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Old 17th Feb 2009, 10:54 am   #2
Skywave
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Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Default Re: Measuring Resonant Frequency

Your overall approach is about right but there are errors in your method of its implementation giving you a false result.
At resonance, a parallel LC cct. will give a rise in the voltage developed across it. This is because at resonance the impedance of this cct. will be high. If I read your Post correctly, you have connected the sig. gen. directly across this LC cct. This gen. will have a low impedance output, which will severely reduce the high impedance of the LC cct. at resonance (impedances in parallel). Thus, the voltage developed across it will be small. To prevent this, insert a high-value R (say 100 k-ohms) between the sig. gen. and the LC cct. Although this will reduce the voltage appearing across the LC cct., it will also reduce the shunting effect of the sig. gen. You'll obviously need to increase the gain at the 'scope to compensate. Similarly, the 'scope will reduce the voltage across the LC cct. if connected directly. The best preventative for this is to use a 10X high-impedance probe.

HTH

Al / Skywave.
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