[SI-LIST] Re: Source impedance?
- From: Witold Teller <Witold_Teller@xxxxxxxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 8 Jan 2002 16:49:39 -0800
Hi,
Lets assume transmission line Zo has perfect far end termination Zload,
Zo = Zload, also, DC resistance of that line is 0 Ohm.
Then ...
signal amplitude at the beginning and at the end of the transmission
line is the same!
Amplitude depends on internal impedance of the source, Zsource.
If Zsource = Zo = Zload, then amplitude at the beginning and at the end
of the line is 50 % of "unterminated" source.
If Zsource = 0 Ohm, then amplitude swing is 100 % across whole transmission
line.
If Zsource is between 0 Ohm and Zo then amplitude decreases.
I am not sure where from 1/4 swing is coming, in Scott's explanation?
Regards,
WT
-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxxxxx]
Sent: Tuesday, January 08, 2002 4:15 PM
To: doug@xxxxxxxxxx
Cc: david.kaiser@xxxxxxxxxx; 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Source impedance?
david and doug,
far end termination decreases the signal amplitude by 50%.
near end termination decreasea the AC signal amplitude by 50%.
this amounts to a 12dB AC attenuation of the signal or 25% of the
source voltage being transmitted to the receiver.
At AC, the maximum voltage swing is 1/4 of the source voltage.
At DC, the maximum voltage swing is 1/2 of the source voltage.
as long as the signaling can withstand this sort of AC and DC amplitude
reduction, then impedance matching on both ends is okay. Otherwise,
the reduced voltage swing is usually unacceptable.
regards,
scott
--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR 97062-3090
(503) 885-1231
http://www.siqual.com
Doug Brooks wrote:
> My opinion is that you are correct. Matching the impedance at the front end
> would be important, of course, if you wanted to maximize the transfer of
> power to the load. But if power is not an issue, and if crosstalk is not an
> issue, and assuming zero trace lengths within the source, then source
> impedance is not a factor.
>
> Doug Brooks
>
> At 03:38 PM 1/8/2002 -0700, David Kaiser wrote:
>
> >Theory question. Assume perfect world with no parasitics.
> >
> >If a 50 Ohm trace on a board is terminated with a 50 Ohm resistor at the end
> >of the trace, is there any advantage in terms of SI
> >to driving the trace with a source that has 50 Ohm output impedance compared
> >to a source with 10 or 20 Ohms of output impedance?
> >
> >It seems to me that the only thing the series source impedance would do to a
> >50 Ohm line that is terminated with a 50 Ohm resistive load would be to
> >divide the amplitude in half without improving the signal integrity.
> >Correct?
> >
> >David
> >
>
> _____________________________________________________________________
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