[time-nuts] pulse height

[SAIDJACK at aol.com]

Hi Didier,
 
yes, if you put a 50 Ohm termination at the far end all looks good, but you 
 are still driving a 91mA DC current through the cable during the high 
times, and  that will have rippling effects on the driver board by loading the 
5V power  supply down with a 1Hz period.
 
And if you forget to switch on the 50 Ohms end-termination, you get  ~10V 
as shown in your plots, and you might just blow up your expensive  instrument 
:) One reason I don't like fast edges being driven by << 50  Ohms series 
resistance.
 
Also, if you use 50 Ohms series termination, AND 50 Ohms end-termination,  
you still get a 2.5V pulse, enough voltage to cleanly switch most of  
today's logic inputs (either TTL or 3.3V LVCMOS).
 
There are just so many things wrong with the 5 Ohms termination, for  
example what happens if you short that output to ground? What happens if you  
feed a parasitic pulse into the line, say from a nearby lightning  strike or 
EMI or ESD event etc? With "proper" 50 Ohms series  termination the pulse 
should simply be absorbed if it is not  unreasonably high and the resistor is 
large enough. With 5 Ohms, the driver will  likely fly off the PCB..
 
In terms of the incident wave switching issue that Hal mentioned, the  wave 
will stay at about 2.5V for a while, then go to 5V, - again enough voltage  
to switch TTL and LVCMOS logic inputs cleanly. But then again it is never a 
good  idea to daisy-chain inputs via BNC-T's anyway's.
 
bye,
Said
 
 
In a message dated 11/26/2012 13:11:21 Pacific Standard Time,  
shalimr9 at gmail.com writes:

Said,

I agree. I intended to complete the page by doing more  tests, but the 
interesting point of the demonstration is that it is sufficient  to match the 
cable at the far end, and in doing so, you preserve the full  amplitude of the 
pulse. If you put 45 ohms in series and terminate in 50 ohms  at the other 
end, you end up with half the signal. However you do not need to  do that.

That was the reason why I wrote the page in the first place. I  will try to 
clarify that when I get a  chance.

Didier