RF/Microwave Solid State Switches

                                                    RF/Microwave Solid State Switches

Introduction
Solid state switches are ubiquitous in modern RF/microwave systems. They are utilized to control signal flows, select signal sources and for many other applications. These switches are implemented either with positive-intrinsic-negative (PIN) diodes or with field effect transistors (FETs) such as pseudomorphic high electron mobility transistors (pHEMTs), each of which offers relative advantages and disadvantages. Part 1 of this article includes an overview of RF/microwave switches, theory of operation for PIN diodes and some representative PIN diode switch circuits. In Part 2 we will discuss more PIN diode switch configurations, the theory of operation for FETs, some representative RF/microwave FET switch topologies, the relative advantages of PIN and FET switches and some criteria by which one of these technologies may be selected over the other for switch applications.

RF/Microwave Solid State Switches Overview
RF/microwave switches utilize variable impedance circuit elements to direct the flow of signals. A switch either allows a signal to propagate through a specific signal path or it blocks the signal from following this path. In an ideal switch, circuit paths are either closed via an ideal conductor with 0 Ω impedance or opened by an ideal open circuit with infinite impedance. Modern semiconductor elements cannot quite meet those ideal impedance values, but they can come sufficiently close to produce excellent switch performance.

Reflective or Absorptive?
Switches in the RF realm can be implemented as reflective or absorptive structures.
An ideal reflective switch places either an open circuit in cascade with a signal path (Figure 1), or a short circuit across the signal path (Figure 2).

These two extreme impedances produce maximum mismatch of impedance, which produces reflection of the entire incident signal back to its source.

An ideal absorptive switch (sometimes called a "matched switch") produces isolation by placing a termination whose resistance is exactly equal to the characteristic impedance, Z0, of the transmission line across the transmission line, as shown in Figure 3. In this case, all of the incident signal energy is absorbed by the terminating resistance and dissipated as heat, leaving no remaining energy to be reflected back to the signal source. In the other state, the termination is disconnected from the transmission line, thereby allowing all of the incident energy to propagate through the switch.

Common Switch Configurations Switches may be implemented in many configurations. These configurations are described in terms of the number of poles and the number of throws implemented in the switch. The number of poles describes the number of signal paths controlled by the switch. The number of throws indicates the number of potential directions into which a pole may be placed. For example, the simplest switch configuration is a single pole, single throw (SPST) switch. This configuration has one signal path which can either be completed by the switch or interrupted by the switch. A single pole double throw switch (SPDT or SP2T) can connect a single transmission line to either of two other transmission lines. The number of poles and throws, and the combinations thereof, is unlimited in the ideal sense, but has practical limitations that will be described later in this paper. Ideal vs. Practical Switches In practical solid state RF/microwave switches, it is not possible to produce a perfect open impedance nor a perfect short circuit. Consequently, there is always some small amount of incident signal that is absorbed by the switch and a bit more reflected by the switch's nonideal impedance when the switch is in the state in which it should ideally pass all incident signal energy. This small reduction in signal amplitude is known as insertion loss (IL) and is typically described in terms of decibels (dB). Insertion loss is simply the ratio of the output power to the input power. Likewise, there is always some small amount of energy that propagates past the switch when it is in the state in which an ideal switch should produce infinite isolation. The measurement of this characteristic is known as isolation and is also described in terms of dB. Isolation is also the ratio of the output power to the input power. Multiple semiconductor elements can be used in a single switch to increase the isolation that the switch produces. Often, these elements are placed in series with the signal path and in shunt with the path. Absorptive switches typically include multiple switching elements: some to complete or interrupt the signal path and others to disconnect or connect the termination resistance to the signal path. PIN Diode Theory of Operation1 The PIN diode is a current controlled resistor at radio and microwave frequencies. It is a semiconductor diode in which a high-resistivity intrinsic I region is sandwiched between a P-type and an N-type region, as shown in Figure 4. When the PIN diode is forward biased, holes and electrons are injected into the I region from the P and the N layers, respectively. These charges do not immediately annihilate each other; instead they stay alive for an average time, called the carrier lifetime, τ or TL. This results in an average stored charge, Q, in the I region which lowers the effective resistance of the I region to a value RS. When the PIN diode is at zero or reverse bias (and assuming it was not forward biased in the immediate past – more about this later) there is no stored charge in the I region and the diode appears as a larger impedance comprising a capacitance, CT, shunted by a parallel resistance, RP. PIN diodes are specified for the parameters listed in Table 1. By varying the I region width and diode area, it is possible to construct PIN diodes of different geometries to result in the same RS and CT characteristics. These devices may have similar small signal characteristics. However, the thicker I region diode would have a higher bulk or RF breakdown voltage and better distortion properties. On the other hand, the thinner device would have faster switching speed. There is a common misconception that τ is the only parameter that determines the lowest frequency of operation and the distortion produced by the diode. This is indeed a factor, but equally important is the thickness of the I region which relates to the transit time frequency of the PIN diode. Neyker Stewart Zambrano CRF http://www.mpdigest.com/issue/Articles/2009/may/skyworks/Default.asp