On Feb 12, 10:37 pm, "werty" <w...@swissinfo.org> wrote:> 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 !Ah, yes, reality. The reality is that we do need the vias for our 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
Building Coaxial transmission line on PCB?
Started by ●February 12, 2007
Reply by ●February 13, 20072007-02-13
Reply by ●February 13, 20072007-02-13
werty wrote:> 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. >> > ---------------------------------------------------------- > > 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" .<snip> Seems video distribution is not one of your areas of expertise ? Note that John was talking about crosstalk, in the analog domain, and yes, what he did certainly will have a measurable improvement, and is somewhat 'industry practise' when minimising crosstalk. Note he says 'cured', that means he is on both sides of the problem, and it is a brave (or something else?) person that counters such direct experience I imagine in extreme digital cases, such as where you are worried not only about sending the clock, but also about the ps/fs of jitter, then this design approach would also help : it's not hard to do. -jg
Reply by ●February 13, 20072007-02-13
On Tue, 13 Feb 2007 04:55:03 GMT, Robert Baer <robertbaer@earthlink.net> wrote:>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"...--- No, he said: ...:"as much like a coaxial cable as possible.", which I took to mean electrically. Otherwise, why not just use a piece of micro-coax and treat it like just another component on the PCB? -- JF
Reply by ●February 13, 20072007-02-13
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?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
Reply by ●February 13, 20072007-02-13
On Mon, 12 Feb 2007 22:30:39 +0100, john jardine <john@jjdesigns.fsnet.co.uk> wrote:> 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
Reply by ●February 13, 20072007-02-13
On 12 Feb 2007 22:37:14 -0800, "werty" <werty@swissinfo.org> wrote:> 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 ! >When you need, say, 40x the crunch power of a decent DSP processor, those relics come in handy. John
Reply by ●February 13, 20072007-02-13
On Tue, 13 Feb 2007 20:46:04 +1300, Jim Granville <no.spam@designtools.maps.co.nz> wrote:>werty wrote: >> 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. >>> >> ---------------------------------------------------------- >> >> 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" . ><snip> > >Seems video distribution is not one of your areas of expertise ? > > Note that John was talking about crosstalk, in the analog domain, >and yes, what he did certainly will have a measurable improvement, and >is somewhat 'industry practise' when minimising crosstalk. > Note he says 'cured', that means he is on both sides of the problem, >and it is a brave (or something else?) person that counters such direct >experience > >I imagine in extreme digital cases, such as where you are worried not >only about sending the clock, but also about the ps/fs of jitter, then >this design approach would also help : it's not hard to do. > >-jg >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
Reply by ●February 13, 20072007-02-13
Thanks for your help, so far. Really appreciate it. I'm already a step further in my considerations. Now I'm looking for a diagram like frequency (some MHz to 10 GHz for example) versus loss tangent and / or epsilon R for FR4. 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 hard to find a graph... Does anybody know where I can find that?
Reply by ●February 13, 20072007-02-13
"Uwe Hercksen" <hercksen@mew.uni-erlangen.de> wrote in message news:op.tnournbqni00lm@hercksen3...> On Mon, 12 Feb 2007 22:30:39 +0100, john jardine > <john@jjdesigns.fsnet.co.uk> wrote: > > > 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? > > ByeWould have been ideal. At the time was thinking about a method to do this and sorted a setup that may have been worth talking to the PCB people about but a customer was paying to clear down his urgent problem and not to start up a research project :). No ... I don't remember what I figured out. Ideas are easy, it's the implementation that's a problem :). No doubt it'll surface again if I'm I'm under pressure. john -- Posted via a free Usenet account from http://www.teranews.com
Reply by ●February 13, 20072007-02-13
Uwe Hercksen wrote:> On Mon, 12 Feb 2007 22:30:39 +0100, john jardine > <john@jjdesigns.fsnet.co.uk> wrote: > >> 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?To do that would need plated slots, and slots have the PCB fabs luke-warm at best : hard to do cleanly, they also weaken the PCB if long, and also have minimum router sizes, plus machine time...... A better direction would be thinner laminates, and using the space you would have lost to the slot anyway, as wider GND webs on the same plane, coupled with stitching vias (which can be smaller dia than slots) -jg
