T-Pad Topology
Port 1 ─── R1 ─── Node ─── R3 ─── Port 2
│
R2
│
GND
For a symmetric attenuator (Z_in = Z_out = Z₀): R1 = R3
Let K = 10^(A_dB / 20) [voltage attenuation ratio]
R1 = R3 = Z₀ · (K − 1) / (K + 1) [series arms]
R2 = Z₀ · 2K / (K² − 1) [shunt arm]
Step-by-Step: 6 dB Attenuator at 50 Ω
K = 10^(6/20) = 1.995
R1 = R3 = 50 × (1.995 − 1)/(1.995 + 1)
= 50 × 0.995 / 2.995 = 16.6 Ω
R2 = 50 × 2×1.995 / (1.995² − 1)
= 50 × 3.990 / 2.980 = 66.9 Ω
Choose nearest E96 values: R1=R3=16.5 Ω, R2=66.5 Ω
Resulting attenuation: A = 5.98 dB (error < 0.1 dB)
Standard Attenuator T-Pad Values (Z₀ = 50 Ω)
| A (dB) | R1=R3 (Ω) | R2 (Ω) | S11 (dB) |
|---|---|---|---|
| 1 | 2.88 | 433.3 | −∞ (matched) |
| 3 | 8.55 | 141.9 | −∞ |
| 6 | 16.6 | 66.9 | −∞ |
| 10 | 26.0 | 35.1 | −∞ |
| 20 | 40.9 | 10.1 | −∞ |
| 30 | 48.4 | 3.17 | −∞ |
Ideal T-pad: S11 = S22 = 0 (perfect match), S21 = −A dB. In practice, connector and pad parasitics cause S11 to degrade at higher frequencies.
PCB Layout for RF Attenuators
- Use 0402 thin-film resistors for best RF performance above 1 GHz
- Match all resistors to ±1% for attenuation accuracy <0.5 dB
- Minimize trace lengths between resistors — parasitic inductance creates resonances
- Ground via directly below R2 shunt resistor — minimum inductance path
- For >6 GHz: use short stub compensation for pad capacitance at T-junction
RF View Attenuator Calculator: Compute T-pad and Pi-pad resistor values for any attenuation and Z₀ in RF View's Utilities tab. Load result directly into the Circuit Simulator for verification. Free on Android.