[SI-LIST] Re: RF Layout - Via spacing
- From: "Brad Brim" <bradb@xxxxxxxxxxx>
- To: "'arvind yadav'" <arvind.yad1983@xxxxxxxxx>, <wolfgang.maichen@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 22 Oct 2009 10:41:45 -0700
hi Arvind,
We too often forget there is *NO* such thing as "ground traces" or a "GND
track". Rather you have two traces that just happen to have a via to ground
placed periodically along their length. This view of your geometry enables a
more intuitive argument for via spacing. If you allow the spacing to become
a half wavelength then you will see a very large resonance so you know it
should be much less than 1/2 wavelength. You need to make the via spacing
"close", which is a subjective term. You could treat these traces as signal
traces (e.g. microstrip coupled lines) and do a circuit or EM simulation to
see the effect of different via spacing. The classical microwave
rule-of-thumb is to use 1/10 to 1/20 wavelength for such via spacing.
In some microwave applications you are trying to emulate a solid metal wall
with this "fence" of vias. In that case the more tightly you space them the
less leakage you will see through the fence. In that case, 1/20 wavelength
is going to yield a better shield than 1/10. However, keep in mind that even
if you space them 1/100 wavelength you will still have energy leak over the
top of these periodically grounded shield traces - i.e. you can beat a dead
horse.
Rules of thumb exist for lateral spacing of the so-called ground traces and
are typically 3X to 5X the maximum of substrate height or line width. These
are based on how much the impedance of the line would be perturbed if you
put a metal wall at the same location as the ground trace. 3X is okay if you
can handle a few ohms difference, 5X probably won't perturb much at all.
cheers,
-Brad
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of arvind yadav
> Sent: Thursday, October 22, 2009 10:13 AM
> To: wolfgang.maichen@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: RF Layout - Via spacing
>
> 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
> >
> >
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