qualified ...
NXP launches its line of multi-gate, multifunction, ...
Doug Bailey, VP of marketing at Power Integrations, gives a ...
See how to ease software bring-up with DesignWare IP ...
DesignWare IP Prototyping Kits enable fast software ...
This video explores the LT3086, a new member of our LDO+ ...
In today’s modern electronic systems, the need for power ...