RC Filter Calculator
Example Data Table
| Filter | R | C | Frequency | Input | Expected cutoff |
|---|---|---|---|---|---|
| Low-pass | 1 k ohm | 100 nF | 1 kHz | 5 V | 1591.55 Hz |
| High-pass | 10 k ohm | 10 nF | 500 Hz | 3.3 V | 1591.55 Hz |
| Low-pass with load | 2.2 k ohm | 47 nF | 2 kHz | 12 V | depends on load |
Formula Used
Time constant: τ = R × C
Ideal cutoff frequency: fc = 1 / (2πRC)
Angular cutoff: ωc = 2πfc
Capacitive reactance: Xc = 1 / (2πfC)
Low-pass gain: |H| = 1 / √(1 + (f / fc)²)
High-pass gain: |H| = (f / fc) / √(1 + (f / fc)²)
Output voltage: Vout = Vin × |H|
Gain in decibels: dB = 20 log10(|H|)
When load resistance is entered, the calculator uses complex impedance. This gives a more realistic output for loaded circuits.
How to Use This Calculator
Choose low-pass or high-pass mode first. Enter resistance, capacitance, frequency, and input voltage. Select the correct units for each value. Add load resistance when the next circuit stage affects the filter. Enter zero when no load is needed. Press Calculate. The result appears above the form and below the page header. Use CSV or PDF buttons to save the report.
RC Filter Guide
What an RC Filter Does
An RC filter uses one resistor and one capacitor. It changes a signal by frequency. Low frequencies and high frequencies do not pass equally. This makes the circuit useful in audio, sensors, timing circuits, and signal conditioning.
Low-Pass Operation
A low-pass RC filter passes slower signal changes. It reduces faster changes. The capacitor becomes easier for high frequency current to pass. So high frequency content is sent toward ground. The output then looks smoother. This is useful for noise reduction and simple averaging.
High-Pass Operation
A high-pass RC filter blocks steady voltage and slow changes. It passes quick changes better. The capacitor opposes low frequency current more strongly. This helps remove offset and drift. It is often used before amplifiers and measuring circuits.
Cutoff Frequency Meaning
The cutoff frequency is the main design point. At this frequency, the ideal gain is about 0.707. This equals about minus 3 dB. It marks the transition between pass band and roll-off region. Real circuits may shift because of load resistance.
Why Phase Matters
Phase shows time shift between input and output. A low-pass RC filter gives negative phase shift. A high-pass RC filter gives positive phase shift. Near cutoff, phase is important. It can affect feedback, waveform shape, and timing accuracy.
Using Load Resistance
A real output usually drives another stage. That stage has input resistance. This resistance loads the filter. Loading changes gain and cutoff. The calculator includes this option. Use the load field for better practical estimates. Use zero for ideal open output.
Design Advice
Start with the required cutoff frequency. Choose a common capacitor value first. Then solve for resistance. Keep resistance within a practical range. Very high values may collect noise. Very low values may waste power. Always check the output voltage at your working frequency.
Interpreting Results
The gain ratio shows output compared with input. The dB value shows attenuation. The time constant shows charging speed. The impedance value helps estimate loading on the source. These numbers together give a fuller design view.
FAQs
What is an RC filter?
An RC filter is a circuit made from a resistor and capacitor. It changes signal strength based on frequency. It can pass low frequencies or high frequencies depending on the connection.
What is cutoff frequency?
Cutoff frequency is the point where ideal output gain becomes about 0.707. This is also called the minus 3 dB point. It defines the filter transition area.
What is a low-pass RC filter?
A low-pass RC filter passes low frequency signals. It reduces high frequency signals. It is often used for smoothing, noise reduction, and sensor signal cleanup.
What is a high-pass RC filter?
A high-pass RC filter passes high frequency signals. It reduces low frequency signals and direct voltage. It is useful for coupling and offset removal.
Why does load resistance matter?
Load resistance changes the effective circuit resistance. This can shift cutoff frequency and reduce output voltage. Use the load field when another device is connected.
What does phase angle mean?
Phase angle shows how much the output waveform leads or lags the input waveform. It matters in timing, feedback circuits, audio filters, and waveform analysis.
Can this calculator handle practical circuits?
Yes. It includes load resistance, impedance, reactance, gain, phase, output voltage, and step response. These options help model many practical RC filter cases.
How can I export the results?
After calculation, use the CSV button for spreadsheet data. Use the PDF button for a simple report. Both options use the current calculated result.