[time-nuts] AC Connector On HP 5061B

[Jeffrey Okamitsu]

A better way to cut coax to the right delay lengths is to use a network analyzer.
 
>From my experience, basing your calculations on the assumed value of the dielectric constant of the cable can lead to significant errors based on the actual quality of the cable.
 
Even so called "high quality" cable can have large variations within a given manufacturing run of cables.
 
Amateur radio operators often cut delay lines for antennas.  Even on HF ( below 30 MHz) I have seen errors up to 10% between calculated length and measured length.
 
Jeff

Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402

--- On Fri, 10/3/08, Hal Murray <hmurray at megapathdsl.net> wrote:

From: Hal Murray <hmurray at megapathdsl.net>
Subject: Re: [time-nuts] AC Connector On HP 5061B
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Date: Friday, October 3, 2008, 1:51 PM

> 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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