Building Coaxial transmission line on PCB?

[Uwe Hercksen]

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

 

[Uwe Hercksen]

Again, I'm looking for a diagram like frequency (some MHz to 10 GHz for
example) versus loss tangent and / or epsilon R for FR4 or other usual
PCB
material. 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 hardto
find a graph but as noone replied to my previous question so far it does
seem to be hard!

Hello,

look here for FR408
http://www.isola.de/d/ecomaXL/index.php?site=ISOLA_DE_product_additional_information&sid=235&p=10

bye

 

[John Larkin]

Hello,

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

Exactly.

John

 

[werty]

work. I can imagine that John J. may well also. When you're aiming

at 100+dB isolation among traces, you do have to be pretty careful,
even at "low" frequencies.

Cheers,
Tom- Hide quoted text -

- Show quoted text -

______________________________________

Its all theory . If its open , the RF will leak
out . Like the holes in a MicroWave dish
----------------
10:1 SWR , open lines ,
and NO radiation .
-------------------
Transmission lines repulse , but that
does NOT mean magnetic fringing
and sending mag flds everywhere .
----------------------

BTW
Study CAT5e Ethernet cable .
garbage !
USB cables are much faster .


I think ppl limit themselves to
whats avail in PCB , then complain
when it dont work , but if they'd
experiment , they'd find the problem
is using thin PCB .
Then they limit on putting down
100 transmission lines per mm .
You cant learn , unless you experiment.

you can't choke the dimensions
and get good results , a transmission
line needs exact dimensions , or you
lose .

In coax for 2.5 Ghz , for example ,
it WILL have a large diameter and
the center will have an exact dia and
ratio .. No substitutes .

Sending signals that will be amplified
use high Z ( ~65 ohms ) and
sending power needs low Z .
These rules can't be bent .
Thats what you're doin , is bending
rules ...









..

 

[werty]

I do stuff down to a few ps RMS jitter on a regular 6 or 8-layer
board, microstrip traces, with switching supplies and uPs and display
drivers on the same board. Picoseconds aren't tough these days.

John- Hide quoted text -

- Show quoted text -

____________________________________

You are hoping that we believe switchers
cause lots of noise ....

Zero ripple is what switchers do !
The sudden pulse of current is only
around a very short loop , it does
not cause noise .
They dont even have "ground loops"

 

[Geronimo Stempovski]

But FR-4 varies a lot, so there's no definitive data.

What are you trying to do?

Hi John, I'm trying to compare several loss tangent values (I think that's
the decisive parameter..!?) from several materials, FR4-, PE, etc. over the
frequency. But it's hard to find such diagrams.

 

[MassiveProng]

____________________________________

You are hoping that we believe switchers
cause lots of noise ....

Zero ripple is what switchers do !
The sudden pulse of current is only
around a very short loop , it does
not cause noise .
They dont even have "ground loops"

We cannot use switchers to feed to rails on our 2 to 12 GHz designs.
NOISE CAN AND DOES get injected into such systems BY SWITCHING POWER
SUPPLIES.

YOU may not be aware of it, but those of us that work in such bands
are aware of high frequency switching noise, and it DOES show up under
spectrum analysis.

Your brain has a ground loop.

 

[Grant Edwards]

BTW
Study CAT5e Ethernet cable .
garbage !
USB cables are much faster .


I think ppl limit themselves to
whats avail in PCB , then complain
when it dont work , but if they'd
experiment , they'd find the problem
is using thin PCB .
Then they limit on putting down
100 transmission lines per mm .
You cant learn , unless you experiment.

Shite man, what are you using for a computer, a 40-column
Commodore PET?

[...]

Sending signals that will be amplified
use high Z ( ~65 ohms ) and
sending power needs low Z .
These rules can't be bent .
Thats what you're doin , is bending
rules ...

Even a PET wouldn't explain the random indentation.

 

[John Devereux]

BTW
Study CAT5e Ethernet cable .
garbage !
USB cables are much faster .


I think ppl limit themselves to
whats avail in PCB , then complain
when it dont work , but if they'd
experiment , they'd find the problem
is using thin PCB .
Then they limit on putting down
100 transmission lines per mm .
You cant learn , unless you experiment.

