RE: [SI-LIST] : Source termination of transmission line

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From: Degerstrom, Michael J. (degerstrom.michael@mayo.edu)
Date: Mon Apr 23 2001 - 11:47:11 PDT


Dave,

You will have to bite the bullet somewhere since you can't
do anything about the 40 ohm connector. Yes you can use
a 75 ohm path from your driver to the connector but then
you have one big reflection at that junction.

If your driver can tolerate the lower impedance and
your receiver can live with the lower voltage swing
then you can use 40 ohm paths and a 40 ohm far end termination.

Another way to go about this is to use no far end
termination and provide a shunt termination of 80
ohms at the far end. This should give you much more
swing and the source impedance in parallel with 80 ohms
is 40 ohms. So use 40 ohm paths and you may have
a workable solution.

Not sure why you don't want to do impedance matching
at the connector but I guess my second approach
is doing the same thing only at the driver.

Mike

_______________________________________________________________
Mike Degerstrom Email: degerstrom.michael@mayo.edu
Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
Phone: (507) 538-5462 FAX: (507) 284-9171
WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
_______________________________________________________________

> -----Original Message-----
> From: david_instone@uk.xyratex.com
> [mailto:david_instone@uk.xyratex.com]
> Sent: Monday, April 23, 2001 4:35 AM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Source termination of transmission line
>
>
> All,
> Source (series) termination of transmission lines to absorb
> reflections is well known. However, how long does the
> transmission line
> have to be to be recognised ,by the signal, as a transmission line?
> Say, for example, I have a driver with an integrated series
> terminating resistor to give a total driver impedance of 75
> ohms, and I
> want to drive a 50 ohm line via a connector with an impedance
> of 40 ohms
> without using a matching pad or other form of impedance matching.
> If I terminate the the far end of the 50 ohms line with 50
> ohms then I
> will get no reflection from there. However, I will get a reflection
> back to the driver from that 40 ohm connector to 50 ohm line
> interface.
> Assuming I can't put the driver hard on the connector, the
> driver source
> terminates at 75 ohms so if I have a section of 75 ohm line
> between the
> driver and the connector the source termination should absorb those
> reflections.
> But how long does that 75 ohm section have to be?
> Intuitively I feel
> it should be greater than the launched signals risetime, is this
> correct?
>
>
> Regards
>
> Dave Instone. Compliance Engineer
> Storage Systems Development, MP24/22
> Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
> Tel: +44 (0)23-92-496862 (direct line)
> Fax: +44 (0)23-92-496014
> http://www.xyratex.com Tel: +44 (0)23-92-496000
>
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