[SI-LIST] Re: Source impedance?
- From: "Chuck Hill" <chuckh@xxxxxxxxxxx>
- To: <scott@xxxxxxxxxxxxxxxx>, <doug@xxxxxxxxxx>
- Date: Tue, 8 Jan 2002 18:04:36 -0700
Scott,
I disagree.
Case 1: voltage source (zero impedance) and terminated load
source reflection coefficient mag = 1
load reflection coefficient mag = 0
100% power transfer
0 dB loss
Case 2: matched voltage source (impedance is same as line ) and
terminated
load
source reflection coefficient mag = 0
load reflection coefficient mag = 0
25% power transfer (relative to case 1)
3 dB loss in source, 3 dB loss in load, 6 dB total
1/2 open circuit source voltage appears on the load
Case 1 has a source with infinite available power which isn't realiztic.
Now to the question. I view the field of SI encompassing applications such
as gigabit serial links, RF systems, as well as the TTL and LVDS digital
systems. In the former application areas, source match is beneficial to the
system SI--but only because it reduces standing waves caused by non-ideal
match in connectors and the load. In the traditional digital systems,
source termination is sometimes used at low values to limit ringing, but not
at values high enough to match the line because of the reduction in signal
voltage.
Charles Hill, consultant
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Scott McMorrow
Sent: Tuesday, January 08, 2002 5:15 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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