Reply by glen herrmannsfeldt●February 26, 20072007-02-26
Geronimo Stempovski wrote:
> 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
Reply by ●February 18, 20072007-02-18
On Sat, 17 Feb 2007 07:37:06 -0500, CBFalconer <cbfalconer@yahoo.com>
Gave us:
>MassiveProng wrote:
>> Vladimir Vassilevsky <antispam_bogus@hotmail.com> Gave us:
>>
>>> Please find a quote or go take your pills.
>>
>> I got a pill for your ass, fuckhead.
>
>This sort of foolish language will sooner or later get you PLONKed
Like I give a fat flying fuck if you read my posts.
Reply by John Larkin●February 17, 20072007-02-17
On Sat, 17 Feb 2007 10:17:33 -0800, MassiveProng
<MassiveProng@thebarattheendoftheuniverse.org> wrote:
>On Sat, 17 Feb 2007 08:17:07 -0800, John Larkin
><jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:
>
>>On Fri, 16 Feb 2007 19:52:36 -0800, MassiveProng
>><MassiveProng@thebarattheendoftheuniverse.org> wrote:
>>
>>>On Sat, 17 Feb 2007 02:10:34 GMT, Vladimir Vassilevsky
>>><antispam_bogus@hotmail.com> Gave us:
>>>
>>>>Please find a quote or go take your pills.
>>>
>>> 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.
>
> 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
Reply by ●February 17, 20072007-02-17
On Sat, 17 Feb 2007 08:17:07 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:
>On Fri, 16 Feb 2007 19:52:36 -0800, MassiveProng
><MassiveProng@thebarattheendoftheuniverse.org> wrote:
>
>>On Sat, 17 Feb 2007 02:10:34 GMT, Vladimir Vassilevsky
>><antispam_bogus@hotmail.com> Gave us:
>>
>>>Please find a quote or go take your pills.
>>
>> 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.
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.
Reply by John Larkin●February 17, 20072007-02-17
On Fri, 16 Feb 2007 10:14:41 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>On Fri, 16 Feb 2007 18:58:54 +0100, "Geronimo Stempovski"
><geronimo.stempovski@arcor.de> wrote:
>
>>
>>"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> schrieb im
>>Newsbeitrag >
>>> 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?
>>
>
>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
Reply by John Larkin●February 17, 20072007-02-17
On Fri, 16 Feb 2007 19:52:36 -0800, MassiveProng
<MassiveProng@thebarattheendoftheuniverse.org> wrote:
>On Sat, 17 Feb 2007 02:10:34 GMT, Vladimir Vassilevsky
><antispam_bogus@hotmail.com> Gave us:
>
>>Please find a quote or go take your pills.
>
> 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
Reply by CBFalconer●February 17, 20072007-02-17
MassiveProng wrote:
> Vladimir Vassilevsky <antispam_bogus@hotmail.com> Gave us:
>
>> Please find a quote or go take your pills.
>
> I got a pill for your ass, fuckhead.
Reply by Michael A. Terrell●February 17, 20072007-02-17
MassiveProng wrote:
>
> On Fri, 16 Feb 2007 18:17:02 GMT, "Michael A. Terrell"
> <mike.terrell@earthlink.net> Gave us:
>
> >John Larkin wrote:
> >>
> >> Please post the schematic of a zero-ripple switcher.
> >>
> >> John
> >
> >
> > Sorry, but that feature is only available on the 0 volt model.
>
> 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
Reply by ●February 17, 20072007-02-17
On Fri, 16 Feb 2007 20:21:34 -0600, Vladimir Vassilevsky
<antispam_bogus@hotmail.com> Gave us:
>
>
>MassiveProng wrote:
>
>>>>We also made an xray supply that made 4kV at 1.5uA, and that had only
>>>>2mV of ripple at rated loading.
>>>
>>>1.5uA is not much.
>>
>> You don't know much about X-ray flux, or the generation thereof.
>
>Sorry, dude. At one time I was working with the X-ray crystallograph.
>The tube was typically operating at 40...50kV, draining the current
>about 10..15mA. So here we go.
And THAT supply ALSO had a pretty low ripple figure, I'll guarantee
you.
The whole idea behind clean DC for the X-ray tube is that the X-ray
flux is clean and pure. THAT yields the best contrast ratio in the
imagery.
The supply I mentioned feeds a small device meant for looking at gas
pipes in city streets.
I also made a supply that was 50kV at 250Watts That's only 5mA.
So your supplies were leaning toward a kW, eh? That must have been
some X-ray flux.
Still, you should have known that the ripple figure I gave for the
CAT supply was exceedingly low. By ALL standards.
Reply by ●February 16, 20072007-02-16
On Sat, 17 Feb 2007 02:10:34 GMT, Vladimir Vassilevsky
<antispam_bogus@hotmail.com> Gave us: