by Tim Galla, Product Manager, Active RF/Microwave Components, Pasternack Enterprises
RF and microwave switches route signals through transmission paths with a high degree of efficiency. Four fundamental electrical parameters characterize how these switch designs perform.
Several electrical parameters are associated with the performance of RF and microwave switch designs, but four are considered to be of fundamental importance to the designer because of their strong interdependence: isolation, insertion loss, switching time and power handling.
Isolation is a measure of how effectively a switch is turned off. It’s the attenuation between the input and output ports of the circuit. Insertion loss, or transmission loss, is the total power lost through the switch in its “on” state. Insertion loss is the most critical parameter to a designer because it may add directly to the system’s noise figure. Switching time is the period a switch needs for changing state from “on” to “off” and “off” to “on.” This period can range from several microseconds in high-power switches to a few nanoseconds in low-power, high-speed devices. The most common definition of switching time is the time measured from 50% of the input control voltage (TTL) to 90% of the final RF output power. Power handling is the maximum RF input power that the switch can withstand without any permanent degradation in electrical performance.
RF and microwave switches can be categorized into two primary groups: electromechanical (EM) relay switches and solid state (SS) switches. There are several design configurations possible that can range from single-pole/single-throw (SPST) to single-pole/sixteen-throw or higher (SP16T), where one input can switch between 16 different output states. Transfer switches are double-pole/double-throw (2P2T) designs. They have four ports with two possible switch states and have the capability to switch a load between two sources.
EM RF switches are usually larger assemblies because they incorporate a series of coils and mechanical contacts. As with ordinary relays, electrically energized coils move the relay contacts. EM relay switches have low insertion loss (< 0.1 dB), high isolation (>85 dB), and can switch signals at speeds in the milliseconds. Major benefits are they can operate down to dc and up into millimeter wave frequencies (50+ GHz), and are not susceptible to electrostatic discharge (ESD). They can handle high power levels (up to several thousand watts of peak power) and have no video leakage.
There are some operational Issues to be aware of when it comes to EM RF switches. Their standard operating life can be limited to about one million cycles, and the assembly can be sensitive to vibrations. Operating life refers to the number of cycles the EM switch will complete while meeting all RF and repeatability specifications. Applications that need a higher operational lifetime can make use of higher quality or high-rel EM switches, which offer exceptional reliability and performance, with operating lives up to 10 million cycles. The longer life comes from a more ruggedly designed actuator and transmission link that has been optimized for magnetic efficiency and mechanical rigidity. They are also designed to withstand more stringent environmental conditions for sine and random vibration and mechanical shock in accordance with MIL-STD-2002.
For example, Pasternack Enterprises offers standard EM RF switches with operating lives of one million cycles, as well as high-rel EM RF switches that will last from 2.5 to 10 million cycles. One such device is the model PE71S6064, an SP2T high-rel switch design which operates from dc to 40 GHz and is guaranteed for up to 10 million cycles.
In contrast, solid-state RF switches have lower package profiles and are usually physically smaller than EM versions because the circuit assembly is planar and contains no bulky components. The switching element is either a high-speed silicon PIN diode, a field-effect transistor (FET), or integrated silicon or FET MMIC (monolithic microwave integrated circuit). These switchers are integrated onto a circuit board assembly with other discrete chip components like capacitors, inductors and resistors.
Switches that use PIN diode circuits can handle more power, and FET-based switches usually have faster switching speed. Of course, solid-state switches have no moving parts, so their operating life is infinite. They have high isolation levels (60 to 80+ dB), ultra-fast switching speeds (<< 100 nsec), and the circuit assembly resists shock and vibration quite well.
Other factors to consider with SS RF switches include their insertion loss. It is not as good as with EM versions and they are of limited use at low frequencies, with operation only down to the kilohertz range (they do not operate down to dc). This limitation comes from the inherent carrier lifetime properties of the semiconductor diode.
