[SI-LIST] Re: RF Layout - Via spacing
- From: arvind yadav <arvind.yad1983@xxxxxxxxx>
- To: wolfgang.maichen@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 22 Oct 2009 22:42:51 +0530
Hello Wolfgang ,
Thanks for the reply .
In my case i dont have a signal via . What i have is a RF signal in top
layer and around that two strips of ground traces .
In that GND track we have placed vias and some guidelines says to
maintain lambda/20
rule between the two same gnd vias .
Can you please explain this .
Thanks
Arvind.H
On Thu, Oct 22, 2009 at 10:34 PM, <wolfgang.maichen@xxxxxxxxxxxx> wrote:
>
> Hello Arvind,
>
> the goal in designing a clean (reflection-free) signal path is to have
> homogeneous characteristic impedance all along the path (typically ZoP Ohm
> unless you are working with TV signals that use 75 Ohms).
>
> The characteristic impedance is determined by the ratio of inductance Lu
> per unit length and capacitance Cu per unit length:
>
> Zo=sqrt(Lu/Cu)
>
> A signal via and its closest return via (or vias) are just part of that
> path. Changing the distance d between signal and return via changes both
> capacitance C and inductance of that via structure (C decreases with d, and
> L increases with D), so you can use that to tune the impedance of the via
> structure. Ideally you'll achieve 50 Ohms although this is hard to do with
> just a single return via. In that ideal case (ZoP Ohms) the via structure
> becomes completely transparent to the signal, i.e. it only causes delay
> (delta_t = sqrt(C x L)) but no reflections.
>
> Designing a well-matched via structure is a challenge and typically need
> either a good 3D simulation tool or a few test boards to get it right at
> high data rates. Rules of thumb ar hardly sufficient although they can
> provide at least a goot starting point as well as show the "knobs" you can
> use to adjust the impedance (for via structure, there a are many knobs - via
> diameter and distance, stub or stub drilling, pad/antipad diamaters, and so
> on).
>
> The lambda/20 rule you mention comes from the fact that typically
> structures that are very short against the shorted wavelength (highest
> frequency) of interest only have negligible influence on the waveform, i.e.
> produce only minimal reflections even when they are mismatched (Zo <> 50
> Ohms). This is of course just a crude rule of thumb.
>
> Whatthe lambda/20 rule achieves very nicely is that it forces you to place
> a return via close to every signal via. This is important - current is
> always flowing in a loop so if there is no return via close by, the return
> current has to "go looking" for the nearest return path which may be quite a
> detour - this will cause a large parasitic inductance in the path (because
> the current now encloses a large loop are) and resulting large reflection
> and reduced bandwidth.
>
> Wolfgang
>
>
>
>
>
>
> *arvind yadav <arvind.yad1983@xxxxxxxxx>*
> Sent by: si-list-bounce@xxxxxxxxxxxxx
>
> 10/22/2009 09:45 AM
> To
> si-list@xxxxxxxxxxxxx cc
> Subject
> [SI-LIST] RF Layout - Via spacing
>
>
>
>
> Hello All,
> I am working on a RF Layout. I looked into some design guidelines and had
> some doubt on gnd via spacing requirement .
>
> Guideline said that ë/20 distance has to be maintained between gnd vias
> that
> are stitched on either side of the RF signal
>
> Can any one please let me know the reason for this requirement ?
>
> I also would like to know what would be the gnd backoff distance from the
> RF signal and the reason .
>
> Thanks
>
> Arvind.H
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
>
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
