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Old 17th Feb 2009, 1:42 pm   #8
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
Default Re: Measuring Resonant Frequency

John,

According to your circuit, you have connected the parallel LC circuit in series with the signal generator's output.

With this connection, the frequency that the 'scope flat-lines is the resonant frequency (because the LC circuit is putting maximum impedance between the sig-gen and the 'scope).

To be honest, I wouldn't have thought that you'd get a very sharp null, because with normal 'scopes, the input impedance is itselt high so you'll need a massively effective LC circuit to seriously attenuate the signal. But, if you shunt the 'scope input with 10 kilohms or so, you may sharpen it up. Don't worry about the 'scope's input capacitance, because it won't affect the null frequency (as it would if you connected the LC circuit as suggested by ronbryan).
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