Calculator
Formula used
For a first-order RC high-pass filter with output across the resistor:
- fc = 1 / (2πRC)
- H(jω) = jωRC / (1 + jωRC)
- |H| = (ωRC) / √(1 + (ωRC)²)
- Phase = 90° − arctan(ωRC) = arctan(1 / (ωRC))
- Gain(dB) = 20·log10(|H|)
- Vout = Vin · |H|
How to use this calculator
- Select a mode, like cutoff, sizing, or response.
- Enter known R, C, and optional target cutoff frequency.
- Provide a single frequency or a list to test.
- Press Submit to see results above the form.
- Use CSV or PDF buttons to export your report.
Example data table
| R | C | fc | Test f | |H| | dB | Phase (°) |
|---|---|---|---|---|---|---|
| 1 kΩ | 0.1 µF | 1.59 kHz | 200 Hz | 0.125 | -18.06 | 82.87 |
| 1 kΩ | 0.1 µF | 1.59 kHz | 1.59 kHz | 0.707 | -3.01 | 45.00 |
| 10 kΩ | 10 nF | 1.59 kHz | 10 kHz | 0.987 | -0.11 | 9.04 |
Values are illustrative. Real circuits may vary with source/load impedance and component tolerances.
FAQs
1) What does a high-pass filter do?
It reduces low-frequency content and passes higher frequencies. In a simple RC design, the corner point is the cutoff frequency where the output reaches about 70.7% of the input.
2) Why is the cutoff called the −3 dB point?
At cutoff, the output magnitude is 1/√2 of the passband level. Converting 0.707 to decibels gives approximately −3.01 dB.
3) Which component should I change to shift fc?
Increase R or C to lower fc, and decrease either to raise fc. The relationship is inverse, so doubling R halves the cutoff frequency if C stays the same.
4) Why does phase start near 90 degrees?
At very low frequencies, the capacitor blocks current and the output lags less, producing a phase approaching +90°. As frequency increases, phase moves toward 0°.
5) Can I use this for active filters?
This tool models a first-order RC high-pass stage. Active high-pass filters can follow different equations depending on the topology, op-amp limits, and gain settings.
6) Why might real measurements differ from calculations?
Component tolerances, capacitor ESR, and source or load impedance shift the effective R and C. Wiring and measurement equipment also introduce stray capacitance and resistance.
7) What frequency list format works best?
Use numbers separated by commas or spaces, like “100 200 500 1000”. You may also type shorthand like “1k” or “2.2M” and it will be converted automatically.
8) What’s a practical way to pick parts?
Start with a convenient resistor range, compute the needed capacitor, and then choose the closest preferred value. Recheck fc and verify response at frequencies important to your design.