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Re: 3rd harmonic trap. Apology

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Hi Jolyon,

> I have not yet read Paul Nicholson's work

Don't bother.  I haven't written anything on this topic,
except in a few list posts which I wouldn't recommend you
try to find.  I can summarise here briefly...

First, be sure there's no confusion between '3rd harmonic',
which is a component of the drive signal at exactly three
times the fundamental, and the 3/4 wave resonance of a coil,
which is the first overtone resonance of the system, usually
between 2.2 and 2.8 times higher than the fundamental in a
typical unloaded coil.

Some situations that have been discussed in the past include:

a) A self-oscillating driver picks on the coil's first overtone
resonance and oscillates there instead of at the quarter wave

b) The tuning of the coil just happens to make the first overtone
equal to the 3rd harmonic of the drive signal.  This can happen
if just the right amount of toploading is present, since unloaded
coils have a ratio of first overtone to fundamental of less than
three, while loaded coils often have a ratio of more than three.
So in between, some modest toploading value must make the ratio
exactly three.

To avoid (a), you must make sure the overall loop gain is less
than unity at these frequencies.  The narrowness of the primary
response may be sufficient providing coupling is modest, but it
may depend on the feedback arrangement.  I believe some people
also use a PLL to set hard limits on the frequency range.

As for (b), if this situation arose, there would be greater primary
current at the 3rd harmonic.  Probably not much, again, depending
on coupling.  It can be calculated if necessary.

I don't think any solid state coilers take (b) to be any great worry.
So far there is no evidence that any of this matters.  Has anyone
deliberately tuned their coil's first overtone to the drive 3rd
harmonic, just to see what happens?  All you need is just the right
amount of toploading.   Does anything nice or horrible happen?
Do you get brighter streamers, or smoke, or just a slightly squarer
looking drive current?
Paul Nicholson
Manchester, UK.