Electrical Engineering - Building Coaxial transmission line on PCB?

Author

Building Coaxial transmission line on PCB?

Geronimo Stempovski

2007-02-12, 9:25 am

I think transmitting high-speed signals is very easy when you have a
360-degree ground reference, round conductors,
and no other nearby signals like in coaxial cables. My aim is to design PCB
tracks as much like a coaxial cable as
possible. Anyone tried this before? Is it possible with regular FR4, anyway?
Thanks for your help.

Gero

John Fields

2007-02-12, 9:25 am

On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
<geronimo.stempovski@arcor.de> wrote:

>I think transmitting high-speed signals is very easy when you have a
>360-degree ground reference, round conductors,
>and no other nearby signals like in coaxial cables. My aim is to design PCB
>tracks as much like a coaxial cable as
>possible. Anyone tried this before? Is it possible with regular FR4, anyway?
>Thanks for your help.

---
http://en.wikipedia.org/wiki/Microstrip

--
JF

Geronimo Stempovski

2007-02-12, 9:25 am

"John Fields" <jfields@austininstruments.com> schrieb im Newsbeitrag
news:urq0t2533bdm5e2t2ui82b7fo8ppvsbqs8@4ax.com...
>
> http://en.wikipedia.org/wiki/Microstrip
>

Microstrip has absolutely nothing to do with the coaxial structure I had in
mind.

Meindert Sprang

2007-02-12, 9:25 am

"Geronimo Stempovski" <geronimo.stempovski@arcor.de> wrote in message
news:45d06e12$0$30327$9b4e6d93@newsspool1.arcor-online.net...
>
> "John Fields" <jfields@austininstruments.com> schrieb im Newsbeitrag
> news:urq0t2533bdm5e2t2ui82b7fo8ppvsbqs8@4ax.com...
>
> Microstrip has absolutely nothing to do with the coaxial structure I had
in
> mind.

But a microstrip will achieve the same results without the technical
difficulty of a real coaxial line. It is even easier to use
balanced/differential connections.

Meindert

Fred Bloggs

2007-02-12, 9:25 am

> I think transmitting high-speed signals is very easy when you have a
> 360-degree ground reference, round conductors,
> and no other nearby signals like in coaxial cables. My aim is to design PCB
> tracks as much like a coaxial cable as
> possible. Anyone tried this before?

Nope- in all the decades of high speed PC circuit design, you are the
first to think of it!

>Is it possible with regular FR4, anyway?

Not even close, the "phase velocity," or speed to you, will be less than
that of free space by a factor of 1/sqrt(epsilon-sub-r), so go figure.

Henning Paul

2007-02-12, 9:25 am

Geronimo Stempovski wrote:

>
> "John Fields" <jfields@austininstruments.com> schrieb im Newsbeitrag
> news:urq0t2533bdm5e2t2ui82b7fo8ppvsbqs8@4ax.com...
>
> Microstrip has absolutely nothing to do with the coaxial structure I
> had in mind.

http://en.wikipedia.org/wiki/Stripline

You won't get any more TEM-like on a PCB.

regards
Henning

F'up2 d.s.e. (Da darfst Du dann auch wieder deutsch sprechen.)

a7yvm109gf5d1@netzero.com

2007-02-12, 9:25 am

On Feb 12, 6:10 am, "Geronimo Stempovski"
<geronimo.stempov...@arcor.de> wrote:
> I think transmitting high-speed signals is very easy when you have a
> 360-degree ground reference, round conductors,
> and no other nearby signals like in coaxial cables. My aim is to design PCB
> tracks as much like a coaxial cable as
> possible. Anyone tried this before? Is it possible with regular FR4, anyway?
> Thanks for your help.
>
> Gero

You can build a wave guide out of a multi-layer board with lots of
micro vias. The performance gain was non-existent vs traditional micro/
strip/line, and cost a lot of money. These days you can satisfy
yourself of this reality with a 3D field solver. Back when we tried
this, workstations were as slow as building it "for real", and more
expensive.
Just break out to a connector, use a coax assembly, and connect back
in. Easy peasy.

Fred Bloggs

2007-02-12, 9:25 am

a7yvm109gf5d1@netzero.com wrote:
> On Feb 12, 6:10 am, "Geronimo Stempovski"
> <geronimo.stempov...@arcor.de> wrote:
>
>
>
> You can build a wave guide out of a multi-layer board with lots of
> micro vias. The performance gain was non-existent vs traditional micro/
> strip/line, and cost a lot of money. These days you can satisfy
> yourself of this reality with a 3D field solver. Back when we tried
> this, workstations were as slow as building it "for real", and more
> expensive.
> Just break out to a connector, use a coax assembly, and connect back
> in. Easy peasy.
>

There is such a thing as microcoax, so he can chisel out a little groove
in a thick pc and stuff that in there.

