On Mon, 12 Jan 2009 10:13:44 -0700, Larry Benko wrote:
>I may not have been super careful when I cal'd the analyzer and 0.5pF
>seems about right from the numbers I posted. Everyone is correct about
>1pF being deal at 30Mhz when trying to measure high impedances.
Yes. With ferrites, the primary effect is to shift the resonance down in
frequency. Because the capacitance of the DUT is so small, a rather
small value of stray C is not insignificant. My estimate of the stray C
of my test setup is 0.4 pF (also based on measuring a resistor). The
shift that causes is shown in the data plot for one of the coax chokes.
A couple of years ago, Danny, K6MHE, instituted a round robin to measure
a choke of about 14 turns on #61. That's a very high Q material, and it
was clear from the results that some of them had an error of about 2pF.
This shifted the resonance of the DUT by nearly 50%! My results
correlated pretty well with what I believe to be the best measurement
when I subtracted out my 0.4 pF.
73,
Jim K9YC
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