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Re: [TCML] Oscilloscope Tuning, Need help, please look

Andrew Robinson wrote:
Looking again for more help with scope tuning. After a fairly successful afternoon in the lab I think we may actually be getting somewhere with this, but could some veterans out there confirm our results please. Might be helpful for any new guys out there too so take a look! What we're going for here is either you looking at it and laughing at how far off we are, or actually (hopefully) telling us we're on the right path. The link below is for a PDF doc we created describing our results. There are some questions in there too. Any help would be greatly appreciated!


As Paul already said, the two frequencies below and above the main resonance don't have relation with the coil. Maybe some external interference. The signal generator in this test shall use a sinusoidal signal, although any waveform will produce similar results. The main resonance is where it should be. Two or more extra resonances are easy to find, somewhat below 3 and 5 times the first resonance.
An interesting test to make is as follows:
Connect the oscilloscope at the bottom of the coil, looking at the output of the signal generator, that has a nonzero output resistance, probably 50 ohms. At the first resonance you will see a dip in the signal level, as well as in the second, third, and so on, resonances. Set the signal generator to the second resonance, and slide a finger along the coil (pointing directly to the coil). The dip is reduced due to the detuning caused by the extra capacitive loading, but you will notice that there is a point where the dip reappears, at about 2/3 of the length of the coil. Set the generator to the third resonance and repeat the procedure. You will find two points where the dip reappears, at about 2/5 and 4/5 of the length. With some care you can repeat the procedure at higher resonances, finding more points where the dip reappears.

Antonio Carlos M. de Queiroz

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