[SI-LIST] Re: Series termination value

Scott I beg to differ. If the load is real , then the best thing to do is to
match the transmission characteristic impedance to the load which will
result in zero reflection for instance if the load is 71 then the
characteristic impedance should be 71. The reflection coeficient is equal to
the difference between the load and characterisitic impedance divided by the
sum of the two, if they are equal then you get zero reflection. But I see
every body making all these assumptions when the question simply indicated a
90 MHz signal. We can complicate the issue by making all these assumptions .
I mean we can start talking about impedance matching networks also .

Justin

-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxx]
Sent: Friday, March 22, 2002 3:20 PM
To: justin.tabatchnick@xxxxxxxxx
Cc: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Series termination value



All,

In looking back at the original inquiry, I believe that it is reasonably 
safe to assume
that the application is one where a digital driver is being used in a 
standard PCB
transmission line signalling environment.  90 MHz refers to the 
frequency of operation
of the system, but says nothing regarding the edge rate, and therefore 
the harmonic
content of the signal being delivered into the board.  If these 
assumptions are correct,
and the edge rate is unknown, one can assume reasonably that it is "damn 
fast" like
most devices these days, and has switching harmonics well into the GHz 
region.  In this
case, any trace longer than a few inches will require some form of 
termination to control
positive and negative overshoot and ringing from a driver with a 20 ohm 
output impedance.

Since the board is targeted at 71 ohms, the appropriate series 
termination resistor would
be about 50 ohms.  For a point-to-point bus, this would be the correct 
termination
value.  For a point-to-many-point bus, with one driver and several 
receivers, the termination
value may have to be reduced do compensate for the negative reflection 
coefficient of multiple
devices with paralled input capacitance.

regards,

scott


-- 
Scott McMorrow
Principal Engineer
SiQual Interconnect Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com


Tabatchnick, Justin wrote:

>I stand corrected , I was thinking 9 MHz instead of 90. Even so, to treat
>the problem in the DC realm , as long as your are talking about cw , is a
>good approximation.
>
>Thanks
>
>Justin
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