Re: resonant freq.

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "D.C. Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<DR.RESONANCE-at-next-wave-dot-net>
> 
> No, nothing wrong.  Your technique is another valid and clever way of doing
> it.  We just always used the "antenna" approach and it seems to work well
> for us.
> 
> Best regards,
> 
> Dr. Resonance
> 
> ----- Original Message -----
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Wednesday, July 10, 2002 5:47 PM
> Subject: Re: resonant freq.
> 
> > Original poster: "Sean Taylor by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <taylorss-at-rose-hulman.edu>
> >
> > Is there something wrong with this method:
> >
> > 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.  Am I missing
> > something here?  Thanks,
> > Sean Taylor

	Nothing wrong for frequency measurement, although the 1 k resistor will
result in a fairly low Q circuit and the frequency for exact minimum may
be hard to determine accurately.  A simple way around this is to connect
the direct signal from the signal generator to the horizontal axis of
the scope and the voltage at the bottom of the secondary to the vertical
axis.  Provided the frequency response of the scope is good enough the
resonant frequency is that at which the resultant lissajou pattern is a
straight line rather than an ellipse.  This can be a very sensitive test
and is one I use in adjusting the resonant frequency of audio filter
circuits.  Good idea to verify that you get a straight line by
connecting the two inputs together and seeing what pattern you get. 
Adjustment of the compensation of the probe for a straight line will
insure that the phase shifts of both deflection axes are the same. 

	However, you can get a pretty good idea of the equivalent series
resistance of the secondary at resonance by measuring the voltage at the
minimum, and then replacing the secondary with a variable resistor when
it adjusted to give the same voltage then it will be close to the
desired resistance.  This same general method can be used to measure the
equivalent parallel resistance at resonance of a very high C circuit
like a primary.  In this case the generator is adjusted to the frequency
of peak voltage, and the parallel resistance measured by the
substitution.  Of course the series resistance in this case needs to be
large enough so that there is significant voltage drop across it at
resonance.

	I usually measure the resonant frequency and Q of a secondary by using
a very small resistor to feed the secondary and observing the resultant
secondary voltage; as long as it is much smaller than the equivalent
series resistance the Q will be unaffected by it.  The signal generator
is connected to that resistor by a series resistor of large enough value
to give an undistorted waveform.  The Q is measured by observing the
frequency difference between the two frequencies which give an output
voltage down 3 dB (0.707 times the peak) as measured by a very loosely
coupled scope probe.

	Kind of long and rambling discussion, I'm afraid.

Ed