About

This tool designs first-order RC filters using standard component values. The results apply to both low-pass and high-pass filters, since their cutoff frequencies follow the same formula.

Instructions

To design an RC filter, enter your desired cutoff frequency (fc). The results will appear in the table below your inputs. Each row shows one result, containing the cutoff frequency (fc), resistor value (R), capacitor value (C), and cutoff frequency error. The results are sorted from best to worst by the error in cutoff frequency.

By default, the resistor values are selected from the (E24) series of standard values - a set of 24 values commonly available in 5% and better tolerances. Capacitors are selected from the E6 series. You can select a different series for either component using the R series and C series menus. The results in the table will update automatically.

Finally, the range of resistance (R) values are restricted to 1k to 10k by default. You can define your own range by adjusting Min. R Value and Max. R Value.

Tips

To get results with more common resistor values, try changing to a smaller resistor series such as E12.

There are several tradeoffs to consider in selecting the range of resistance values - Min. R Value and Max. R Value. Here are some guidelines:

1. The filter's input impedance approaches R at frequencies well above fc. Thus filters with smaller resistance values require more drive current.
2. While larger resistor values reduce drive current, they make the circuit more prone to electrical signal pickup.
3. You'll also want to be sure that the load connected to the output of the filter has an impedance that is far greater (such as 10x higher) than the resistance value.
4. Using a larger resistor range increases the amount of solutions available, which may help to better match your target cutoff frequency.

Theory

An RC (resistor-capacitor) filter is a simple type of filter that can pass certain frequencies while blocking others. Both low-pass and high-pass filters can be built with a resistor and capacitor as shown below.

A low-pass RC filter allows low-frequency signals to pass through while attenuating high-frequency signals. As frequency increases, the capacitor's impedance decreases, causing more signal to be dropped across the resistor. The cutoff frequency $f_c = 1/(2πRC)$ marks where signal power drops to half its original value. This half-power point corresponds to a voltage ratio of: $V_{out}/V_{in} = \sqrt{2}$. Low-pass filters are commonly used in audio systems to remove high-frequency noise, smooth DC power supplies, and as anti-aliasing filters in digital sampling.

A high-pass RC filter blocks low frequencies while allowing high frequencies to pass. At low frequencies, the capacitor acts like an open circuit, blocking signals. As frequency increases, more signal passes through to the output. High-pass filters share the same cutoff frequency formula as low-pass filters. They're used to remove DC offsets, reduce low-frequency noise like power line hum, and in audio crossovers to direct high frequencies to tweeters.

References

1. Horowitz, P., & Hill, W. (2015). The Art of Electronics (3rd ed.). Cambridge University Press. (See especially section 1.7.8 & 1.7.9 for discussion of RC filters.)