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Re: Measuring secondary resonant frequency
Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>
Tesla list wrote:
> Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
> Tesla list wrote:
> > Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> > << Excite the base of the
> > secondary with a square wave, with a frequency about 20 times smaller
> > than the resonance frequency >>
> > I have used the sine wave output of the sig gen to find Fo.
> > What is the advantage in using a square wave to find Fo?
> In terms of precision, none. To start, the generated signal is much
> smaller. But looking at the ringing transients at each transition of
> the square wave you can see what happens when you make changes in the
> environment around the coil without having to search again for the
> resonance with a sinusoidal signal.
> Antonio Carlos M. de Queiroz
Another way to excite the base is with short pulses (which have all of
the energy you are using out of the square wave in your example). This
can be done very easily with a buzzer circuit [the primary circuit is
series connected and placed across the contacts of the buzzer]; I've
done it and looked at the resultant waveform with the scope and it shows
the impulse response of the system from which you can calculate the
frequency and the damping. This tells you something, but not all, about
the behavior when the coil is excited with high power and the
transformed energy is being dissipated in the form of sparks or
The use of buzzers for the purpose of "wireless" measurements goes back
to the beginnings of the field, and is described in most old treatises
including Zenneck's "Wireless Telegraphy" Bureau of Standards Circular
No. 74. I have a 79 year old General Radio "wavemeter" which employs
this scheme and generates a signal which can be used for testing and
calibrating radio receivers. It tunes from 100 kHz (3000 meters) to 2
MHz (150 meters) and the output waveform has a half-power bandwidth of
the order of 5 to 10 percent, as observed with a spectrum analyzer.
P.S. The buzzer is, of course, powered from a battery.