[RFI] Fair Rite #31 Mix Material

[Jim Brown]

On Sun, 11 Jan 2009 21:51:58 -0700, Larry Benko wrote:

>Ok, you made me fire up the network analyzer (HP8753B). 

Before you place too much confidence in data from your network 
analyzer, study my measurement travails, documented in the Coaxial 
Chokes Power Point. Ferrite chokes are VERY difficult to measure 
accurately, because of the values of impedance you're trying to 
measure. 

Here are two elements of the problem: 1) the HF equivalent circuit 
is a parallel equivalent circuit with capacitance on the order of 
0.5pF to 4 pF, well within the range of stray capacitance in most 
analyzer setups. Yes, some analyzer software attempts to subtract 
it out, but most are not very good at it when the unknown DUT is 
in this range. 2) The impedance at resonance is typically 500 - 
5,000 ohms. This is far outside the range of unknown impedance 
that can be measured with any accuracy by a reflection-based 
measurement (that is, S11). That's because the equations that 
compute the unknown Z are differences of quantities that are very 
nearly equal, so a small difference in any quantity makes a very 
large difference in the value of the unknown.  

What you CAN do and get good data is to make measurements of S21 
with the unknown Z in the series leg. My Power Point shows how 
I've done that, and my data for coaxial chokes was obtained using 
that technique. 

But don't trust my measurements of these parameters. Do the curve-
fitting that I've illustrated to find R, L, and C for the parallel 
equivalent circuit that produces the impedance curves in the Fair-
Rite data. That is, write the equation for the Z of the parallel 
RLC circuit, plot it on the same scale as that for Fair-Rite data. 
When the two curves match, you now know R, L, and C. 

73,

Jim K9YC