MassiveProng

2007-02-12, 1:25 pm

On Mon, 12 Feb 2007 14:17:50 GMT, Fred Bloggs <nospam@nospam.com> Gave
us:

>
>
>a7yvm109gf5d1@netzero.com wrote:
>
>There is such a thing as microcoax, so he can chisel out a little groove
>in a thick pc and stuff that in there.

Semi-rigid is the term, and if it is in coax, it doesn't need to be
IN the PCB from point to point.

I wish you guys would stop cross posting all over the place. Many
ISPs have group inclusion limits.

It is also not considered proper Usenet practice.

John Larkin

2007-02-12, 1:25 pm

On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
<geronimo.stempovski@arcor.de> wrote:

>I think transmitting high-speed signals is very easy when you have a
>360-degree ground reference, round conductors,
>and no other nearby signals like in coaxial cables. My aim is to design PCB
>tracks as much like a coaxial cable as
>possible. Anyone tried this before? Is it possible with regular FR4, anyway?
>Thanks for your help.
>
>Gero
>

Sure. Microstrip, stripline, coplanar waveguide, or even the very
strange slotline.

John

John Fields

2007-02-12, 1:25 pm

On Mon, 12 Feb 2007 14:39:30 +0100, "Geronimo Stempovski"
<geronimo.stempovski@arcor.de> wrote:

>
>"John Fields" <jfields@austininstruments.com> schrieb im Newsbeitrag
>news:urq0t2533bdm5e2t2ui82b7fo8ppvsbqs8@4ax.com...
>
>Microstrip has absolutely nothing to do with the coaxial structure I had in
>mind.

---
Well, Mister Nasty-XXX, what exactly did you have in mind, then?

--
JF

Grant Edwards

2007-02-12, 1:25 pm

On 2007-02-12, Geronimo Stempovski <geronimo.stempovski@arcor.de> wrote:
>
> "John Fields" <jfields@austininstruments.com> schrieb im Newsbeitrag
> news:urq0t2533bdm5e2t2ui82b7fo8ppvsbqs8@4ax.com...
>
> Microstrip has absolutely nothing to do with the coaxial
> structure I had in mind.

If you're such an expert, why are you asking here?

In my experience working with stuff in the low end of the
microwave region (~ 1GHz), microstrip is pretty much what
everybody uses as a PC board alternative to coax.

--
Grant Edwards grante Yow! NANCY!! Why is
at everything RED?!
visi.com

Joel Kolstad

2007-02-12, 1:25 pm

"Geronimo Stempovski" <geronimo.stempovski@arcor.de> wrote in message
news:45d04b34$0$27624$9b4e6d93@newsspool2.arcor-online.net...
>I think transmitting high-speed signals is very easy when you have a
>360-degree ground reference, round conductors,
> and no other nearby signals like in coaxial cables. My aim is to design PCB
> tracks as much like a coaxial cable as
> possible. Anyone tried this before?

I haven't tried it myself, but it's not entirely uncommon in microwave designs
to do something like this -- it's essentially a stripline design with a
"picket fence" worth of vias to serve as the sidewalls. I suspect the reason
it isn't particular popular is that the performance isn't that much better
than a stripline, the models for it aren't found in ADS/Microwave
Office/Ansoft Designer, and the manufacturing costs may be higher. ("Maybe"
because I've seen a lot of people who've started transitioning from microstrip
to a co-planar waveguide, which requires a bazillion drill hits as well. You
trade off the number of drill hits for isolation...)

---Joel

Austin Lesea

2007-02-12, 5:25 pm

Gero,

Why?

Seems others have already progressed to the "abuse" phase, however I am
curious.

If microstrip, or stripline constrains the electric fields such that for
all practical purposes the matched line does not radiate, why is a
coaxial line any better? The only benefit of a coaxial line is that
unmatched, it can not radiate (all the RF energy has to come out of the
ends).

Is this a concern for radiated emissions?

Again, unmatched microstrip or stripline structures can be engineered
with adjacent shielding such that EMI/RFI should not be an issue.

Trying to create a coaxial guide by placing many vias and metal lines is
just too ugly to even think about without a valid reason. Once the
reason is known, the first or second approximation to the structure is
probably completely adequate.

