Resistor-Capacitor (RC) Filter
Calculated Cutoff Frequency (ƒc):
1591.55 Hz
Resistor-Inductor (RL) Filter
Calculated Cutoff Frequency (ƒc):
1591.55 Hz
Circuit Diagram
Frequency Response (Bode Plot)
Core Concepts
Understand the fundamental principles behind low-pass filters.
What is a Low-Pass Filter?
A low-pass filter is an electronic circuit that allows signals with a frequency lower than a specific cutoff frequency to pass through it, while attenuating (reducing the amplitude of) signals with frequencies higher than this cutoff frequency. In essence, it blocks high frequencies and lets low frequencies pass, which is useful for removing noise from a signal or for audio applications like subwoofers.
The Cutoff Frequency (ƒc)
The cutoff frequency is the point at which the filter begins to work. Specifically, it's the frequency at which the output signal's power is reduced to half the power of the input signal. This is also known as the -3dB point. Frequencies below ƒc are passed with little to no attenuation, while frequencies above ƒc are progressively attenuated at a specific rate (the "roll-off").
RC Filter Explained
The RC low-pass filter uses a resistor and a capacitor. At low frequencies, the capacitor has a high impedance, so most of the input voltage appears at the output. As the frequency increases, the capacitor's impedance drops, creating a voltage divider with the resistor that attenuates the output signal. The formula is:
ƒc = 1 / (2 * π * R * C)
RL Filter Explained
The RL low-pass filter uses a resistor and an inductor. At low frequencies, the inductor has a very low impedance, acting almost like a wire, so the input signal passes to the output easily. As frequency increases, the inductor's impedance rises, forming a voltage divider with the resistor that blocks the high-frequency components from reaching the output. The formula is:
ƒc = R / (2 * π * L)
Real-World Applications
Where you'll find low-pass filters in action.
Audio Systems
In audio crossovers, low-pass filters direct low-frequency signals to subwoofers, ensuring they only reproduce bass sounds they are designed for.
Signal Processing
Used to remove high-frequency noise or "hiss" from signals, cleaning up data from sensors or in telecommunications before further processing.
Image Processing
Analogous filters are used in software for image blurring and smoothing, effectively removing high-frequency details or noise from an image.
Design Checks for Low Pass Filter Calculator
low pass filter calculator design checks - Use these checks before trusting the calculator result in a PCB, repair, or reverse engineering workflow.
For a first-order RC low pass filter, fc = 1/(2*pi*R*C). Confirm source impedance, load impedance, capacitor dielectric tolerance, and op-amp GBW if the passive filter is buffered or cascaded.
- Confirm the formula assumptions against the real circuit topology, component tolerances, and parasitic PCB effects.
- Check operating limits such as voltage, current, bandwidth, temperature rise, or safety margin before selecting parts.
- When a measured board disagrees with the calculator, inspect the surrounding components and test points before changing the design.
Keep the calculation result with the schematic revision, measured values, and component datasheets.
