Switching RF And Microwave Signals

July 14, 2011
Switches for RF and microwave applications are fabricated with a number of different device technologies and available with performance levels in a wide range of signal-path configurations.

Switches control signal flows in RF/microwave circuits. They provide signal routing through a number of different configurations and technologies, and are characterized by many different parameters for comparison, including frequency range, insertion loss, switching speed, power-handling capability, power consumption, linearity, and on/off isolation. They can be as simple as a single-pole, single-throw (SPST) device to a complex multithrow switch or switch matrix. Selecting an RF switch is a matter of matching its electrical and mechanical characteristics to the needs of the application. RF/microwave switches come in many shapes, sizes, and technologies, from tiny integrated circuits (ICs) to larger, electromechanical switches. As with many comparisons of different technologies, solid-state and electromechanical switches each have their benefits and their shortcomings. Solid-state switches, which include discrete and IC components that employ PIN diodes and GaAs MESFETs as switching devices, are known for their long operating lifetimes, fast switching times, and small size. Electromechanical switches are larger and heavier, with considerably slower switching times, but boast excellent electrical performance.

The complexity of a switch is a function of the number of signal paths through the component. A relay or switch with a single path is known as a single-pole, single-throw (SPST) component, while a component used, for example, to connect a single antenna to four receivers is a single-throw, four-pole (SP4T) switch. The number of poles and throws will be dictated by the needs of an application. Switches can be designed to be absorptive or impedance matched to the surrounding circuitry when in their "off" state (high-isolation state), through the use of terminating resistors. They can also be configured as reflective short or reflective open when in the high-isolation state.

Switch electrical performance is characterized by a number of parameters, including its frequency range; the insertion loss when the switch is in the "on" state; the isolation when the switch is in the "off" state; its return loss; its switching speed from on to off or off to on; how well it is impedance matched (its VSWR) for use with associated components; its power-handling capability (usually defined as signal power where the insertion loss increases by 1 dB, or 1-dB compression); its linearity (usually in terms of its second- and/or third-order intercept point); and its durability or reliability, typically specified as the number of expected switching operations. For all their moving parts, for example, it is not unheard of for electromechanical switches to be rated for 100 million or more switching operations with minimal degradation in insertion loss.

Electromagnetic switches and relays make use of the magnetic field created by current flow to make or break a switch contact. They are typically comprised of a coil, some form of armature mechanism, and electrical contacts on the armature. When current flows through the coil, the armature moves due to the force of the magnetic field that is formed, and an electrical path is formed through the contact. When current flow ceases, the armature shifts back to its original position, and the electrical contact is broken. By providing adequate contact area, electromechanical switches can be made for high power levels. But as the contact size is increased, the size of the switch or relay also increases. Because of the mechanical motion of the armature, an electromechanical switch is limited in switching and settling time to about typically 5 to 20 ms. Electromechanical switches and relays are available in latching and nonlatching configurations. A latching switch requires continuous current flow to maintain a closed circuit; once closed, a nonlatching switch continues to maintain electrical connections even without current applied.

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A much faster form of signal-switching component is the PIN diode or GaAs FET switch, although both are limited in the amounts of power they can handle. A PIN diode is basically a current-controlled resistor. It consists of a high-resistivity intrinsic (I) region sandwiched between positive (P) and negative regions. With no current applied, a PIN diode is like a capacitor (high resistance, high isolation); with current applied, it is like an inductor (low resistance, low insertion loss). Increasing current leads to lower resistance and insertion loss. PIN diode switches can be designed for high-frequency use, to 40 GHz and beyond, with good linearity and low loss. They require driver circuitry for switching control, which contributes a great deal to the ultimate switching speed that is possible with a PIN diode switch.

In a GaAs FET, an RF signal flows from source to drain, with the gate providing the switching function. The off state (high impedance) is when the gate is fed a voltage that is more negative than the FET's pinchoff voltage. The on state (low impedance) is achieved by applying zero bias to the gate. A typical high-frequency switch is formed from multiple FETs in a series or shunt configuration. The current- or power-handling capability of a FET switch is linked to the gate periphery of the devices, essentially to the size of the transistors in the switch. GaAs FET switches, which suffer much less video leakage than PIN diode switches, are ideal for applications in which video leakage must be at a minimum, such as in measurement applications. As with a PIN diode switch, the switching speed is a function of the speed of the driver circuitry and the settling time of the semiconductor devices. The settling time is the amount of time needed for the active devices to reach a specific amplitude stability value, such as 0.1 dB or 0.01 dB.

In recent years, RF/microwave switches have been fabricated with other device structures, including silicon-on-sapphire (SOS) CMOS and microelectromechanical-systems (MEMS) technologies. Both approaches provide extremely small switches with good high-frequency performance and reasonable power-handling capabilities. Peregrine Semiconductor is a leading supplier of the former type of switch, using silicon-on-sapphire substrates and their patented UltraCMOS process technology to fabricate switches for use in 75- and 50-Ω systems through about 13.5 GHz. The firm recently announced a line of digitally tuned capacitors (DTCs) based on the same process technology.

The latter technology has been of particular interest to the US military, with generous funding provided by the Department of Defense's (DoD's) Defense Advanced Research Projects Agency (DARPA). Some of this has supported studies of electrical performance as well as operating lifetime. Several of these MEMS switch studies, performed on automated test systems, have demonstrated switching lifetimes in the billions for MEMS switches, including 900 billion switching cycles for devices from Radant MEMS and more than 100 billion switching cycles for devices from MEMtronics Corp.

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