Austin

Robert Baer

2007-02-13, 3:25 am

Geronimo Stempovski wrote:
> I think transmitting high-speed signals is very easy when you have a
> 360-degree ground reference, round conductors,
> and no other nearby signals like in coaxial cables. My aim is to design PCB
> tracks as much like a coaxial cable as
> possible. Anyone tried this before? Is it possible with regular FR4, anyway?
> Thanks for your help.
>
> Gero
>
>
I would think that would be difficult and/or costly.
Working from bottom to top:
Start with (say) 6 mil laminate 0.5/0.5 at bottom; bottom layer has a
narrow stripe to emulate the lowest part of a coax, and top part is a
wider stripe: both ground (shield).
Next layer (#2) is (say) 6 mil laminate 0/0.5 (ie bottom has no
copper and top is 0.5 ounce); stripe is wider and is ground.
"Middle" or next layer is (say) 6 mil laminate 0/0.5 (ie bottom has
no copper and top is 0.5 ounce); 3 stripes: ground / center conductor /
ground.
Next layer is (say) 6 mil laminate 0/0.5 (ie bottom has no copper and
top is 0.5 ounce); stripe is as wide as layer #2.
Then use (say) 6 mil laminate 0/0.5 at top; where the top copper has
a "wide" stripe same as first laminate top stripe as ground.
Finish with (say) 6 mil laminate 0/0.5 at top; where the top copper
has a "narrow" stripe same as first laminate bottom stripe as ground.

Use more layers if they are thinner.
Use vias liberally for tying the ground stripes together.
Note the 6 mils is a wild guess.

Robert Baer

2007-02-13, 3:25 am

John Fields wrote:

> On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
> <geronimo.stempovski@arcor.de> wrote:
>
>
>
>
> ---
> http://en.wikipedia.org/wiki/Microstrip
>
>
He said "coax"...

john jardine

2007-02-13, 3:25 am

Robert Baer

2007-02-13, 3:25 am

Fred Bloggs wrote:

>
>
>
> Nope- in all the decades of high speed PC circuit design, you are the
> first to think of it!
>
>
>
> Not even close, the "phase velocity," or speed to you, will be less than
> that of free space by a factor of 1/sqrt(epsilon-sub-r), so go figure.
>
>
Gee, coax cables, even those that use spiral teflon seperators, are
like that...

Bob

2007-02-13, 3:25 am

"John Fields" <jfields@austininstruments.com> wrote in message
news:aab1t2dlh31fogqb48o6uptri6qr4at2ml@4ax.com...
> On Mon, 12 Feb 2007 14:39:30 +0100, "Geronimo Stempovski"
> <geronimo.stempovski@arcor.de> wrote:
>
>
> ---
> Well, Mister Nasty-XXX, what exactly did you have in mind, then?
>
>
> --
> JF

How DARE you not know what he had in mind!

Bob

John Fields

2007-02-13, 9:25 am

On Tue, 13 Feb 2007 04:55:03 GMT, Robert Baer
<robertbaer@earthlink.net> wrote:

>John Fields wrote:
>
> He said "coax"...

---
No, he said: ...:"as much like a coaxial cable as possible.", which
I took to mean electrically. Otherwise, why not just use a piece of
micro-coax and treat it like just another component on the PCB?

--
JF

CBFalconer

2007-02-13, 9:25 am

Robert Baer wrote:
> John Fields wrote:
> He said "coax"...

Which is just one more way of implementing a line. As is a piece
of wire in some medium.

Ridiculous cross-post level reduced by setting follow-ups.

--
<http://www.cs.auckland.ac.nz/~pgut0.../vista_cost.txt>
<http://www.securityfocus.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

Uwe Hercksen

2007-02-13, 9:25 am

On Mon, 12 Feb 2007 12:10:43 +0100, Geronimo Stempovski
<geronimo.stempovski@arcor.de> wrote:

> I think transmitting high-speed signals is very easy when you have a
> 360-degree ground reference, round conductors,
> and no other nearby signals like in coaxial cables. My aim is to design
> PCB
> tracks as much like a coaxial cable as
> possible. Anyone tried this before? Is it possible with regular FR4,
> anyway?

Hello,

the technology used to build multilayer PCB does not allow to build a
round coaxial transmission line.
You may have structured planes of copper foil separated by insulation
material, but nothing like the shield of a coaxial cable. Even a
rectangular shield around a center conductor is not possible with the
existing methods to build multilayer PCB.

bye

Uwe Hercksen

2007-02-13, 9:25 am

On Mon, 12 Feb 2007 22:30:39 +0100, john jardine
<john@jjdesigns.fsnet.co.uk> wrote:

> Had trouble with crosstalk on a mass of video signals. Cured with a
> multilayer board where each signal was 'boxed in' by ground plane to the
> sides, above and below. Sort of square coax.

Hello,

but how about a real closed square shield around the center conductor?

Bye

Geronimo Stempovski

2007-02-13, 1:25 pm

Thanks for your help, so far. Really appreciate it. I'm already a step
further in my considerations.

