[Elecraft] K3 6m pre-amp

[Jim Brown]

On Thu, 31 Jul 2008 09:10:54 -0700, Alan Bloom wrote:

>When you wire two chokes of different value in series, you almost always
>get a series-resonant "hole" in the attenuation somewhere between the
>parallel-resonant frequencies of the two chokes.  It's not hard to see
>why that's true if you model each choke as an ideal inductor in parallel
>with a capacitor. 

But that's not an accurate model! If ferrite chokes are wound on the 
"right" ferrite material (the mix), they have a dominant RESISTIVE 
component in the frequency range of interest. While it IS important to 
consider the series L-C possibility, in REAL chokes, the resistive 
component can prevent this from being a problem. Indeed, this goes back to 
the original comment that the important property of a ferrite choke is 
RESISTANCE, NOT inductance!  

A properly designed choke (that is, right number of turns, winding style, 
and using the "right" material) can be made to have a high resistive 
impedance over at least an octave, and Fair-Rite's #31 is good for at 
least two octaves.  Again, see my tutorial, which also shows how simple 
curve fitting in a spreadsheet can be used to come up with actual R, L, 
and C values from magnitude-only impedance data. It also shows a method of 
measurement that is much better than commonly accepted "standards," like 
the HP Z meters and network analyzers when the unknown Z is high. 

73,

Jim Brown K9YC