[SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs

  • From: icer world <icermail@xxxxxxxxx>
  • To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>, Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Mon, 19 Oct 2009 21:36:34 -0700 (PDT)

1) length matching must be consided  seriously and mismatch should be below 
100mils;
2) generally speaking,the differential pairs impedance must keep 100ohm and 
  the ground return path must not be choped ;
3) the ground guarding trace should keep two times of differential trace width 
away from the differantial pairs,which can not influences the       
differential pairs impedance and avoid crosstalk issues;
4) using serial and parallel matching resistors simultaneously   for debug ;
5) changing layers is not expected,but if necessary ,you should place a groung 
via near the signal via ;
the above is just an advice ,and you'd better do a simulatiom to decide the 
rules of matching resistors and crosstalk;



________________________________
From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
To: Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx>; si-list@xxxxxxxxxxxxx
Cc: chundi srikanth <chundis@xxxxxxxxx>; Lambert Simonovich 
<bertsimonovich@xxxxxxxxxx>
Sent: Mon, October 19, 2009 11:46:36 PM
Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs

Length matching does not need to be tighter than 100 mils.

It is not necessary to back drill vias used to connect component pins to
traces or to change layers.  The 0.6 pF or so capacitance does not
adversely affect the signals at this data rate.

It is not necessary to put "ground" vias next to routing vias as long as
the PDS is well designed."

"Guard"  traces have no value.  Proper spacing to avoid crosstalk does. 
The "rule of thumb" for spacing given, while crude, is not far off.

Lee Ritchey


> [Original Message]
> From: Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Cc: chundi srikanth <chundis@xxxxxxxxx>; Lambert Simonovich
<bertsimonovich@xxxxxxxxxx>
> Date: 10/18/2009 5:45:08 PM
> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
>
> Srikanth,
>
> Additional to Bert's comments:-
>
> 1) Match the lengths of the pairs exactly
> 2) Use a solid ground return
> 3) Avoid stubs by design or by using stub-drilling
> 4) If you have vias in the path use a small drill and a large anti-pad and
> place a ground via next to each signal via
>
> Regards
>
> Paul
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of Lambert Simonovich
> Sent: 16 October 2009 21:57
> To: 'chundi srikanth'; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
>
>
> Srikanth,
>
> A rough rule of thumb is greater than 3 times trace width separation
between
> pairs. Using a 2D field solver will ultimately give you the crosstalk
> coupling factor for the exact geometry in your stackup, and dictate the
> routing rules you need to follow to satisfy your noise budget. 
>
> Adding GND guarding will more often than not present more issues than it
> solves. You should stitch these guard traces at regular intervals
> approximately 1/10 of the wavelength of the highest frequency component of
> the aggressor signal to avoid it from resonating and coupling noise back
> onto other adjacent traces. This further reduces routing density of the
> board. By the time you factor in the additional space of one trace width
> between the guard trace and Diff pair, plus the additional via stitching,
> you will find you are already at 3 times separation between pairs and you
> would gain back more real estate.
>
> Regards,
>
> Bert Simonovich
>
>
>
>    
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of chundi srikanth
> Sent: October-16-09 12:40 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Routing guidelines for 3.12Gbps LVDS pairs
>
> Hi Techies,
> We have a 12-layer board in which we have lot of differential LVDS pairs
> operating at several hundreds MHz. And we have SERDES signals
(differential
> TX & RX pairs) operating at CPRI rate-4 i.e., 3Gbps. So can you just share
> me some inputs on exactly what are the guidelines to be followed  while
> routing these signals. And is GND gaurding between the differential pairs
> improve the SI?Please share or refer me any documents in which i can get
> Good information on High-Speed design guidelines.
>
> Thanks
>
> With Best Regards
> Srikanth
>
>
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