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Building Coaxial transmission line on PCB?

  • Thread starter Geronimo Stempovski
  • Start date
M

Michael A. Terrell

Jan 1, 1970
0
MassiveProng said:
Are they fooly regulated?


As regulated as a dead short can be. ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
J

John Larkin

Jan 1, 1970
0
I got a pill for your ass, fuckhead.

Still got that anal/fecal thing going, I see. I'm sure glad I wasn't
born with a fetish like that.

John
 
J

John Larkin

Jan 1, 1970
0
A couple of things make pcb's lossy: the loss tangent of the material
(and FR4 is pretty bad) and the copper losses. Copper loss gets bad on
conventional FR4 boards because

1. FR4's Er is high, so for a given impedance traces are skinny.

2. The underside of the copper is treated to bond to the epoxy/glass,
and the treatment (black oxide or something) greatly increases skin
losses. Peel some up and look... it's gross.

3. In the case of microstrip, the current is concentrated on the
underside (the dirty side) of the trace, so losses are that much
worse... the shiny topside of the copper is underutilized. Stripline
would be better, with balanced current density, except that the trace
will be much thinner, which has its own penalty.


A good microwave pcb has a low Er, low loss dielectric; is thick, for
low current density and wide traces; has very smooth copper, which
means traces and pads peel off easily.

I don't think any simple geometry tricks (ie, emulating coax) will
make FR4 any better, and would probably make it worse. For low losses,
microstrip on a thick board is probably as good as it gets.

John

Suppose you did an FR4 pcb with a wide microstrip on the top, then
route out most of the material below the trace, namely route a trench
under the trace, from the opposite side, say 90% of the board
thickness. Now we'd have a sort of suspended substrate trace, much
wider for a given Z, with much lower losses. We could call it
LarkinLine.


-------------- copper trace
=================================================
================== ================= epoxy-glass
================== air =================
------------------ ----------------- copper gnd



I haven't a clue how to calculate the impedance; field solver, like
ATLC maybe?

John
 
M

MassiveProng

Jan 1, 1970
0
Still got that anal/fecal thing going, I see. I'm sure glad I wasn't
born with a fetish like that.

Well, you're wrong, and that makes the fetish you were born with
insisting on being stupid, and then showing your ass to the rest of
the world.
 
J

John Larkin

Jan 1, 1970
0
Well, you're wrong, and that makes the fetish you were born with
insisting on being stupid, and then showing your ass to the rest of
the world.

I keep my posterior components discretely covered in public.

And I'm as smart or a stupid as I was born, so am blameless in that
respect. Luckily, the world seems to want my brand of stupidity, so it
all works out. How is your brand of stupidity holding up?

John
 
M

MassiveProng

Jan 1, 1970
0
This sort of foolish language will sooner or later get you PLONKed


Like I give a fat flying **** if you read my posts.
 
G

glen herrmannsfeldt

Jan 1, 1970
0
Geronimo said:
Ah, okay, what I am actually trying to find out is what makes FR4 act worse
than e.g. teflon at data rates beyond 2,5 Gbps. Is it the loss tangent or
the epsilon r?

It is the imaginary part of the dielectric constant. I believe that
can be described as a loss tangent, though I haven't done that.
How is the frequency-dependent attenuation physically
describable? Where does the energy go? Heat, ...?

Yes, heat. You can consider it as electrical friction.
It was my opinion that
higher frequencies can be transmitted over coax but not over FR4 because of
the geometry. Because in a coax there is (almost) no energy loss because the
TEM wave is "captured" by the outer shield and in a planar setup like
stripline or microstrip there are E-field and H-field lines vanish into the
air environment (or somewhere else...). Therefore I'm trying to design a
coax on a PCB. Am I right with my thoughts, anyway?

I don't believe that is true. It may be that coax tends to use better
dielectrics. Other than that, even if the wave isn't "captured" as you
say, as long as it doesn't find anything else to induce current into, it
doesn't contribute to loss. (Well, twisted pair cable is twisted to
minimize the radiation. Microstrip isn't twisted. Radiation will be
mostly in the plane of the board, and minimized by keeping the board
thin.) The very low loss coax cables use a spiral dielectric such that
most of the space is air and very little is plastic. There has to be
enough material to hold the center conductor in place.

The other loss increase with frequency is due to the skin effect, where
the current travels in a thin layer on the surface of the wire,
increasing the resistance as seen by the current. I believe FR4
is chosen for its thermal and strength properties, and ability to bind
to metal, all not normally required by a coax cable dielectric.
(And also cost.)

-- glen
 
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