# Resonant Frequency

#### paul_alan

Joined Nov 5, 2011
43
how do you calculate resonant frequency if you only have a capacitor? i know the formula fr = (6.28 x the square of LC), but what if there is no inductor?

#### thatoneguy

Joined Feb 19, 2009
6,359
A capacitor and a resistor don't have a resonant frequency, they do have a cutoff frequency when used as a filter. Same is true for an inductor and resistor.

In some cases of spiral wound capacitors, there is some self inductance, but having the inductance at a high enough value to self resonate with the capacitance is unlikely. It is taken into consideration in some cases, if parasitic inductance of capacitor is listed.

#### paul_alan

Joined Nov 5, 2011
43
do you find the cutoff frequency by trial and error? and what really is the difference between a high pass and low pass filter?

#### thatoneguy

Joined Feb 19, 2009
6,359
High pass lets frequencies above the cutoff point through

Low pass lets frequencies below the cutoff point through

Click on the impedance graph paper in my sigline and you'll see it with some examples.

#### t_n_k

Joined Mar 6, 2009
5,455
One should not overlook the phenomenon of self-resonance in capacitors.

#### crutschow

Joined Mar 14, 2008
27,736
do you find the cutoff frequency by trial and error? and what really is the difference between a high pass and low pass filter?
The cutoff (-3dB) frequency in Hertz of a simple RC filter is 1/(2$$\pi$$RC)

#### paul_alan

Joined Nov 5, 2011
43
it's as simple as calculating a formula? in the lab, we're hooking up simple LC and RC circuits and supposed to keep adjusting the function generator and oscilloscope trying to determine the cutoff. one of the instructors said we need to look for the resonant frequency, but one of the responses to my initial question was that there isn't a resonant frequency in an RC or LC circuit. also, i keep adjusting the time of the oscilloscope as i change frequency and i always can find a sine wave. how can i determine the cutoff by making adjustments on the equipment?

#### thatoneguy

Joined Feb 19, 2009
6,359
it's as simple as calculating a formula? in the lab, we're hooking up simple LC and RC circuits and supposed to keep adjusting the function generator and oscilloscope trying to determine the cutoff. one of the instructors said we need to look for the resonant frequency, but one of the responses to my initial question was that there isn't a resonant frequency in an RC or LC circuit. also, i keep adjusting the time of the oscilloscope as i change frequency and i always can find a sine wave. how can i determine the cutoff by making adjustments on the equipment?
Assuming in all cases, you start your signal generator at 20Hz or less:

For a Capacitor, set the scope so the waveform covers 4 divisions. Increase the frequency until the waveform covers 8 divisions, and that is the 3dB point.

For an inductor, set the scope so the waveform covers 8 divisions, increase the frequency until it covers 4 divisions, that is the 3dB cutoff for that LR pair.

Similar with series LC circuits, except there will be a peak, so start out with the wave covering 2 divisions, sweep the frequency until you find the peak, and that is the resonant frequency.

For parallel LC circuits, it's the reverse, start so the waveform fills 8 divisions, and adjust the frequency until the waveform is at minimum, then read the frequency.

You'll have a good idea of what the resonant frequency will be using the impedance cheat sheet in my sigline.