[time-nuts] AC Connector On HP 5061B

Hal Murray hmurray at megapathdsl.net
Fri Oct 3 17:51:47 UTC 2008

> On the DA you will want to ensure all your output cables are of
> identical length as this will ensure that all signals are in phase at
> the timebase input(s).   It's a small thing really a fraction of a
> picosecond but it's there and easily compensated for. 

If you are after picoseconds, you have to match cable lengths very carefully.

The rule of thumb is a foot per nanosecond.  Coax is slower: roughly 1/2 for 
junk but faster for good (low loss) coax.

The way I remember the number for fibers (and good coax) is that 1 ft/ns 
would be 5 microseconds per mile.  Fiber is 5 microseconds per kilometer.

All you need to calculate the speed of light is the dielectric constant.  
(I'd have to dig out a book to find the formula.  It's probably k/squt(e))  
Good coax is mostly air which reduces the dielectric constant and speeds up 
light.  Similarly, traces on the outside layers of a PCB (half air if you 
neglect the solder mask) are faster than traces on inside layers.

Good coax uses foam rather than solid dielectric.  It's mostly air which 
reduces the dielectric constant.

These are my opinions, not necessarily my employer's.  I hate spam.

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