[SI-LIST] : Source termination of transmission line

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From: David Instone (david_instone@uk.xyratex.com)
Date: Mon Apr 23 2001 - 02:34:46 PDT


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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