GM Jim,
I certainly hope you or anyone else did not think that since I posted
the data to the ohm that any high level of accuracy was implied. If
accuracies were better than 10% I would be amazed BUT 10-20% is good
enough to tell materials 61, 77, 31, & 43 apart.
I have read all the documents you have published this topic and on page
47 of the PowerPoint slide you mention that the VNA doesn't seem to
cancel out the stray capacitance of the fixture so I decided to see how
my fixture would perform with a couple of resistors. I just measured
two 0805 SMT 1% resistors (1.00K & 10.0K) which were soldered across the
calibrated text fixture directly. I was doing an S11 measurement.
1.8MHz: 1006 & 10,210 ohms magnitude respectively
30MHz: 984 & 7,100 ohms magnitude respectively (phase for 1k was
slightly inductive and for 10k was moderately capacitive)
I guess I am not seeing the magnitude of the effect you saw.
I have measured many common mode chokes over the years both by doing S11
measurements and S21 measurements (in different calibrated fixtures of
course). The biggest problem I have seen for chokes of higher common
mode impedance is repeatability with the winding placement. Once a
choke was picked for a particular use occasional QA checks were done on
incoming chokes to prove suppliers were using the correct material.
73, Larry W0QE
Jim Brown wrote:
> 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
>
>
>
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