Building Coaxial transmission line on PCB?

On 2007-02-12, Geronimo Stempovski <geronimo.stempovski@arcor.de> wrote:
>
> "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. 

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            

"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

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

"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? Is it possible with regular FR4,
anyway?
> Thanks for your help.
>
> Gero
>
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.



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

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.

John Fields wrote:

> 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
> 
> 
   He said "coax"...

Fred Bloggs 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? 
> 
> 
> 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.
> 
> 
   Gee, coax cables, even those that use spiral teflon seperators, are 
like that...

Robert Baer wrote:
> John Fields wrote:
>> <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?
>>
>> http://en.wikipedia.org/wiki/Microstrip
>>
> 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/~pgut001/pubs/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

On Feb 12, 2:30 pm, "john jardine" <j...@jjdesigns.fsnet.co.uk> wrote:
> "Geronimo Stempovski" <geronimo.stempov...@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? Is it possible with regular FR4,
> anyway?
> > Thanks for your help.
>
> > Gero
>
> 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.
>
> --
> Posted via a free Usenet account fromhttp://www.teranews.com

----------------------------------------------------------

  Boxed !   the wavelength is far greater than
 your dimensions , thus higher modes can not
 exist , thus you do NOT need sides .
    When you reach 10 Ghz , then maybe
  you need sides in ur boxed "coax" .

  But the big joke , is in the real world ,
  they use cheap PCB to xmit 2.5 Ghz .
   No strip line , no microstrip , nada ..
   It works well , so quit arguing reality .

    BTW , i saw some novice , trying to
   use juice cans to launch WiFi .
  He figured the more cans , the more
   gain .  He had 3 cans , T'd .
   to divide the  power .
     Gain is not in cans , its in size of
   the dish .

    Another book worm said all i needed
    was $26 for 100 meters of  blah blah
   coax at 2.5 Ghz ..

     10 times that price !
     and 1.8" dia  hard line !

    At these wavelengths , its lower loss
   to  send it TEM  and thru the air ,
   not thru a coax .

 This is goin to FPGA ?  Do those relics
  still exist ?!  Oh well , i supose ya gotta
  try to "protect" your firmware by reinventing
  the CPU !

"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:
>
>>
>>"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.
>
> ---
> Well, Mister Nasty-Ass, what exactly did you have in mind, then?
>
>
> -- 
> JF

How DARE you not know what he had in mind!

Bob