Now I'm looking for a diagram like frequency (some MHz to 10 GHz for
example) versus loss tangent and / or epsilon R for FR4. I only found a poor
black-and-white copy from 1991 in a paper which I searched with Google. I
wouldn't have thought it to be so hard to find a graph... Does anybody know
where I can find that?

john jardine

2007-02-13, 5:25 pm

"Uwe Hercksen" <hercksen@mew.uni-erlangen.de> wrote in message
news:op.tnournbqni00lm@hercksen3...
> On Mon, 12 Feb 2007 22:30:39 +0100, john jardine
> <john@jjdesigns.fsnet.co.uk> wrote:
>
>
> Hello,
>
> but how about a real closed square shield around the center conductor?
>
> Bye

Would have been ideal. At the time was thinking about a method to do this
and sorted a setup that may have been worth talking to the PCB people about
but a customer was paying to clear down his urgent problem and not to start
up a research project

.
No ... I don't remember what I figured out. Ideas are easy, it's the
implementation that's a problem

.
No doubt it'll surface again if I'm I'm under pressure.
john

--
Posted via a free Usenet account from http://www.teranews.com

Jim Granville

2007-02-13, 5:25 pm

Uwe Hercksen wrote:

> On Mon, 12 Feb 2007 22:30:39 +0100, john jardine
> <john@jjdesigns.fsnet.co.uk> wrote:
>
>
>
> Hello,
>
> but how about a real closed square shield around the center conductor?

To do that would need plated slots, and slots have the PCB fabs
luke-warm at best : hard to do cleanly, they also weaken the PCB if
long, and also have minimum router sizes, plus machine time......

A better direction would be thinner laminates, and using the space you
would have lost to the slot anyway, as wider GND webs on the same plane,
coupled with stitching vias (which can be smaller dia than slots)

-jg

Uwe Hercksen

2007-02-14, 9:25 am

On Tue, 13 Feb 2007 18:41:00 +0100, Geronimo Stempovski
<geronimo.stempovski@arcor.de> wrote:

> Now I'm looking for a diagram like frequency (some MHz to 10 GHz for
> example) versus loss tangent and / or epsilon R for FR4. I only found a
> poor
> black-and-white copy from 1991 in a paper which I searched with Google. I
> wouldn't have thought it to be so hard to find a graph... Does anybody
> know
> where I can find that?

Hello,

I am sorry to tell that, but for frequencies of 1 to 10 GHz, FR4 ist not
the right material, there are other PCB materials which are better for
high frequencies, take a look here
http://www.andus.de/Leiterplatten/Impedanz/hfmat.htm
They write there that FR4 may be used up to 4 GHz.

Bye

John Larkin

2007-02-14, 1:25 pm

On Wed, 14 Feb 2007 15:36:15 +0100, "Uwe Hercksen"
<hercksen@mew.uni-erlangen.de> wrote:

>On Tue, 13 Feb 2007 18:41:00 +0100, Geronimo Stempovski
><geronimo.stempovski@arcor.de> wrote:
>
>
>Hello,
>
>I am sorry to tell that, but for frequencies of 1 to 10 GHz, FR4 ist not
>the right material, there are other PCB materials which are better for
>high frequencies, take a look here
>http://www.andus.de/Leiterplatten/Impedanz/hfmat.htm
>They write there that FR4 may be used up to 4 GHz.
>
>Bye

FR4 can be used at 20 GHz, depending on what you're trying to do.

John

Del Cecchi

2007-02-14, 1:25 pm

John Larkin wrote:
> On Wed, 14 Feb 2007 15:36:15 +0100, "Uwe Hercksen"
> <hercksen@mew.uni-erlangen.de> wrote:
>
>
>
>
>
> FR4 can be used at 20 GHz, depending on what you're trying to do.
>
> John
>

Also you need to narrow down which variety of FR4 you are interested in.

--
Del Cecchi
"This post is my own and doesn’t necessarily represent IBM’s positions,
strategies or opinions.”

John Larkin

2007-02-14, 1:25 pm

On Wed, 14 Feb 2007 10:27:58 -0600, Del Cecchi
<cecchinospam@us.ibm.com> wrote:

>John Larkin wrote:
>
>Also you need to narrow down which variety of FR4 you are interested in.

FR4 is a commodity that's not well controlled. There really aren't
controlled varieties, and most pcb houses don't guarantee they'll
always use the same stuff. If you want loss or Er consistancy, you
usually have to call out something more specific. Or design so that it
doesn't matter.

John

Uwe Hercksen

2007-02-15, 9:25 am

On Wed, 14 Feb 2007 17:10:34 +0100, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>
> FR4 can be used at 20 GHz, depending on what you're trying to do.

Hello,

well, if the transmission line is very, very short.....

bye

John Larkin

2007-02-15, 1:25 pm

On Thu, 15 Feb 2007 16:15:49 +0100, "Uwe Hercksen"
<hercksen@mew.uni-erlangen.de> wrote:

>On Wed, 14 Feb 2007 17:10:34 +0100, John Larkin
><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>
>
>Hello,
>
>well, if the transmission line is very, very short.....
>
>

Exactly.

John

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