MEMS RF switch, working, types and advantages


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The RF MEMS switch, in essence, is a miniaturized version of the venerable toggle switch. MEMS switches can be a thin metal cantilever, an air bridge or a diaphragm; From the point of view of the configuration of the RF circuit, it can be connected in series or in parallel with an RF transmission line.

Working of Contact switches:

The basic structure of a switch in the MEMS series of contacts consists of a conductor beam suspended during an interruption in the transmission line. The application of DC polarization induces an electrostatic force in the beam, which lowers the beam through space, short-circuiting the open ends of the transmission line.

When the DC polarization is removed, the restoring force of the mechanical spring in the beam returns it to its suspended position (upwards). The losses in the closed circuit are low (dielectric and I2R losses in the transmission line and DC contacts) and the open circuit isolation of the gap of ~ 100 μm is very high up to 40 GHz. Because it is a contact switch Direct, it can be used in low-frequency applications without compromising performance.

Types of MEMS switches:

Capacitive switches:

In this case, the RF signal is shorted to ground by a variable capacitor. Specifically, for RF MEMS capacitive shunt switches, a grounded beam is suspended over a dielectric pad on the transmission line.

There is a thin layer of dielectric which separates two conducting electrodes. There is no direct metal to metal contact. The capacitive switch is mainly used in a shunt switch configuration where RF signal is shorted to ground by a variable capacitor.

Ohmic switches:

The basic structure has a reactive ohmic beam suspended during an interruption in the transmission line. The ohmic switch can be of two types, side and line. The signal range for the ohmic switches is DC at 40 GHz, with the upper limit established by the parasitic capacitance that is formed between the metallic contacts.

Advantages of MEMS switches:

  • Power consumption close to zero: the electrostatic actuation requires almost 30-80 V, but no current passes

  • Very low insertion loss: the RF MEMS metal contact and capacitive switches have an insertion loss of 0.1 db even at 100 GHz

  • Linearity and Inter-modulation products: MEMS switches are extremely linear devices, and therefore result in low inter-modulation products in switching and tuning operations. Their performance is far better than PIN diode or FETs.

  • Low cost potential: RF MEMS are manufactured with quartz or silicone, which are not expensive materials, but the cost of packaging is high because the net cost of RF switches increases