[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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