[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

  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
  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


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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