[Amps] KK5DR's ferrite loaded

[Jim Brown]

On Tue, 19 Jan 2010 15:50:08 +0000, Manfred Mornhinweg wrote:

>But clearly in each particular case the capacitance and its effects 
has
>to be evaluated. It's easy to run into trouble with it, when the 
circuit
>impedances become higher.

That's really the key. 

Another VERY important point is impedance measurement of ferrite 
inductors. Using traditional reflection-based instruments will often 
give very wrong answers, because the impedance being measured is too 
far removed from the 50 ohm impedance of the measurement system. As you 
can see from the measured data in my tutorial, that is certainly the 
case for these chokes near resonance. 

A simple measurement circuit that provides fair accuracy for impedances 
above about 500 ohms, and increasing accuracy as the impedance gets 
larger, is to set up the unknown as the series element of a voltage 
divider, where the load of the divider is the 50 ohm input of a 
calibrated receiver or spectrum analyzer (or a voltmeter with a 50 ohm 
load). If you had a network analyzer, you would measure S21. You can 
then do simple curve-fitting to determine the values of R, L, and C in 
the equivalent circuit. The tutorial, and an accompanying presentation 
on coaxial chokes, discuss this technique and show some results both 
for chokes I've measured and for data provided by Fair-Rite in their 
data sheets for their parts. 

73,

Jim K9YC