Re: Measuring secondary resonant frequency

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-uswest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Kelly & Phillipa Williams by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <kellyw-at-ihug.co.nz>

> For this test, you connect the "signal" lead from the frequency generator to
> the bottom lead of the secondary coil, and leave then "ground" lead
> disconnected.

Technically, you should ground the ground lead, but it's probably
already
grounded trough the power cord.

> You then suspend the "signal" probe of the ocilliscope about a yard/meter
> away from the toroid, leaving the "ground" lead sitting on top of the
> ocilliscope casing.

Same case.
 
>...
> The precise frequency of the peak amplitude on the ocilliscope is the
> resonant frequency.

Correct.
 
> The resonant frequency of my 9" by 45" secondary coil and 25" by 9" toroid
> is about 55 kHz according to this method. However, if I leave the signal
> generator set on the resonant frequency, and walk close to the toroid (1.5
> meters / yards), the high-amplitude wave on the scope dies down to a
> smallish sine wave. As I walk away again, the wave on the scope goes back up
> to a large amplitude.
> 
> Question - does this mean that when my coil is operating, and I walk closer
> to it and further away from it, (while remaining safe from arcs) I am
> actually changing the resonant frequency slightly?

Yes. You change the capacitance from the coil to its surroundings, and
change the resonance frequency a bit.
 
> Also, if you touch the toroid during the test, the wave on the scope cuts
> completely to a straight line. I suspect that this is similar to what
> happens when the secondary coil is arcing to a grounded rod, when the Q of
> the coil and the output voltage drops.

True, but in this condition you also change the resonance frequency
significantly. 

If you have a signal generator, there is a simpler way to see resonance
and Q without having to search for the peak. Excite the base of the
secondary with a square wave, with a frequency about 20 times smaller
than the resonance frequency. You will see a series of small oscillation
bursts in the oscilloscope. The frequency seen is the resonance
frequency, and from the time required for decay you can deduce the Q.
The advantage is that you can see in real time what happens when you
move close to the coil or touch the terminal.
With only a coil without a toroid, or with a small one, you may see 
strange waveforms, because the other resonances of the coil will appear
too, and measurements may be inconclusive.
The signal generator must have low output impedance for this. Shunt
it with a small resistor (10 Ohms) if it hasn't.

Antonio Carlos M. de Queiroz