[time-nuts] pulse height
Hal Murray
hmurray at megapathdsl.net
Mon Nov 26 21:11:13 UTC 2012
saidjack at aol.com said:
> Nice page, but your first plot with R1 = 0 ohms and no R2 shows too much
> ringing.
It shows what I expect. I thought it was an example of what happens if you
set things up that way, not how to get the best results.
> With proper impedance matching that setup will give the best results,
> meaning the highest voltage pulse, no ringing, the lowest power consumption,
> etc.
"proper impedance matching" isn't a well defined term.
I'm familiar with 2 approaches. The one I think you are referring to is
generally called "series termination". This is Didier's R2=infinity and
R1=line.
You place a resistor between the source of the signal and the transmission
line. That resistance (correcting for internal resistance in the source)
matches the transmission line impedance. At the start of the pulse, the
source resistance and the transmission line impedance make a simple voltage
divider. The pulse on the line will be half the driving voltage. When the
pulse gets to the far end (no termination), it reflects back. That doubles
the voltage. A scope (or chip) at the far end sees a clean transition from 0
to full voltage. When the reflection gets back to the source, the source
rises to full voltage and due to the termination there are no more
reflections.
One advantage of source termination is that it doesn't use any power except
when switching.
One disadvantage is that only the far end sees a clean signal. If you daisy
chain the cable (or PCB trace) past several receivers, the signal at the
non-terminal receivers will have a flat spot in the middle. It will go up
half way, stay there for the time for the signal to get to the end and back,
then go to full voltage. That flat spot is asking for troubles. Noise may
cause multiple clock edges.
The other approach is to drive the line with a low impedance and put a
terminator at the far end. I don't know of a good term for this. This is
what I think of when I hear an unqualified "termination". In a discussion
like this, I would probably call it "far end termination". It's Didiers
R1=0, R2=line. The signal starts at full voltage, goes down the line at full
voltage. The far end sees a clean transition from 0 to full voltage. There
are no reflections.
The voltage actually depends on the internal impedance of the source.
Sometimes you set the source impedance to match the line. That gives half
voltage, but absorbs any reflections due to manufacturing tolerances or
whatever and provides short circuit protection.
This approach eats power but works with multiple receivers. It's common in
lab gear and used with ECL logic.
--
These are my opinions. I hate spam.
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