[SI-LIST] Re: Why 50 ohms
- From: "Perry Qu" <perry.qu@xxxxxxxxxxx>
- To: Andrew.Ingraham@xxxxxxxxxx
- Date: Tue, 22 Jan 2002 10:39:02 -0500
Andrew:
I agree with your comment about choosing impedance based on driver,
receiver, termination, etc., not necessarily 50 ohm. However, in practice,
it is difficult to do so. For today's PCB with very high density routing, if
we have to use different impedance for different nets, it will be very
difficult to implement on a PCB, e.g., on certain routing layer, use a few
impedance control options. For layout guys, this will be a headache and not
easy for manufacturing also. Thus I think on PCB, 50 ohm impedance is used
because of ease of implementation, sacraficing a little bit on signal
integrity for some cases.
Regards
Perry
"Ingraham, Andrew" wrote:
> > While discussing with my friend, he told me that the PCB trace
> > impedance should be maintained as 50 Ohms (for differential=3D
> > 100 and for some RDRAM buses 27ohms).
>
> Why 50 ohms? There are many parts to this question.
>
> Not really related to what you are asking, is why does so much test
> equipment use 50 ohms, and why are so many coax cables 50 ohms. Some of
> the previous replies answer this. Another reference that I have pointed
> people to for years (decades, really), is in the Trompeter catalog. A
> shortened version of their description is on the first page of
> http://www.trompeter.com/assets/product/PDF/wiring.pdf
>
> I believe that 50 ohm cables came first (to match antennas), and 50 ohm
> test equipment followed.
>
> Now, why does so much PCB wiring also use 50 ohms? A lot of reasons.
>
> When it comes to PC boards, the short answer is that there is no magic
> to 50 ohms. Unless you have a standardized signal like Ethernet or
> SCSI, you DON'T necessarily need to have 50 (or 100 or 27) ohm traces.
> But you DO often need to know what your trace impedances are. Depending
> on risetimes and trace lengths, some terminations may be required, and
> then you should know what are your driver impedances, load impedances,
> and board trace impedances, and what their mismatch does to your
> signals. It may be that the drivers were optimized for driving 50 (or
> 70 or whatever) ohm traces, and then you ought to target that value
> (whatever it is), when you make your PC boards.
>
> If you're doing, say, Ethernet, then you should maintain its cable
> impedance (50 ohms S.E. or 100 differential) on your PC boards, for
> those signals connected to the cable.
>
> And there is some PCB "history." ECL is an older but very fast logic
> family. It often requires terminated lines, even on moderately short
> traces. Many ECL output drivers were designed to drive 50 ohm
> terminated loads, which also happens to be ideal for driving, or being
> driven by, standard test equipment. The need to interface with test
> equipment continues to this day, and can be one reason for sticking with
> 50 ohm traces.
>
> If you ignore all the history, had no standards to adhere to, and no
> test equipment to interface with, you could choose your own impedance.
> You would probably end up with in the vicinity of 30-100 ohms anyway,
> because that's what you get with typical trace widths and materials on
> multi-layer boards.
>
> Designing everything for lower impedances (50 ohms or lower) requires
> somewhat more switching power, bigger transistors, but is better in the
> face of parasitic capacitances. There may be some advantage too in
> electrical crosstalk immunity. On the downside, lower impedances might
> be worse for magnetic crosstalk, and more di/dt noise.
>
> Regards,
> Andy
>
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--
Perry Qu
Product Integrity | 600 March Road
Alcatel Canada | Ottawa, ON K2K 2E6, Canada
DID: (613) 7846720 | FAX: (613) 5993642
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