RF switch basics

There are three types of RF switches. Electromechanical, solid-state, and MEMS.

The operation of an electromechanical switch is based on the simple theory of electromagnetic induction. This switch relies on mechanical contacts for its switching mechanism. In electromechanical switches, the control signal causes the contact to physically change position during the switching process. These types of switches can handle high-power RF signals since they have low insertion loss and high isolation. Even though RF applications can benefit from these features, the disadvantages are that they can be bulky, heavy, and very slow (switching speed in the millisecond range). These part are used mostly in industrial and T&M applications.

Unlike electromechanical switches, nothing moves in the solid-state switch, since it is based on semiconductor technology. This electronic switching device is faster (switching speed in the nanosecond range), smaller, and lighter. These parts are inferior with regard to insertion loss, dc power consumption, isolation, and power handling compared to electromechanical switches. A switch of this kind is made from diodes (low insertion loss) or transistors (faster switching time).

The MEMS switch is a new type of RF switch. It promises better properties over the electromechanical and semiconductor switches. MEMS switches offer the high RF performance and low DC power consumption of electromechanical parts, and the small size, weight, and low-cost features of semiconductor parts.

MEMS switches are surface-micromachined devices that use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission line. These switches are designed to operate at RF to millimeter-wave frequencies (0.1 to 100 GHz). The advantages of MEMS switches over PIN diode or FET switches are:

• Near-zero power consumption

• Very high isolation

• Very low insertion loss

RF MEMS switches also have their disadvantages, and these include:

• Relatively low speed

• High-voltage or high-current drive

RF MEMS switches can replace the SP2T and SP3T switches (implemented by semiconductor technology) that are used in today’s dual-band and triple-band cell phones. Here the benefit is improvement in RF performance, which will reduce dc power consumption.

These types of switches would also benefit satellite applications, which not only demand high switching performance, but also mass and volume reduction. Another possible use is in beam-forming networks, such as in the design of reconfigurable Butler matrices and phase shifters for multibeam satellite communication systems. Down the road, MEMS switches should become more advantageous as frequency is increased.