[time-nuts] An Excellent Tutorial on Precision Clocks
SAIDJACK at aol.com
SAIDJACK at aol.com
Thu Mar 15 18:03:04 UTC 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.
bye,
Said
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