[TowerTalk] Measuring Ferrite

[Jim Brown]

On 4/10/2014 1:58 PM, Earl Morse wrote:
> I confess, I am spoiled.  I have access to a plethora of RF devices such as network and impedance analyzers so I wouldn't have to make do with making this measurement with a rigged up setup.

Actually, measuring ferrite chokes is not nearly as easy as it looks. 
The only stand alone instrument I would trust the the HP impedance 
analyzer. All reflection-based analyzers have the limitation of greatly 
reduced accuracy as the impedance is much more than 1/5 or 5x the 
circuit Z. There are HP applications notes that address this issue.

The equivalent circuit of a ferrite choke is series R and L in parallel 
with C (the interwinding capacitance), which simplifies to parallel RLC 
near resonance. Effective suppression is provided by the resistance, not 
the reactance (because the reactance can resonate with the line it's 
trying to choke). Ferrite materials that provide effective suppression 
are very low Q (on the order of 0.5 for an effective suppression 
material) where they are resonant.

What makes the Z measurement so difficult is that C in that parallel 
resonant circuit for an effective choke is on the order of 2-10 pF and 
the desired R is on the order of 5K.

73, Jim K9YC