[Elecraft] BL2 choking Impedence measurements?

[Jim Brown]

On Sat, 28 Aug 2010 22:41:47 +0100, David Cutter wrote:

>This brings me nicely to the final bit of the puzzle:  say, in a given 
>location, you want to set up your own personal outdoor lab to make these 
>measurements comparing various solutions.  How do you create a constant 
>noise source *which will most effectively demonstrate* the performance of 
>one choke against another?

The choking impedance measurement can give you a first approximation of 
the parallel equivalent circuit, certainly sufficient for this purpose. 
You add that choke (or any other choke of known characteristics) to a 
circuit and observe the noise reduction (if any). You take that reduction, 
in dB, and work backwards to compute the initial series impedance of the 
common mode circuit. If the choking Z is much higher (5:1) than the 
impedance of the common mode circuit, reactance doesn't cause much error. 
Now, knowing the common mode impedance of that particular circuit, you can 
predict what more or less choking Z will do. 

THAT'S why I've always placed so much emphasis on determining the 
equivalent circuit values for the choke. And THAT'S why it was learned at 
least 60 years ago that RESISTANCE is the key to suppression. This 
principle can be found in old references from the 50s and 60s (I know, 
because in the process of publishing my work as an AES paper stating that 
principle, I did a literature search and found some references to it), but 
somehow it never found its way into modern literature, including ARRL 
publications. 

73, Jim K9YC