Larry,
If I did the math correct, your fixture (or the 0805 resistor) has about
0.5pf of stray C (based on the 30 MHz measurement). What are your
calibration standards?
73, Mike W4EF..............
Larry Benko wrote:
>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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>>
>>
>>
>>
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