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Re: resonant freq.
Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>
Tesla list wrote:
> Original poster: "Sean Taylor by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> > 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 circuit.
> So the resistor is still there you mean? What's basically going on is
> you're checking if the two signals are in phase, quite a bit of signal
> gain will most likely be needed for the probe at the base of the
> secondary, or a smaller resistor should be used to make a slightly
> higher voltage at the base. Am I seeing this correctly?
> Sean Taylor
The answer is that I think you are seeing it correctly. If the two
signals are in phase then the input impedance at the base is a pure
resistance, which occurs at the resonant frequency.