Re: Strange sweep results --- perhaps overcoupling

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "D.C. Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <DR.RESONANCE-at-next-wave-dot-net>


After reviewing the data I suspect you have overcoupled the primary and are
producing some sort of forced resonance.  Try just driving the bottom of the
secondary (no primary coil whatsoever) with the sig. generator and pulling
the signal off the top through a 1 meg resistor.  The forced resonance
should go away and give you the true resonance points.

Regards,

Dr. Resonance



----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, June 19, 2002 12:49 PM
Subject: Re: Strange sweep results


> Original poster: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
>
> Sundog wrote:
> > got the expected Fres of around 833khz...But, ... ended up with
> > another spike at 2.5mhz.
>
> Something may be wrong here.
>
> It's a little suspicious that this response is exactly 3 times the
> normal Fres. Usually the 3/4 wave response is some 2.2 to 2.6
> times the 1/4 wave Fres for an unloaded secondary. And the 5/4
> wave is usually between 3.5 and 5.5 times the 1/4 wave.
>
> Perhaps your coil just happens to have the right amount of
> toploading to bring these two coil responses to a frequency ratio
> of 3, which is possible (and may have unpleasant consequences for
> CW operation).  Toploading reduces the frequencies of all the coil
> resonances, but the lower frequency responses are pulled down more
> than the higher frequency ones, so a ratio of 3 could be achieved
> by chance.  It's just a little suspicious.
>
> Is the 2.5Mhz reported from the sig gen frequency setting, or from
> the observed period on the scope?  Whichever, can you check the
> other?  Add a small amount of extra C to the coil top (eg a few
> cm of wire or a door-knob) and re-measure.  If the 2.5Mhz is a
> genuine coil overtone, as opposed to a sig gen harmonic, the ratio
> of the two frequencies should increase above 3.
>
> > At harmonics I get a nice double-sine waveform,
> > with a big and small hump.
>
> Make sure you're using a sine wave drive.  The response will be
> tricky to interpret if you use any other waveform.
>
> >  this one is just as strong as the real Fres.
>
> It is difficult to draw conclusions from the apparent difference
> in the amplitude of the coil's responses.  So many factors are
> involved:  Q factor of the coil, characteristic impedance
> of the coil, probe/pickup response, sig gen output impedance;
> All these have a direct effect on the observed amplitude, and all
> vary quite a lot with frequency.
>
> > Or should I try both and see which gives better performance?
>
> The lower frequency is the one used for normal operation. The
> higher overtones are very much less efficient at producing top
> voltage because the effective capacitance of the secondary at
> these higher frequencies is a great deal bigger - circa three times
> larger for the 3/4 wave overtone than the normal 1/4 wave. Also,
> when driven at the overtone frequencies, voltages equal to or
> exceeding the topvolts will occur within the secondary winding.
>
> Further to earlier answers, it's important to call the higher coil
> resonances 'overtones' or 'modes' rather than 'harmonics'.
> Remember, 'harmonics' refers to the frequency components of a
> repetitive signal, whereas 'overtones' or 'modes' refers to the
> multiple resonant responses characteristic of any sort of
> distributed resonator - mechanical or electrical.  Overtones are
> not (except by chance!) related to one another by integer ratios.
> --
> Paul Nicholson,
> Manchester, UK.
> --
>
>
>
>