Digital Filter Designer: Low-Pass & High-Pass
Design a real RC low-pass or high-pass filter, watch its Bode magnitude plot draw live, and see exactly how much a signal is attenuated at any frequency.
Change R, C, the filter type and order, and watch the Bode magnitude plot and the live waveform update instantly.
f_c = 1 / (2πRC), |H(f)| dB = -20·n·log10(√(1+(f/fc)²)) (low-pass, n = order)About the Digital Filter Designer: Low-Pass & High-Pass
Free digital filter designer: low-pass & high-pass. Design a real RC low-pass or high-pass filter, watch its Bode magnitude plot draw live, and see exactly how much a signal is attenuated at any frequency. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for engineering, the digital filter designer: low-pass & high-pass runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Design a real RC low-pass or high-pass filter, watch its Bode magnitude plot draw live, and see exactly how much a signal is attenuated at any frequency. Use it to explore engineering ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Digital Filter Designer: Low-Pass & High-Pass
- Use the controls to change Resistance R (Ω), Capacitance C (nF), Test / signal frequency (Hz). The simulation reacts instantly.
- Pick an option such as Low-pass, High-pass, 1st order, 2nd order (cascaded) to switch modes or load an example.
- Press "Reset to defaults", "Lab report" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Pick R and C to hit exactly 1000 Hz cutoff.
- Switch from low-pass to high-pass and see the curve mirror.
- Add a second order and watch the roll-off steepen from 20 to 40 dB/decade.
- Try the -35 dB attenuation challenge at 10x cutoff.
Key ideas you can learn
- An RC filter's cutoff frequency is fc = 1/(2 pi R C); above it a low-pass filter's output falls off, and below it a high-pass filter's output falls off.
- Each filter 'order' (an extra RC stage cascaded on) doubles the roll-off steepness: about 20 dB/decade per order.
- A Bode plot shows gain in decibels against frequency on a logarithmic axis, which turns the filter's smooth curve into a nearly straight-line roll-off far from the cutoff.
- Filters also shift the phase of a signal near the cutoff frequency, which matters in control loops and audio crossover design as much as the amplitude response does.
Where this is used in the real world
Audio crossovers, anti-aliasing filters ahead of analog-to-digital converters, power-supply noise filters and sensor signal conditioning circuits all rely on exactly this RC low-pass/high-pass cutoff-and-roll-off design process.
Who is this simulation for?
Engineering and technology students, makers, robotics clubs and teachers of design and technology. It gives a hands-on feel for how machines behave before you build a real one.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why does doubling the filter order double the dB roll-off per decade instead of the cutoff frequency?
Cascading a second identical RC stage multiplies the magnitude response of the first stage by itself, and since decibels are logarithmic, multiplying magnitudes adds their dB values - so the roll-off slope doubles from about 20 dB/decade to about 40 dB/decade, while the -3 dB cutoff point stays close to the same frequency.
Why is a log frequency axis used for a Bode plot instead of a normal linear axis?
A filter's response changes by a similar percentage over each doubling or decade of frequency rather than each fixed increment, so a logarithmic axis spreads that behavior out evenly and turns the far-from-cutoff roll-off into a straight line that is easy to read the slope from.
Is the Digital Filter Designer: Low-Pass & High-Pass free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Digital Filter Designer: Low-Pass & High-Pass work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.