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Re: [TCML] Oscilloscope Tuning, Need help, please look
Your calculations are fine. Question though, in the diagram you show a
meter picking up 356kHz. Obviously incorrect if the coil is built as
shown. If there are objects nearby you could be loading the coil down
with external C between the objects and the coil. Or, you may be picking
up a lower harmonic on the meter. I expected the coil to measure near
517kHz if the secondary is say 5" above the floor. Are the turns
completely calculated or did you count them? If calculated and your
looking at just some measurement fun, then I personally would unwind and
cut off 1 turn as a measurement stick. The length/PI will give you it's
true diameter and a caliper can then verify the wire with insulation
thickness. Helps for fine tuning calculations.
Regarding the toroid and secondary. No, you can't measure Cself of the
secondary with the toroid in place. Cext between the coil & ground, the
toroid & ground, and the coil & toroid will all influence the system.
It's best to measure the system and then use a program like Javatc to
separate the capacitances. Also, there is an high frequency capacitance
and a DC capacitance. When the coil is operating at resonance, the high
frequency will dominate the LC values and if the coil is over say 1.5
h/d (as most are), the effects of the high frequency become more
apparent. These are hard to look at however. For example, when you
measure frequency, your measuring the coil at the resonant frequency.
But when you measure the inductance with a meter, your measuring the DC
inductance. If you then use the high frequency measurement and back
calculate Cself with the DC inductance, then you end up with a Cself
that is truly not the DC capacitance or the high frequency capacitance
(you end up somewhere between the two). So this does get difficult and
why a program is best suited for this task (the programs today have the
ability to accurately predict all those variables).
None of that is needed for tuning a coil, but it does help greatly on
preliminary designs. The old tap around the primary method can always be
done and should be done even after math or programs for best sparks.
Andrew Robinson wrote:
Looking again for more help with scope tuning. After a fairly
successful afternoon in the lab I think we may actually be getting
somewhere with this, but could some veterans out there confirm our
results please. Might be helpful for any new guys out there too so
take a look! What we're going for here is either you looking at it and
laughing at how far off we are, or actually (hopefully) telling us
we're on the right path. The link below is for a PDF doc we created
describing our results. There are some questions in there too. Any
help would be greatly appreciated!
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