[time-nuts] Data collection cycle -- multiple sources to one counter?

Brooke Clarke brooke at pacific.net
Sun Jun 19 13:35:16 EDT 2005


Hi David:

The problem was that you must move the cables some in order to do the 
calibration.  The semi-rigid coax would spring back to the position 
where is was and gave good results, but conventional coax that's 
flexible would never go back to where it was.  The newer crinkled 
surface 141 cable was both flexible and repeatable without the need for 
mandrel bending.

Regular coax if not allowed to move would only have temperature variations.

Have Fun,

Brooke

David Kirkby wrote:

> Brooke Clarke wrote:
> 
>> When working with the HP 4380S Balanced Network Analyzer system I 
>> found that using flexible cables resulted in an unstable system.  My 
>> first improvement was to use 0.141" semi rigid coax (it's much more on 
>> the rigid side) and got a mandrel to bend it.  Later I discovered you 
>> can get flexible 0.141" coax that's not "rigid" and it worked great, 
>> no mandrel needed.  The point is that you probably will need to have 
>> good control on all the coax lines.  The lines will change length with 
>> temperature, so the longer they are the more temperature variation 
>> there will be.
> 
> 
> If its not moved, is semi-rigid any more stable than other coaxes, if 
> its not moved?
> 
> I don't know all the mechanisms for delays in coax, but the temperature 
> coefficient of the dielectric constant of the dielectric must be one of 
> them, and quite possibly the most significant. I would have thought that 
> dominated any change due to the expansion of the metal. This would 
> suggest you need to find the optimal dielectric.
> 
> I've got a bit of cable here with what is basically an air dielectric, 
> with the inner conductor supported on a spiral of some sort, which means 
> 99% of the dielectric is air. However, that cable is not practical at 
> small sizes.
> 
> PTFE might for example be a lot better/worst than polyethylene in the 
> variation of dielectric constant with temperature.
> 
> I'm not sure that RG405 semi-rigid (which is what I guess you mean) is 
> necessarily the best choice.
> 
> I used semi rigid a lot, but prefer the bigger one, since its a lot 
> easier to solder, as my eyses can see it!!
> 

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