Message-ID: <005701bf5c9e$23b33b00$c60dd6cf@test>
From: "Dave Brown" <tractorb@ihug.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <000e01bf5ba5$6bc57f60$60d725c3@194.95.193.10.fen.baynet.de><387ABC73.EB1AE1BB@qsl.net> <eMRVZDANl5e4Ewbl@pickmere.demon.co.uk>
Subject: Re: LF: Calculating distributed C for inductors
Date: Wed, 12 Jan 2000 14:41:22 +1300
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The classic method for determining self capacitance of an inductor is to
measure the resonant frequency (freq) with several values of parallel
capacitance.(C)

Making a plot of 1/(freq)squared on the vertical axis v. resonating
capacitance on the horizontal axis yields a straight line(or should!!) that
intersects the HORIZONTAL or capacitance axis at a negative C  value when
extrapolated.
 The negative C value is the self capacitance of the inductor in question.
As well,  the slope of the straight line is a measure of the  TRUE
inductance value of the coil and can be taken from the relation  L (henries)
= 0.0253M, where M is the slope of the line.(freq in MHz and C in pF).

A more practical method based on the above is the F/2F method where the
inductor is resonated at an initial frequency, F1, the added parallel C
value measured (C1). The inductor is then re-resonated at TWICE the first
frequency, say F2, and again the resonating C measured(C2).
The inductor self-capacitance is then given by
Cself = (C1 - 4C2)/3.

Measuring the inductance and then the parallel resonating capacitance and
trying to predict the stray C from the required result correction will only
give an approximate answer, as will direct measument of the self-resonant
frequency (and thence to the self-C)  but for accurate work the F/2F method
is better.

73
Dave
ZL3FJ