[SI-LIST] Re: PCB tracks
- From: "LATOURRETTE,JEFF (A-SanJose,ex1)" <jeff_latourrette@xxxxxxxxxxx>
- To: "'johnlipsius@xxxxxxxxx'" <johnlipsius@xxxxxxxxx>,SI-LIST <si-list@xxxxxxxxxxxxx>
- Date: Thu, 8 Aug 2002 11:09:41 -0600
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
//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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