[SI-LIST] Re: Why we need to use "Series resistor" at Transmitter ?
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 23 Oct 2002 15:32:30 -0700 (PDT)
All of what Jeff says is very true.
One other thing about the series termination technique which has been
discussed on the si-list before, but which may bear repeating for new
members, is that by slightly "undersizing" the series resistor by a few (say
5 or less) ohms such that Zdrv + Rt < Znom of the transmission line you can
induce a small amount of overshoot at the receiver that has the net effect of
reducing the effective rise time of the signal and reducing the delay by a
small amount. You don't want to get carried away and make the series
termination resistor too small or you will induced too much overshoot and
ringing at the receiver.
As an example, assume the output impedance of your driver is 25 ohms and you
are driving a 50 ohm line. A reasonable value for the series resistor might
be 20 or 22 ohms in lieu of the 25 ohms for a perfect match.
Either set up a spice simulation and see what happens with a specific
value resistor or get out the scope and soldering iron in the lab
and try it there.
-Ray Anderson
Sun Microsystems Inc.
>
>Typically, the series resistor is sized such that the transmitter
>impedance PLUS the series resistor match the trace impedance. This
>will be the reason why you are using a 33ohm resistor. If you check
>the imedance of your transmitter, you should find it is about 20ohms
>give or take a few ohms. This is typical in a LVCMOS type
>transmitter, I would believe. I'm sure folks will correct me if I am
>wrong.
>
>In this manner, you create a transmitter with the desired output
>impedance, so there is little or no impedance mismatch between the
>transmitter and the line trace impedance. For placement, it should
>be placed "as close as possible" to the transmitter. As a rule of
>thumb, it should be well within the rise time of the transmitter
>otherwise, you effectively have a transmission line between the
>transmitter and the series resistor which will give you two
>discontinunities rather than eliminating one.
>
>Hope this helps.
>
>Jeff Reynolds
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