Shite man, what are you using for a computer, a 40-column
Commodore PET?

[...]

Sending signals that will be amplified
use high Z ( ~65 ohms ) and
sending power needs low Z .
These rules can't be bent .
Thats what you're doin , is bending
rules ...

Even a PET wouldn't explain the random indentation.

And the random word and punctuation spacing. Some kind of mobile
phone? The Microsoft Word Usenet Export Filter?

I think it's the usenet equivalent of green ink.

<http://en.wikipedia.org/wiki/Green_ink>

 

[John Larkin]

Hi John, I'm trying to compare several loss tangent values (I think that's
the decisive parameter..!?) from several materials, FR4-, PE, etc. over the
frequency. But it's hard to find such diagrams.

No, what are you trying to *do*? WHY do you want a "coax on a pc
board"?

John

 

[Grant Edwards]

BTW
Study CAT5e Ethernet cable .
garbage !
USB cables are much faster .


I think ppl limit themselves to
whats avail in PCB , then complain
when it dont work , but if they'd
experiment , they'd find the problem
is using thin PCB .
Then they limit on putting down
100 transmission lines per mm .
You cant learn , unless you experiment.

Shite man, what are you using for a computer, a 40-column
Commodore PET?

[...]

Sending signals that will be amplified
use high Z ( ~65 ohms ) and
sending power needs low Z .
These rules can't be bent .
Thats what you're doin , is bending
rules ...

Even a PET wouldn't explain the random indentation.

And the random word and punctuation spacing. Some kind of mobile
phone? The Microsoft Word Usenet Export Filter?

I think it's the usenet equivalent of green ink.

<http://en.wikipedia.org/wiki/Green_ink>

I love it! I'd never heard the phrase "green ink" before. It's
a keeper.

 

[John Larkin]

In coax for 2.5 Ghz , for example ,
it WILL have a large diameter and
the center will have an exact dia and
ratio .. No substitutes .

Never heard of micro hardline, I guess. Or non-TEM propagation modes
in large-diameter coax.

John

 

[John Larkin]

____________________________________

You are hoping that we believe switchers
cause lots of noise ....

Zero ripple is what switchers do !
The sudden pulse of current is only
around a very short loop , it does
not cause noise .
They dont even have "ground loops"

Please post the schematic of a zero-ripple switcher.

John

 

[Geronimo Stempovski]

No, what are you trying to *do*? WHY do you want a "coax on a pc
board"?

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? How is the frequency-dependent attenuation physically
describable? Where does the energy go? Heat, ...? 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?

 

[John Larkin]

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? How is the frequency-dependent attenuation physically
describable? Where does the energy go? Heat, ...? 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?

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

 

[Michael A. Terrell]

Please post the schematic of a zero-ripple switcher.

John

Sorry, but that feature is only available on the 0 volt model.



--
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

 

[Vladimir Vassilevsky]

Sorry, but that feature is only available on the 0 volt model.

Indeed there are the topologies of the switchers with exactly zero or
almost zero ripple, assuming the ideal symmetry of everything.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

 

[CBFalconer]

.... snip ...

Please post the schematic of a zero-ripple switcher.

All it takes is an infinite capacity capacitor. The turn-on time
and inrush current may be high. However it avoids the need for a
UPS. Once they get the breakdown voltage up and get them into
production we can have all the electric cars we want, and dispense
with all batteries.

--
<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

 

[John Larkin]

Indeed there are the topologies of the switchers with exactly zero or
almost zero ripple, assuming the ideal symmetry of everything.

Like a polyphase switcher with *big* inductors?

But I don't want "almost zero ripple", I want the real thing.

John

 

[Vladimir Vassilevsky]

Like a polyphase switcher with *big* inductors?
But I don't want "almost zero ripple", I want the real thing.

Zero ripple is a real thing.
Imagine the two identical bucks operating 50/50 duty with 180 degree
phase shift on the common load. Ideally, there will be no ripple at the
load at all. There are numerous patents on the variations of this idea,
allowing to adjust the duty, different topologies and such.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com