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I designed a RLC circuit. I chose L and C such that resonant frequency become 2KHz. L=100mH, C=0.25*10^-6F and I took R to be 100 ohms. Then I generated Bode plot for range 100Hz to 10KHz. Ideally I should look for a phase angle of zero in oscilloscope out for resonant frequency, but there is none. Now how should I find out experimental resonant frequency? Should I pick up a phase angle nearest to zero and look for corresponding frequency?
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Your assumptions are wrong: the resonance frequency, give the values of L and C, is $$ f= \frac{\omega}{2 \pi} = \frac{1}{2 \pi}\sqrt{\frac{1}{LC}} = \frac{1}{6.28 \cdot 0.1 \cdot 250 \cdot 10^{-9}} \simeq 1 \, kHz $$ If you want it to resonate at 2K, and keeping the value of the inductance, it helds $$ C = \frac{1}{(2 \pi f)^{2}L} = \frac{1}{(6.28)^2 \cdot 4 \cdot 10^{6}*0.1} = 0.4 \, \mu F \simeq 63,4 \, nF $$ To see it on the scope, you have to measure and superimpose the voltage and the current; fortunately, since you have a series circuit, the current will cause a voltage drop on the resistor, so you can just take that drop as Vout. Also, at the resonance frequency, you will have the maximum gain. Check this simulation for demonstration. |
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Excellent Resonant circuits. Discusses the circuit below, which essentially directly matches your problem.
Useful http://www.tina.com/English/tina/course/28resonant/resonant.htm |
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There are many way to confirm with a scope 1) For frequency domain, Use FM sweep gen in X-Y mode. This is for visual confirmation using envelope centered at bottom of screen.using Scope X axis sweep to CH1 and Ch1 out to FM in and CH1 as X axis on scope CH2 as Signal current over R then scope shows frequency response. You can shift X axis with CH1 level and scale FM accordingly. 2)For time domain, pulse the circuit with narrow pulse at low rate eg 100 Hz and measure the ringing resonant frequency. The gain of the filter or Q factor is ratio of reactive to real impedance, in this case 1257ohm /100 =12.6=Q so there should be a few cycles decaying to confirm resonant values. |
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