This Electronics Engineering Seminar Topic deals with the following:
Compound solid state switches such as GaAs MESFETs and PIN diodes are widely used in microwave and millimeter wave integrated circuits (MMICs) for telecommunications applications including signal routing, impedance matching networks, and adjustable gain amplifiers. However, these solid-state switches have a large insertion loss (typically 1 dB) in the on state and poor electrical isolation in the off state. The recent developments of micro-electromechanical systems (MEMS) have been continuously providing new and improved paradigms in the field of microwave applications. Different configured micro-machined miniature switches have been reported. Among these switches, capacitive membrane microwave switching devices present lower insertion loss, higher isolation, better nonlinearity and zero static power consumption. In this presentation, we describe the design, fabrication and performance of a surface micro-machined capacitive microwave switch on glass substrate using electroplating techniques.
RF MEMS TECHNOLOGY
Basically RF MEMS switches are of two configurations-:
- RF series contact switch
- RF shunt capacitive switch
Currently, both series and shunt RF MEMS switch configurations are under development, the most common being series contact switches and capacitive shunt switches.
RF Series Contact Switch
An RF series switch operates by creating an open or short in the transmission line, as shown in Figure 1. The basic structure of a MEMS contact series switch consists of a conductive beam suspended over a break in the transmission line. Application of dc bias induces an electrostatic force on the beam, which lowers the beam across the gap, shorting together the open ends of the transmission line1. Upon removal of the dc bias, the mechanical spring restoring force in the beam returns it to its suspended (up) position. Closedcircuit losses are low (dielectric and I 2 R losses in the transmission line and dc contacts) and the open-circuit is olation from the ~100 μm gap is very high through 40 GHz. Because it is a direct contact switch, it can be used in lowfrequency applications without compromising performance. An example of a series MEMS contact switch, the Rockwell Science Center MEMS relay, is shown in Figure 2.
RF Shunt Capacitive Switch
A circuit representation of a capacitive shunt switch is shown in Figure 3. 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 (see Figure 4). When the beam is in the up position, the capacitance of the line-dielectric-air-beam configuration is on the order of ~50 fF, which translates to a high impedance path.