# Problem calculating resonant frequency [series RLC]

I try to link/prove the theory of resonance in practice. So for test I connected in series 1mH inductor, 1k Ohm resistor and 2.1 uF ceramic capacitor.

Calculated resonant frequency -> $f_{r} = \frac{1}{2 \cdot \pi \cdot \sqrt{LC} } = \frac{1}{2 \cdot \pi \cdot \sqrt{1*10^{-3}*2.1*10^{-6}}} = 3473 Hz$

Okay so, to prove calculated resonance I pluged 4Vpp sinusoidal signal from function generator to my RLC circuit and monitored resistor voltage on my oscilosope. I went from 1Hz up to the point at which I observed change in signal amplitude. From my calculations I expected resonance to happen at 3473 Hz but surprisingly something started to happen at frequency of around 0.5 Mhz. How is that possible? The amplitude went from 3.3V to 330 mV. I started to blame inductor value, since there is no code/indicator on it's body. I got rule for inductance:

$L=\frac{\sqrt{\left( \frac{V}{I}\right)^{2} -R^2}}{\omega}$

and to calculate it I conncted in series resistance of 1k Ohm and my "1mH" inductor. Powered it by 1Khz sinusoid. Therefore I calculated that inductance is 15.8 mH instead of 1 mH. So, in my resonance calulations I changed 1mH with 15.8 and got resonant frequency equal to 873 Hz...not even close to what I observe on my scope. So to summarize here are my questions:

1. Why in my series RLC circuit I only see decrease in amplitude instead of sharp rise?
2. Why I don't see any change in resistor voltage at calculate resonant frequencies?
• try removing that swamping 1k resistor. Then measure capacitor voltage or inductor voltage with your 'scope. – glen_geek Mar 10 '18 at 22:07
• You are doing something wrong or you have the wrong value for inductance. The theory is well trod and works in real life. – Andy aka Mar 10 '18 at 23:34
• Simple question that you've probably accounted for: does your circuit have a ground, and where do you connect the scope ground? – Chu Mar 11 '18 at 0:03
• @glen_geek, if I take the voltage out of the inductor I get aprox. 0V up tp 1khz of frequencies and then it rises slowly to 4+V at around 50kHz. Stays at 4V up to11 Mhz then starts to drop...30Mhz is max of my function generator and at that freq I get 1.8V. On Capacitor it is entirely different story. The voltage starts at 4V and drops with frequency toward 0V. At 20kHz the signal is already gone. So I think your suggestion is wrong. The resistor is there for purpose, at resonance both aditances shall cancel each other and therefore max current shuld flow thru resistor. – DannyS Mar 11 '18 at 11:04
• @chu, good question I don't have any ground in the circuit. When I do my rsonance test I just plug function generator leads to both ends of the circuit and that's all. Scope ground is on one leg of the resistor and the probe is on second leg. Something wrong here? – DannyS Mar 11 '18 at 11:13