Re: resonant freq.

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

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

George Hadle wrote:

> How does one experimentally measure the resonant
> frequency of a coil using physical, not inductive,
> connections.

There are three classes of method, with variations on each:

The least accurate:
 
D.C. Cox wrote:
> Inject a signal into the bottom of the secondary coil and then,
> using a scope probe, hold a small 4-5" long piece of wire near
> the top of the coil's toroid --- 1-2 in. away but NOT TOUCHING
> the sec. coil  This antenna will pick up the signal and give you
> the resonant freq on the scope. 

Probing the coil in this way disturbs the resonant frequency too
much for accurate work. 1-2in is far too close.  Try to keep the
pickup at least a coil length away from the coil. But George
asked for methods involving physical connection, so this doesn't
count anyway.

Medium accuracy:

Sean Taylor wrote:

> Inject a signal into the bottom of the secondary through
> a 1 KOhm (about) resistor, and monitor the voltage at the
> bottom of the secondary (after the resistor).  When this voltage
> goes to a minimum, you have reached resonance, in other words,
> the voltage across the resistor is at a maximum, thus current
> is at a maximum.

This works quite well because you're working at the grounded end
of the coil.  You can also estimate the Q factor - if you make the
resistor variable and adjust so that the voltage in the dip is
half the voltage seen when away from resonance, then the Q factor
is approximately 2*pi*F*L/R. The accuracy is limited by the ability
of the experimenter to discern the minimum of the dip.  To avoid
this, use a dual-channel scope with one probe at each end of the
resistor.  At resonance the two waveforms will be in phase 
(ignore the wave shape and go for aligning the zero crossings).
This frequency doesn't quite coincide with the minimum of the dip,
and the correct frequency is at the phase match, not the minimum
dip.

The most accurate:

Ping the coil with an impulse into the base and record the
resulting base current with a digital scope.  Process the data
through a computer using a program such as

 http://www.abelian.demon.co.uk/tssp/tcma/

which gives in one measurement the frequencies and Q factors
for several of the lowest frequency modes.  When a primary is
present (with shorted gap), this method also gives you the 
coupling coefficient.  With an additional resistor you also get
the Q factor and energy storage inductance.
--
Paul Nicholson,
Manchester, UK
--