RF MEMS Switch Fundamentals
RF MEMS (Micro-Electro-Mechanical System) switches use microscale mechanical contact or capacitive bridges to route RF signals. They offer superior linearity and extremely low insertion loss compared to PIN diode or FET switches, at the cost of slower switching speed and high actuation voltage.
MEMS vs. Semiconductor Switches
| Parameter | RF MEMS | PIN Diode | FET Switch |
|---|---|---|---|
| Insertion loss | 0.1–0.5 dB | 0.5–1.5 dB | 0.5–2 dB |
| Isolation (OFF) | 30–50 dB | 20–40 dB | 20–35 dB |
| P1dB | +30 to +40 dBm | +20 to +30 dBm | +15 to +25 dBm |
| Switching speed | 1–100 μs | 1–100 ns | 1–100 ns |
| Control voltage | 20–80 V | 0–5 V | 0–5 V |
| Power consumption | ~0 (latching) | 10–100 mW | <1 mW |
S-Parameter Analysis: ON State
In the closed (ON) state, the MEMS switch should look like a through connection:
- S21 (insertion loss): −0.1 to −0.5 dB, flat across band
- S11 (input match): <−20 dB (well matched)
- Phase of S21: nearly linear (good group delay flatness)
S-Parameter Analysis: OFF State
In the open (OFF) state:
- S21 (isolation): <−30 dB, may degrade at higher frequencies due to capacitive feedthrough
- S11: approaches −∞ dB at resonance with parasitic L (series MEMS) or near 0 dB (shunt MEMS)
RF View allows you to load both ON-state and OFF-state .s2p files simultaneously and overlay the S21 traces to measure the switching ratio (ON insertion loss − OFF isolation) vs. frequency.
Reliability Considerations
MEMS switches degrade through contact wear (series type) or dielectric charging (capacitive type). Periodically measuring S-parameters and comparing against baseline files in RF View detects degradation before failure. An increase in ON-state S21 loss exceeding 0.3 dB indicates contact degradation.