FreeLists / si-list / [SI-LIST] Re: PCB tracks

["LATOURRETTE,JEFF (A-SanJose,ex1)" <jeff_latourrette@xxxxxxxxxxx>]

John, Philippe:

Another reason for 45 degree angle or curved traces instead of 90 degree bends 
is to reduce what ends up being excess capacitance due to an abrupt bend.  This 
capacitance occurs as the fields change from a nice even distribution on 
straight microstrip to very concentrated on the inside and less concentrated on 
the outside of the bend.  This problem is more pronounced with wider, lower 
(like 30 to 50-ohm) impedance lines, becoming much less significant at 75-ohms 
or 100 ohms.  

If you are stuck doing 90 degrees and working at high frequencies, you can 
miter (remove metal) at the bend to reduce  excess capacitance in your 
high-speed signal traces.  You can also think of it as slightly raising the 
impedance of the line at the bend, which when done correctly can significantly 
improve return loss.  This is probably even harder for PCB houses than 45 
angles, so be sure your application really needs it.

As a quick check you can take that corner, mitered off, calculate its 
capacitance and then reactance at your frequency of interest.  For most 
frequencies below 1 GHz, this becomes insignificant and experiments can show 
that mitering does little good at very low frequencies.  This technique is used 
on most microwave designs where they are ringing out every last dB of Return 
Loss performance from a design.  A lot of digital systems & components don't 
even specify VSWR/Return Loss and may be able to tolerate more mismatch.

From an MTT article on how to optimally miter a microwave bend, mitering can 
improve performance at frequencies as low as 1 GHz, so your comment on not 
needing to worry at 200 MHz is probably right on.  It just depends what kind of 
mismatches (VSWR) your system can handle, and how many harmonics above your 
signaling rate you are designing to.

I think I posted this reference here once before, but for more: 
 
"Experimental Study of Symmetric Microstrip Bends and Their Compensation"  
Douville & James, IEEE Transactions on Microwave Theory & Techniques, Vol. 
MTT-26. No. 3, March 1978, pp. 175-181  

This article, due to its age, only covers out to 3 GHz, but the principles in 
it are applied successfully up into 10's of GHz, although if you're up in that 
range, you can easily verify the effect on your field solver.

I agree with John that the amount of energy launched into evanescent modes is 
probably a 3rd order effect and not worth worrying about.

I'm not sure, however, I agree that you can ignore this at 10 Gb/s.  Since a 10 
Gb/s signal carries with it some very high frequencies and seems to start to 
tax the limits of some PCB materials, cables and connectors, I'd think you'd 
want the best possible Return Loss in your signal traces.

Now you have $0.03,

Jeff LaT.

-----Original Message-----
From: john lipsius [mailto:johnlipsius@xxxxxxxxx]
Sent: Wednesday, August 07, 2002 4:48 AM
To: SI-LIST
Subject: [SI-LIST] Re: PCB tracks



Philippe,

For a detailed answer, you should find it in a textbook.  
There's been much prior discussion on this; see the si-list archives 
http://www.freelists.org/archives/si-list/ . 
Or see perhaps  www.signalintegrity.com or www.gigatest.com .

I assume your problem area is current digital technology.

1. For digital boards, even dealing with 10Gbps, 90 deg. is fine.  
There's just not a significant effect requiring even 45 deg., much less 
curved traces.  

2. That said, why is 45 is "better" than 90 deg., and curvilinear is best?
All for the same reason:  with sudden geometry changes, the TEM 
(transverse electromagnetic) wave sets up "evanescent modes" other 
than the main one to compensate for the change.  Why?  Remember, 
the EM field is a solution to partial differential equations with both 
space and time as variables.  Therefore, sudden spatial boundary changes 
force different boundary conditions, forcing different local micro field 
changes, thus local impedance changes.  The locus of these changes is 
so much less than the wavelengths of importance that the signal 
integrity effect is unimportant.  The energy loss in these evanescent modes 
is, likewise, minimal in the overall noise budget.  This assumes you have a
limited number of trace bends. 

That's my $.02.  Due to inflation and the passing of time, it may now be 
$.015 or less. 

---


----- Original Message ----- 
From: "Philippe Robert" <PhilippeR@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, August 07, 2002 2:22 AM
Subject: [SI-LIST] PCB tracks


> Hi there,
> 
> I am currently in the process of doing a new PCB, at the routing stage. In
> the router that I am using, there is an option to have 90-degree or
> 45-degree angles on the tracks. I know that it is better to have the option
> set to 45 to avoid changing too much the impedance of the tracks. I designed
> few board using that option.
> 
> In the router, there is an other option to have curves instead of angles on
> the tracks. Theoretically, it should be better in terms of impedance as the
> width of the track vary even less.
> 
> Could someone tell me if curves are better than 45-degree angles and why ?
> 
> Best regards.
> Philippe.
> 
> 
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