フィルタカットオフ周波数 計算ツール
RC/RLフィルタの-3dBカットオフ
Required Parameters
Waiting for input data...
Quick Answer
The -3dB cutoff frequency (fc) is where the signal power drops by half. For RC filters: fc = 1 / (2πRC). For RL filters: fc = R / (2πL).
銉曘偅銉偪銈儍銉堛偑銉曞懆娉㈡暟 瑷堢畻銉勩兗銉玚n
鍙楀嫊RC銇娿倛銇矻C銉曘偅銉偪銇?*-3dB銈儍銉堛偑銉曞懆娉㈡暟**銈掕▓绠椼仐銇俱仚銆俙n
RC銉曘偅銉偪銇偒銉冦儓銈儠
fc = 1 / (2pi x R x C)
LC銉曘偅銉偪銇偒銉冦儓銈儠
fc = 1 / (2pi x sqrt(L x C))
銉曘偅銉偪銇ó椤瀈n
| 绋 | 閫氶亷 | 閬柇 |
|---|---|---|
| 銉兗銉戙偣 | fc浠ヤ笅 | fc浠ヤ笂 |
| 銉忋偆銉戙偣 | fc浠ヤ笂 | fc浠ヤ笅 |
| *銉愩兂銉夈儜銈? | f1-f2 | 甯煙澶? |
| *銉愩兂銉夈偣銉堛儍銉? | 甯煙澶? | f1-f2 |
闁㈤€ｃ儎銉笺儷
- *銉偄銈偪銉炽偣瑷堢畻銉勩兗銉?
- 鏅傚畾鏁拌▓绠椼儎銉笺儷
- *555銈裤偆銉炪兗瑷堢畻銉勩兗銉?
Design Notes
First-order passive filters only roll off at -20dB per decade. They are great for simple noise filtering, anti-aliasing before a slow ADC, or debounce circuits, but inadequate for separating close frequencies. When designing RC filters, avoid using extremely large resistors (e.g., >1MΩ) because the input bias current of the next stage (like an Op-Amp or ADC) will cause a DC voltage error. Conversely, avoiding extremely small resistors (<100Ω) will save power and prevent overloading the driving source.
Common Mistakes
- 1
Expecting a sharp cutoff from a simple RC filter. It's a gentle curve, not a brick wall.
- 2
Not balancing the source impedance. If your signal source already has 10kΩ output impedance, and you use a 1kΩ series resistor for your filter, your effective R is 11kΩ.
- 3
Ignoring capacitor tolerances. Cheap Y5V or Z5U ceramic capacitors can vary by +20%/-80% with temperature and DC bias, completely ruining your cutoff frequency.
Engineering Handbox
1. Identify variables: R = 10,000, C = 0.0000001 2. Multiply R × C = 0.001 seconds (Time Constant τ) 3. Multiply by 2π: 0.001 × 6.283 = 0.006283 4. Divide 1 by result: 1 / 0.006283 = 159.15 Hz
Knowledge Base
-3dBカットオフ周波数とは？
出力電力が入力の半分（電圧が70.7%）に低下するポイント。電子工学の標準的な定義です。
RCローパスフィルタの公式は？
fc = 1/(2π×R×C)。例：R=10kΩ、C=100nF → fc = 159.15 Hz。
ローパスとハイパスの違いは？
ローパス：低周波を通過、高周波を減衰。ハイパスは逆。同じカットオフ公式で部品配置が異なります。
1次フィルタの減衰傾斜は？
-20 dB/ディケード（-6 dB/オクターブ）。急峻にするには多段カスケード。
フィルタに適したコンデンサは？
精密用：C0G/NP0。一般用：X7R。Y5V/Z5Uは温度・電圧で容量が大きく変動するため避けてください。
Related Engineering Tools
Reactance Calculator
Inductive and capacitive reactance
Time Constant Calculator
RC and RL transient constant
Series and Parallel Capacitor Calculator
Equivalent capacitance from a capacitor list
Frequency to Wavelength Conversion
Convert frequency and wavelength with velocity factor