# Low Pass Filter Calculator

RC Low Pass Filter

This RC low pass filter calculator calculates the cutoff frequency point of the low pass filter,
based on the values of the resistor, R, and the capacitor, C,
of the circuit, according to the formula **fc= 1/(2πRC)**.

To use this calculator, all a user must do is enter the capacitance, C, of the capacitor and the resistance, R, of the resistor. This calculator allows a user to select the magnitude of the farads of the capacitor, including picofarads (pF), nanofarads (nF), microfarads (µF), and farads (F). After the capacitance and resistance values are entered, the user clicks the 'Calculate' button, and the result is automatically computed. The resultant value of the cutoff frequency calculated is in unit hertz (Hz).

An RC low pass filter is a filter circuit, composed of a resistor and a capacitor, which passes low-frequency signals and blocks high frequency signals. When a resistor is placed in series with the power source and a capacitor is placed in parallel to that same power source, as shown in the diagram circuit above, this type of circuit forms a low pass filter. It forms a low pass filter because of the reactive properties of a capacitor. A capacitor is a reactive device. This means that the resistance that it offers to a signal changes depending on the frequency of the signal. Capacitors are reactive devices that offer very high resistance, or impedance, to low frequency signals. Conversely, they offer lower resistance as the frequency of the signal increases. Thus, a capacitor offers very low impedance to a very high frequency signal. Because they offer low impedance to high-frequency signals, high frequency signals normally go through, as they represent a low-impedance path. Remember that current always takes the path of least resistance. So high-frequency signals normally take the capacitor path, while low-frequency signals don't; they go through to output.

When we calculate the cutoff frequency of the low pass filter, which is what this calculator does, we're calculating the point in the frequency response of the filter, where the gain has dropped by 3dB. Low pass filters pass low frequencies with high gain until it reaches a point in the frequency response curve where it no longer can pass out frequencies with as much gain. As the frequency gets higher, the signals get attenuated. The point at which the low pass filter can longer produce full gain and has dropped the gain by 3dB is referred to as the cutoff frequency. The cutoff frequency is the point where we know that the filter cannot produce full gain for signals. This is why it's crucial and why just knowing the cutoff frequency where the low-pass filter ends. At this frequency, signals begin attenuating greatly, and no longer pass frequencies with very much gain.

As you can see in the above diagram, the low pass filter produces its full gain for low frequency signals and then begins producing lower gain signals.
At the cutoff frequency, there is a 3dB reduction in gain. And as the frequency increases, the gain reduces even more, until it's essentially 0.

RL Low Pass Filter

This RL low pass filter calculator calculates the cutoff frequency point of the low pass filter,
based on the values of the resistor, R, and inductor, L,
of the circuit, according to the formula **fc= R/(2πL)**.

To use this calculator, all a user must do is enter the inductance, L, of the inductor and the resistance, R, of the resistor. This calculator allows a user to select the magnitude of the henry of the inductor, including picohenry (pH), nanohenry (nH), microhenry (µH), and henry (H). After the inductance and resistance values are entered, the user clicks the 'Calculate' button, and the result is automatically computed. The resultant value of the cutoff frequency calculated is in unit hertz (Hz).

An RL low pass filter is a filter circuit, composed of a resistor and a capacitor, which passes low-frequency signals and blocks high frequency signals. When an inductor is placed in series with the power source and a resistor is placed in parallel to that same power source, as shown in the diagram circuit above, this type of circuit forms a low pass filter. It forms a low pass filter because of the reactive properties of an inductor. An inductor is a reactive device. This means that the resistance that it offers to a signal changes depending on the frequency of the signal. Inductors are reactive devices that offer very high resistance, or impedance, to high frequency signals. Conversely, they offer lower resistance as the frequency of the signal decreases. Thus, an inductor offers very low impedance to a very low frequency signal. Because they offer low impedance to low frequency signals, low frequency signals go through, as they represent a low-impedance path. Remember that current always takes the path of least resistance. So low-frequency signals normally take the inductor path, while high-frequency signals don't; they get impeded by the inductor's high impedance.

Just as with the RC filter, the RL low pass filter calculator calculates the cutoff frequency of the filter. This is the point in the frequency response of the circuit where the gain has reduced by 3dB. And as the frequency increases, signals attenuate more and more. The cutoff frequency calculation is important because it shows where the gain has been reduced by half, which is what the 3dB point represents. This is why the cutoff frequency is so crucial; it shows where the amplitude of the signal has been cut in half. At this frequency, signals begin attenuating greatly, and no longer pass frequencies with very much gain.

As you can see in the above diagram, the low pass filter produces its full gain for low frequency signals and then begins producing lower gain signals.
The cutoff frequency is the half power point, the point where the gain is now half of its full strength.

Related Resources

High Pass Filter

High Pass Filter Calculator

LM317 Resistor and Voltage Calculator

What is an Adjustable Voltage Regulator?

How to Connect an Adjustable Voltage Regulator

LM317 Voltage Regulator