I'm trying to experimentally verify my calculations on resonance on a simple series LC circuit.enter image description here

To prove resonance, I'm measuring the amplitude on the series resistor. When the amplitude is the highest, then resonance has occured.

I'm using WE coil:

enter image description here

And a ceramic capacitor in series. Measured both elements values by a LCR meter.

By the formula:

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We can calculate that the resonance frequency should be around 1.03 MHz.

But in reality, the measurements doesnt add up.

Firstly, on the calculated resonance frequency, there is very little voltage drop on the resistor. Secondly there are multiple maximums via the 1-100 MHz frequency range:

At 4.6 MHz = 100 mV

At 26 MHz = 80 mV

At 70 MHz = 100 mV

I'm guessing that at the 26 and 70 MHz peaks, the parasitics kicked in, but what got me more confused is that when I remove the capacitor, the voltage maximums remain at the same frequencies. Worth noting that the 4.6 MHz peak corresponds to self resonant frequency of the coil by the datasheet. What is the reason for this behaviour?

  • 1
    \$\begingroup\$ What kind of generator is "G"? More importantly, what is its output impedance? For your Q calculation, its output resistance is added to R1 (100 ohms). \$\endgroup\$
    – glen_geek
    Commented Nov 22, 2018 at 15:53

1 Answer 1


Your Q is less than one (OK 1.75).

At Fres, the XL is 2 * PI * F * L, or about 7 * 24 or nearly 200 (the Mega and Micro cancel). The Q is XL / R = 200 / 100 = 2.

How about reducing the resistor, to 10?

Get a feeling for moderate Q peaking first.


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