Also, SS RF switches are more sensitive to ESD. Their power handling capability depends on the switch design and connector type as well as operational frequency and temperature exposure. Some PIN-diode switch designs can handle power levels up to a few thousand watts of peak power, but there is a tradeoff with slower switching speed. An example is Pasternack’s model PE7167, an SP4T PIN diode switch design that operates from 500 MHz to 40 GHz and has a 100-nsec maximum switching speed with up to +20 dBm input power handling.
All in all, SS RF switches are more reliable, last longer and switch faster than EM versions. So they are preferred in applications where fast switching speed and high reliability are critical. Applications characterized by wide frequency coverage down to dc and a need for low insertion loss are candidates for EM RF switches, with high-rel versions preferred if longevity is absolutely essential.
Designers should also be aware of additional features related to these switch designs. 50-Ω resistive loads are one example. Any unused open transmission line in a switch circuit has the possibility to resonate at microwave frequencies. Resonance could cause power to reflect back to the active source and damage it, especially in a system that works at 26 GHz or higher where isolation drops off considerably. Many transmission lines are designed to have a 50-Ω impedance so RF switches that incorporate 50-Ω resistive loads reflect little energy.
EM RF switches are categorized as terminated or unterminated. In terminated versions, the selected path is closed when all paths are terminated with 50-Ω loads, so all current is cut off or isolated. Incident signal energy is absorbed by the terminating resistance so none reflects back. Unterminated switches do not have 50-Ω loads. So the impedance match must take place at some other part of the system to reduce reflections. But unterminated switches have the benefit of lower insertion loss.
Also important with EM RF switches is the armature relay mechanism. When the coil energizes, the induced magnetic field moves the armature coils, which open or close the contacts. Some models are non-latching and have a normally-closed initial position. The force of a spring or magnet holds the switch closed while no current flows. Normally-closed devices are useful for applications where a switch must return to a known state if power is lost.
Other models have latching mechanisms. They have no default position and maintain the last position without power applied. Latching relays are useful where power consumption and dissipation are an issue. The coil for a contact consumes power only for an instant while the relay switches off.
Some models have a fail-safe mode that always returns the RF path to the de-energized position when there is no voltage applied to the coil. But this feature requires that voltage be continuously applied to the coil to maintain the energized position. This sort of continuous energization can cause a lower mean time between failure (MTBF) than with latching switch designs.
Another feature to consider in EM RF switches consists of a set of auxiliary dc contacts linked to the coil that switches the RF paths. The aux contacts normally control indicators or pilot lights signaling the position of RF paths. They may also be used to give status information to an external control system.
SS RF switches can be categorized as absorptive or reflective. Absorptive switches use a 50-Ω load termination in each output port which results in a low VSWR (voltage standing wave ratio) in both on and off states. In the terminated port, the terminating resistance absorbs the incident signal energy that would otherwise reflect from an unterminated port. When there is a signal on the input that must propagate through the switch, the open port is disconnected from the termination so all the signal energy can propagate through. Absorptive switches are used in applications where it’s important to minimize reflections back to the RF source.
In contrast, reflective switches have no termination resistors. So their open ports have a lower insertion loss. Reflective switches go into applications where high off-port VSWR isn’t critical. The impedance match is provided at another point in the system.
Another important feature to consider with SS switches is the driver circuit. Some SS switch designs have integrated drivers with TTL logic states available for specific control functions. The TTL driver supplies all the necessary currents to ensure diodes have either reverse or forward bias voltage.
RF EM or SS switches come in a variety of package sizes and connector configurations. Most coaxial switch designs use SMA connectors for operation up to 26 GHz. Designs that operate up to 40 GHz typically use 2.92-mm or K connectors. Designs that work up to 50 GHz use 2.4-mm connectors, while designs of up to 65 GHz employ 1.85-mm connectors.
Switches with waveguide ports are widely used for high-power communication signals covering microwave and millimeter-wave frequency bands. They give the lowest possible insertion loss. Coaxial switch designs that handle more power (up to several hundred watts of CW power) might use larger N-Type or TNC connectors. Package styles can range from commercial grades which are not environmentally sealed, to high-rel grades which are hermetically sealed to withstand harsh conditions.