[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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