impedance control steps

Hi folks,

Just wondering how to answer a question that will posed in an interview. An
understanding of what impedance is, and how to control it with trace
width/thickness and planes is one thing. Having never actually done it, what
is the proper reply to "how do you go about it?"

Lots of web info on what it is, not much on the step by step of how a designer
goes about implementing it. Guess I am looking for an example of the actual
process. I haven't done it, but I am gonna be asked to, so is there any help
here, or any URLs or sources that can gimme a hand?

Thanks,

Mark.

 

[Jon Elson]

mountains@iname.com wrote:

Hi folks,

Just wondering how to answer a question that will posed in an interview. An
understanding of what impedance is, and how to control it with trace
width/thickness and planes is one thing. Having never actually done it, what
is the proper reply to "how do you go about it?"

Lots of web info on what it is, not much on the step by step of how a designer
goes about implementing it. Guess I am looking for an example of the actual
process. I haven't done it, but I am gonna be asked to, so is there any help
here, or any URLs or sources that can gimme a hand?

I don't know about procedure. Generally, you can look it up in various

texts. Knowing the thickness of the dielectric (space between ground
plane and signal trace) and assuming microstrip (signals on outer copper
of board) you just use a graph to figure out the proper trace width.
Motorola had a classic chapter on this in their MECL design handbook
from about 1970 or so. It had formulas as well as charts to select
the proper width. National Semiconductor, I think, made a slide rule
(cardboard) for this. That was probably a decade ago, too.

Jon

 

[Ian Buckner]

"Jon Elson" <elson@pico-systems.com> wrote in message
news:8a374$413d3cb2$4503e66a$21081@msgid.meganewsservers.com...

mountains@iname.com wrote:
Hi folks,

Just wondering how to answer a question that will posed in an
interview. An
understanding of what impedance is, and how to control it with
trace
width/thickness and planes is one thing. Having never actually
done it, what
is the proper reply to "how do you go about it?"

Lots of web info on what it is, not much on the step by step of
how a designer
goes about implementing it. Guess I am looking for an example of
the actual
process. I haven't done it, but I am gonna be asked to, so is
there any help
here, or any URLs or sources that can gimme a hand?

I don't know about procedure. Generally, you can look it up in
various
texts. Knowing the thickness of the dielectric (space between
ground
plane and signal trace) and assuming microstrip (signals on outer
copper
of board) you just use a graph to figure out the proper trace width.
Motorola had a classic chapter on this in their MECL design handbook
from about 1970 or so. It had formulas as well as charts to select
the proper width. National Semiconductor, I think, made a slide
rule
(cardboard) for this. That was probably a decade ago, too.

Jon

There are a number of free transmission line impedance calculators

available. Do a Google search using those terms. Agilent has a tool
called AppCAD, there's also Ultracad and TxLine.

Regards
Ian