Cutoff Frequency
The time constant is related to the cutoff frequency fc, an alternative parameter of the RC circuit, by
or, equivalently,
where resistance in ohms and capacitance in farads yields the time constant in seconds or the frequency in Hz.
Short conditional equations:
- fc in Hz = 159155 / τ in µs
- τ in µs = 159155 / fc in Hz
Other useful equations are:
- rise time (20% to 80%)
- rise time (10% to 90%)
Standard time constants and cutoff frequencies
for pre-emphasis/de-emphasis RC filters:
| Organization |
Time constant in µs |
Cutoff frequency fc in Hz |
|---|---|---|
| RIAA | 7958 | 20 |
| RIAA, NAB | 3183 | 50 |
| — | 1592 | 100 |
| RIAA | 318 | 500.5 |
| — | 200 | 796 |
| — | 140 | 1137 |
| MC | 120 | 1326 |
| NAB | 100 | 1592 |
| MC | 90 | 1768 |
| RIAA, FM | 75 | 2122 |
| FM | 50 / 75 | 2122 / 3183 |
| NAB, PCM | 50 | 3183 |
| DIN | 35 | 4547 |
| — | 25 | 6366 |
| AES | 17.5 | 9095 |
| PCM | 15 | 10610 |
| — | 12.5 | 12732 |
| — | 10 | 15915 |
| Ortofon | 3.5 | 45473 |
| RIAA | 3.18 | 50000 |
In more complicated circuits consisting of more than one resistor and/or capacitor, the open-circuit time constant method provides a way of approximating the cutoff frequency by computing a sum of several RC time constants.
Read more about this topic: RC Time Constant
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