[time-nuts] An Excellent Tutorial on Precision Clocks

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Mar 15 14:03:04 EDT 2007

In a message dated 3/14/2007 23:20:54 Pacific Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

Matt  Ettus wrote:
> I was just investigating this very issue.  It seems  that every program
> you use to compute these impedances comes up with  different answers,
> sometimes wildly different.  Anyone have a  free program they trust
> with this sort of thing?
>  Matt

Hello Matt,
try Agilents excellent freeware AppCad, available at:
_http://www.hp.woodshot.com/_ (http://www.hp.woodshot.com/) 
It can do striplines, microstrip, coplanar waveguides etc. and a lot  more.

Unfortunately it doesn't do differential lines.
The results match the PCB's quite well if you have a good PCB  factory.
Keep in mind that you will not know the exact Dielectric constant of the  PCB 
you are getting (changes with every PCB) and consistency highly depends on  
factors such as how well the factory cleans the layers before pressing, how 
much  solvent is left on the PCB, how many impurities (air, solvents, etc) are 
left in  the layers, how well the material reacts to the soldering  trauma, etc.
Then again there is very little change in actual signal quality for say a 5  
- 10% change in transmission line impedance. Howard Johnson has a good  
discussion about that, as well as how impurities in the PCB affect loss tangent  and 
dielectric constant.

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