RFアッテネータ 計算ツール
T型、π型、ブリッジT型減衰器
Required Parameters
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Quick Answer
An RF attenuator reduces signal power by a precise dB amount while maintaining impedance matching. For a T-pad in 50 ohm: R_series = Z0 x (K-1)/(K+1), R_shunt = 2Z0K/(K^2-1), where K = 10^(dB/20).
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- R1 = Z0 x (K - 1) / (K + 1) K = 10^(A/20)
- R2 = 2 x Z0 x K / (K^2 - 1)
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Design Notes
Resistive attenuators are frequency-independent in theory, but parasitic inductance and capacitance of physical resistors limit the usable frequency range. Through-hole pads work to about 100 MHz; SMD 0402 pads extend to 6 GHz. For broadband precision, use thin-film attenuator chips rated for your frequency range. Always verify return loss (>20 dB) and ensure each resistor can handle its share of the dissipated power.
Common Mistakes
- 1
Using a simple voltage divider instead of a matched-impedance pad — this causes reflections and VSWR problems.
- 2
Ignoring resistor power dissipation — at high attenuation, nearly all input power is dissipated as heat in the resistors.
- 3
Using wirewound resistors at RF frequencies — parasitic inductance destroys the flat frequency response above a few MHz.
Engineering Handbox
1. Calculate K = 10^(6/20) = 10^0.3 = 1.995\n2. R_series = 50 x (1.995-1)/(1.995+1) = 50 x 0.332 = 16.6 ohm\n3. R_shunt = 2 x 50 x 1.995 / (1.995^2 - 1) = 199.5 / 2.98 = 66.9 ohm
Knowledge Base
πアッテネータの用途は？
特性インピーダンス（50Ωまたは75Ω）を維持しつつ信号電力を減衰。反射とVSWR問題を防止。
T型とπ型の選び方は？
T型：低いシャント容量（高周波向き）。π型：PCB実装が容易。1GHz以下では差は最小限。
T型の計算式は？
R_series = Z0×(K-1)/(K+1)、R_shunt = 2×Z0×K/(K²-1)、K = 10^(dB/20)。
抵抗の精度は？
測定用：1%金属皮膜以上。量産RF：20dB超は0.1%精密抵抗推奨。
カスケード接続は？
dB値を単純加算。3dB + 6dB = 9dB。各段がシステムインピーダンスに整合していること。
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