Building Coaxial transmission line on PCB?

  • Thread starter Geronimo Stempovski
  • Start date
G

Geronimo Stempovski

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
 
J

John Fields

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.
 
M

Meindert Sprang

Geronimo Stempovski said:
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
 
F

Fred Bloggs

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.
 
J

John Larkin

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
 
G

Grant Edwards

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.
 
J

Joel Kolstad

Geronimo Stempovski said:
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
 
A

Austin Lesea

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
 
J

john jardine

Geronimo Stempovski said:
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.
 
R

Robert Baer

Geronimo said:
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.
 
R

Robert Baer

Fred said:
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...
 
C

CBFalconer

U

Uwe Hercksen

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
 
U

Uwe Hercksen

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
 

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