From: Paul
Date: Tue, 09 Jan 2001 23:05:27 +0000
Subject: Re: [TSSP] L measurements
Marc wrote:
> coil #2
> dia.= 4.254"
> W.L.= 42.4"
> 24g wire
> 60.022 ohms
> 29.12 mh
> Q = 2.6
Using your measured inductance gives us
mm2: h/d 10.11 bare
measured modeled
f1 276.9kHz 273.8kHz -1.1%
f3 711.8kHz 692.6kHz -2.7%
which is quite good. We can use this coil for further
measurements, which is great because its the highest
h/d we have.
The next step with #2 will be to measure the Q factor,
and it should be in the region of 270 at f1. This is
the one where you find the frequencies at either side
of f1 at which the signal from your pickup loop is
71% of its value at resonance. These are the half power
frequencies, and the difference between them is the
half power bandwidth, BW. The Q is then f1/BW.
When I do this measurement I like to set the scope gain
so that the signal at resonance spans 7 vertical divs.
Then I tune the generator till it drops to 5 divs, which
is near enough the 71%. I find it helpful to turn the
timebase down, so instead of the sine wave you get a
broad band, and position it so it sits +/- 3.5 divs
either side of the zero line at resonance.
When doing the Q measurement, it's important to drive
the base with a low impedance source which means using
those two resistors I mentioned in an earlier post, which
'pad' the generator output impedance, which might be
50 ohms, down to 1 ohm or thereabouts. The base input
impedance of coil #2 will be around 120 ohms, so that
remaining 1 ohm will give less than 1% error. You'll
only be able to read the levels from the scope to 5%
anyway, unless you have one of those fancy digital things,
but even 5% is more accurate than we can calculate Q at
the moment. I'm predicting 270, but really it could come in
at anything from say 200 to 350.
Could you let me know how far away the nearest wall is
from the test coils? Sorry if you've already told me that.
Cheers,
--
Paul Nicholson,
Manchester, UK.
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.