[time-nuts] Why are 1PPS signals so skinny?

SAIDJACK at aol.com SAIDJACK at aol.com
Tue May 15 22:24:34 UTC 2012


Yes, you are right of course. My bad. This should have been written  as:
 
"The Thunderbolt has less than 5 Ohms output impedance, so if you get  a 
reflection coming from the cable stubs or non-end-terminated cable  back into 
the Thunderbolt, then you get ringing on the cable because the  impedance is 
mismatched!"
 
On a properly series terminated device, any reflections on the open-ended  
cable coming back to the source will end in the sources' 50 Ohms  
terminator, and be removed. One more advantage I didn't mention for series  
termination versus the Thunderbolt used with end-termination.



In a message dated 5/15/2012 15:03:12 Pacific Daylight Time,  
hmurray at megapathdsl.net writes:


SAIDJACK at aol.com said:
> Also, the Thunderbolt has less  than 5 Ohms output impedance, so you  get 
a
> reflection going back  from the 50 Ohms end-termination anyway because the
> impedance is  mismatched! 

I think that's a different problem.

If the far end  termination matches the cable there won't be any reflection.

If the far  end isn't terminated correctly, there will be reflections from 
the 
far  end.  There may also be reflections from joints in cables or a Tee and 
 
input load if you are daisy chaining multiple instruments.  When  those 
reflections get back to the typical low impedance driver, they will  get 
reflected back again.

It's not uncommon to use both  source/series and end/parallel terminations. 
 The series terminator drops  the signal level by 2 but minimizes 
reflections if you are working in a less  than ideal setup.  It also provides a 
current limit on the driver in case  something gets shorted.




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