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Modern RF instrumentation is heavily dependent on switch technology. As both power consumption and space requirements shrink, selecting the appropriate switch solution becomes increasingly important. This article provides an overview of RF switches and considerations for selection based on function within the RF instrument.
OVERVIEW OF TECHNOLOGIES
Solid State Switches
Solid state switches can be divided into two primary categories: diode and field-effect transistor (FET). PIN diode switches are generally realized using discrete implementations and are known for higher power handling and fast switching speed, but the fairly complex biasing schemes and high levels of DC power required are significant disadvantages, especially for battery-operated instrumentation. PIN diodes require a forward current through the device to establish a low series resistance. This direct injection of DC current into the RF channel limits low-frequency operation.
FET-based switches are more commonly found in integrated solutions because the switching behavior is voltage-dependent. The control voltage applied to the gate of the FET "switches" the channel from a low-resistance "on" state to a high-resistance, capacitive "off' state. Three-terminal FET devices feature minimal DC power consumption and separate gate control of the channel. The high impedance of the gate supports a broadband response, but the FET is still frequency limited by finite capacitance between the RF channel and the gate terminal, as well as channel capacitance when the device is in the "off' state.
There are two distinct FET types commonly used in industry today: MESFETs and MOSFETs. MESFETs are fabricated using gallium arsenide (GaAs) and gallium nitride (GaN) processes, while MOSFET devices are commonly silicon-based. A primary distinction between the two types is the asymmetric behavior of the MESFET gate to applied voltage.
Electromechanical and MEMS Switches
Electromechanical (EM) switches use a different mechanism than their solid state counterparts. In an EM switch, a metal contact is actuated to make or break the connection. This technology offers the advantages of low insertion loss, high isolation, and high linearity.1 Emerging microelectromechanical systems (MEMS) switch technology attempts to deliver the advantages of traditional EM switches, but in a small form factor. MEMS switches employ micro-miniaturized mechanical contacts controlled by electrostatic forces to make RF connections.12
PERFORMANCE CATEGORIES
No single switch solution can satisfy all of the various requirements for each...