How do you design an RF attenuator?
A matched resistive attenuator reduces signal level while keeping input and output impedance at Z0. With K = 10A/20 for attenuation A in dB, a Pi pad has shunt resistors Rsh = Z0(K + 1)/(K − 1) and series resistor Rse = Z0(K² − 1)/(2K); a T pad has series arms Z0(K − 1)/(K + 1) and shunt Z0·2K/(K² − 1).
What values do common 50 Ω pads use?
| Attenuation | Pi shunt (Ω) | Pi series (Ω) | T series (Ω) | T shunt (Ω) |
|---|---|---|---|---|
| 3 dB | 292.4 | 17.6 | 8.5 | 141.9 |
| 6 dB | 150.5 | 37.4 | 16.6 | 66.9 |
| 10 dB | 96.2 | 71.2 | 26.0 | 35.1 |
| 20 dB | 61.1 | 247.5 | 40.9 | 10.1 |
Use nearest standard values and check return loss; thin-film chip attenuators are better above a few GHz.
Why use attenuators?
- Improve match: an attenuator of A dB improves return loss seen at its input by 2A dB.
- Protect instruments and set levels.
- Stabilize amplifiers and reduce mismatch ripple between stages.
What are variable attenuators?
PIN-diode attenuators provide analog control; digital step attenuators (DSAs) switch fixed pads with FETs in steps such as 0.25 or 0.5 dB. Their insertion loss, step accuracy, phase shift versus state, and IP3 are key specs.
Frequently asked questions
Does an attenuator add noise?
Yes; at 290 K its noise figure equals its attenuation.
Pi or T?
Both work; choice depends on available resistor values and layout.
How much power can a pad take?
The first resistor dissipates most; derate per manufacturer data.