Re: Inductance measurements of a flat spiral coil

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Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

> I'm always keeping my eyes open for the occasional
> lucky find. I have some 1% 2W 0.1 Ohm resistors I use
> for current measurement, and a 0.1% capacitor, I think
> it is 680pF.

Keep in mind that if you combine a bunch of randomly selected resistors of
the same value, the tolerance goes as 1/Sqrt(N).  So, 100 1% resistors in
series will be a 0.1% resistor.  The key is that the errors have to be
independent (i.e. if they're all from the same mfr lot, and uniformly 0.05%
high, it doesn't work...)

> >
> But the way I do it, the 1000pF doesn't matter! :'))
> That is the beauty of this method. All that matters is
> the accuracy of the capacitance you add.

Indeed...
>
> Ignoring resistive losses: A given inductor in an LC
> resonant circuit will shift frequency a fixed Hz when
> a 100pF cap is added, regardless of other parallel
> capacitance in the circuit, including parasitic, self,
> and circuit capacitance. Maybe I should do a webpage
> on this.
>
> Just use some algebra with the resonant LC formula.
> One formula uses f1, the other f2. One uses L1*C1, the
> other L1*(C1+C2), where C2 is the precision 100pF
> capacitor.
>
>
> I've been experimenting with antenna configurations
> for a Theremin design I'm working on. This experience
> makes me question David Thomson's relying on 0.2pF
> difference in readings of several hundred pF when that
> much capacitance can be from shifting a hand a few
> inches closer even from several feet. The larger the
> object, the further away you can affect it by the same
> amount of capacitance change. So something as simple
> as the curve in the test lead wire can easily affect
> it by that much.
>