Fixed attenuators are the cheapest, most useful RF component on any bench or board. They improve match, protect inputs, set gain, and isolate oscillators from load pulling. This note covers the numbers you need to design one from chip resistors.

1. Design equations (Z0 = 50 ohm, K = 10^(A/20))
Pi pad: shunt R = Z0 (K + 1)/(K - 1), series R = Z0 (K^2 - 1)/(2K)
T pad: series R = Z0 (K - 1)/(K + 1), shunt R = 2 Z0 K/(K^2 - 1)

Worked values:
- 3 dB: Pi shunt 292.4, series 17.6 ohm. T series 8.55, shunt 141.9 ohm.
- 6 dB: Pi shunt 150.5, series 37.4 ohm. T series 16.6, shunt 66.9 ohm.
- 10 dB: Pi shunt 96.2, series 71.2 ohm. T series 26.0, shunt 35.1 ohm.
- 20 dB: Pi shunt 61.1, series 247.5 ohm. T series 40.9, shunt 10.1 ohm.
Pick the topology that gives standard E96 values with the least error. At 3 dB, the 8.55 ohm T series resistor is awkward, so the pi version (287 or 294 ohm, 17.4 or 17.8 ohm) is easier. At 20 dB the T shunt of 10.1 ohm is fine, but the pi series of 247.5 ohm becomes more sensitive to its own parasitic capacitance.

2. The return loss bonus
A matched pad of A dB improves the return loss seen at its input by 2A dB, because the reflection passes through it twice. A load with VSWR 2.0 (return loss 9.5 dB, |Gamma| = 0.33) behind a 3 dB pad looks like 15.5 dB return loss (|Gamma| = 0.17, VSWR 1.40). This is why a 3 dB or 6 dB pad on a mixer port or a VNA test port often stabilises a measurement more than any amount of tuning.

3. Where the heat goes
Power is not shared evenly. For a 10 dB pi pad with 1 W in:
- Input shunt resistor: 0.52 W
- Series resistor: 0.33 W
- Output shunt resistor: 0.05 W
- Delivered to the load: 0.10 W
For a 3 dB pi pad with 1 W in, the series resistor takes 0.24 W, the input shunt 0.17 W and the output shunt 0.09 W. So derate the input-side parts and do not assume a pad with three 0.25 W resistors handles 0.75 W. If the pad can see power from either side (for example, between two amplifiers or in a reflectometer), size both shunt resistors for the worst case. Remember that chip resistor ratings assume a specific terminal temperature, typically 70 C.

4. Frequency limits
In 0402 thin-film parts, a pad built from discrete resistors stays reasonably flat to a few GHz. Above that, the series element's parasitic capacitance and the shunt vias to ground start to dominate. A 20 dB pad is the first to degrade, because its large series resistor is bypassed by about 0.03–0.05 pF. For work above about 6 GHz, use integrated thin-film attenuator chips or distributed designs, and simulate the ground vias.

5. Practical checks
- Measure S21 and S11 together. Good flatness with poor S11 usually means a via or pad-to-ground problem, not the resistor values.
- If an attenuator is used for power-level calibration, characterise it at the actual operating temperature.

What pad values do you keep on your boards as standard "tuning sites", and have you seen a pad burn on the input shunt first?

— rfforum 编辑部 / Editorial Team