How to Select RF Switches
Image credit: AKON Inc. | JFW Industries | Mouser
RF switches are designed to route radio frequency (RF) signals between various inputs and outputs. They are very commonly used in RF and microwave test systems for routing signals between instruments and the devices under test (DUT). By using RF switches, multiple tests can be performed without changing the setup, resulting in increased efficiency due to infrequent connecting and disconnecting.
The use of RF switches to simplify complex signal routing can be illustrated by considering matrix switching. The image below shows the use of six single pole, triple throw (1P3T) switches to route three RF signals. The colored lines denote the path of each input signal; by adjusting each switch the signals can be rerouted via any of the black lines in the image. Because this design uses only switches and does not require power dividers or attenuators, it features low insertion loss and very high signal isolation.
Image credit: JFW Industries
RF switches can be classified into two types, based on their switching action:
Electromechanical switches rely on the physical movement of mechanical contacts to make or break circuits.
Solid state switches make or break circuits using semiconductor technology such as diodes and transistors.
Electromechanical switches have metal contacts which can either be physically open — meaning that no current or signal will flow through them — or closed, resulting in the completion of the circuit and allowing current to flow. This type of switch can be described in terms of the number of circuits which can be connected — referred to as "poles" — and the number of distinct positions — or "throws" — the switch can adopt. For example, a single pole, single throw (SPST) switch is a simple on-off device: it simply makes or breaks one circuit. A double pole, single throw (DPST) switch is a slightly more complicated on-off switch in that it makes or breaks two connected circuits simultaneously. In contrast to these two switch types, a double throw (DT) switch is capable of switching between two separate circuit paths. Several electromechanical switch schematics are shown below.
(left to right) SPST; SPDT; DPST. Image credit: Wikipedia
Electromechanical switches can be used over wide frequency ranges and feature low signal loss and minimal cross talk between channels. Because electromechanical switches require physical movement to operate, their use is accompanied by several disadvantages as well:
Unreliable or slow switching due to contact bounce
Arcing between contacts
Electromagnetic interference caused by contact sparking; often occurs when switching inductive loads
Solid State Switches
Solid state switches are semiconductor devices which do not require mechanical switching to operate; for this reason they typically switch very quickly. Due to their superior resistance to shock, vibration, and mechanical wear, they are regarded as more reliable and long-lasting than their electromechanical counterparts. Solid state switches are also prone to higher insertion loss when compared to electromechanical switches.
Solid state switches may use three primary types of semiconductor technology to effectively switch signals.
PIN diodes act as variable resistors at RF and microwave frequencies. They are current-controlled, meaning that varying amounts of input current produce lower resistance (ON or "make" state) or higher resistance (OFF or "break"). PIN diodes are able to control large RF signals using much smaller control current levels. Despite this fact, PIN diode switches are typically limited to lower frequency (< 1 MHz) operation. Within this range, they tend to provide the best isolation characteristics at higher frequencies. The image below shows three different SPDT PIN diode switch schematics: series PIN (top), shunt PIN (middle), series-shunt PIN (bottom).
Image credit: Microwave Journal
Field effect transistor (FET) switches use semiconductor devices that control the shape — and hence the conductivity — of a channel in the semiconductor material. FET switches consume less current and have higher insertion loss than PIN diode types, and feature excellent isolation at low frequency ranges; although they also switch more slowly than PIN diode switches. Isolation characteristics tend to degrade at higher frequencies because of drain-to-source capacitance.
An SPDT FET switch. Image credit: Microwave Journal
Hybrid switches are designed to incorporate both PIN diodes and FETs as well as the inherent advantages of both switch types. They consume moderate amounts of power, have excellent isolation at high frequencies, and average switching speeds.
In addition to solid state and electromechanical types, some RF switches may be manually operated. These devices require a human operator to turn a knob or move a slide in order to switch. Manual switches are typically capable of handling high power signals and are frequently used to switch between multiple antennas and RF transmitters/receivers.
Impedance and VSWR
Impedance and voltage standing wave ratio (VSWR) are two important specifications to consider when selecting RF switches.
Impedance describes a circuit's opposition to the passage of current when a voltage is applied and is measured in Ohms (Ω). In order to maintain maximum signal transmission, the input signal's impedance (also known as source impedance) must match the switch's load impedance. Common electrical impedances include 50 and 75 Ω.
Voltage standing wave ratio (VSWR) measures the efficiency of an electrical transmission. When impedance mismatches occur in transmission lines, RF energy is reflected back to the source, resulting in a smaller percentage of the original transmission reaching its intended destination. Buyers specify VSWR as a single number, which represents the first half of the ratio indicating the relative size of the reflected transmission waves. For example, a VSWR of 1.2 can also be expressed as 1.2:1. A VSWR of 1 is ideal and results in 100% transmission efficiency. As impedance mismatch increases, the VSWR also increases as the transmission efficiency decreases.
RF switches may be manufactured, tested, and used based on various standards. A few examples of standards pertaining to RF switches are:
MIL-DTL-25879 (RF switch for coaxial transmission lines)
MIL-DTL-83739 (Solid state RF antenna switch)
SMD 5962-98553 (16-bit FET switch)