RE: [SI-LIST] : 50 or 60 ohm impedance

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From: Ron Miller (rmiller@Brocade.COM)
Date: Tue Dec 12 2000 - 15:15:33 PST


> -----Original Message-----
> From: Moran, Brian P
> Sent: Tuesday, December 12, 2000 10:03 AM
> To: 'Sampson, Scot'; Si-Questions (E-mail)
> Subject: RE: [SI-LIST] : 50 or 60 ohm impedance
>
> Scot,
>
> The 50 ohm controlled impedance board was sort of the original impedance
> standard back in the ECL days and it carried over to this day. People have
> generally found, however, that there are advantages to using 60 ohms, in
> terms of speed and power, and it plays better with most CMOS logic.
> Current PCB fabrication technology generally allows either, with the high
> impedance end being squeezed in high layer count designs. If you just use a
> typical a 4/4/4 dual stripline stack up with 4 mil traces, which is pretty
> standard, you get about 56 ohms, give or take. As someone mentioned
> previously, some high speed logic and hybrids do have internal terminations
> at 50 ohms, which can force the issue. What I have generally done has been
> to use 56-60 ohm stack ups and add a little trace width to those signals
> requiring 50 ohms.
        [Ron Miller]

> One problem with using 50 ohms across the board is you
> often lose some routing density, if you stack up requires say 5 mil traces
> to get 50 ohms.
        [Ron Miller]
        This statement is illogical. As compared with 60 ohms the, and for the minimum
        manufacturable width at ~4-5 mils the 50 ohm trace can use a thinner dielectric
        which allows for more dense packaging not less dense.

> The one place where 50 ohms can benefit is that it tends to
> have less impedance variation wth capacitive loading which can help if you
> have irregularily loaded busses or multidrop interconnects. There are other
> pros and cons as far as coupling and very high frequency characteristics,
> but this is the basic argument, at least as far as I am concerned.
        [Ron Miller]
        The big pro for 50 ohms is that it is standard, so semirigid coaxes, adapters,
        and thest equipment are all designed for that impedance.

        Semiconductor manufacturers are going down to 25 ohms and less to get better
        packaging density. NOT UP>

        Ron Miller

> Brian P. MOran
> Signal Integrity Engineering
> Intel Corp.
>
> -----Original Message-----
> From: Sampson, Scot [mailto:ssampson@sonusnet.com]
> Sent: Tuesday, December 12, 2000 7:58 AM
> To: Si-Questions (E-mail)
> Subject: [SI-LIST] : 50 or 60 ohm impedance
>
>
>
> Hello All
>
> What are some of the criteria that is used when selecting 50 or 60 ohms
> for board impedance?
>
> Thanx Scot
>
>
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