RF switches are used extensively in radio-frequency and microwave test systems to route signals between instruments and devices under test (DUT) and change routing without having to connect or disconnect equipment. Adding RF switches at key points allows the designer to automate the test flow, increasing throughput in high-volume production environments.
Two RF switch technologies are commonly available, each with different performance characteristics.
Electromechanical (EM) RF switches route the signal between input and output using a relay: an electrically-controlled mechanical switch. EM RF switches have low insertion loss when closed and high port-to-port isolation when open, so they can transmit high-power, high-frequency signals; 200 W at 1 GHz, for example. As a mechanical technology, EM RF switches are relatively slow. They can change states in 10 milliseconds (ms) typically. RF switches can pass signals over an extremely wide frequency range, from DC up to 40 GHz, depending on model. They do exhibit mechanical wear that limits their lifetime to 1 million operations typically.
Solid-state RF switches use semiconductor technology to switch the signals between input and output. They have fast switching speed (a few microseconds), high reliability (MTBF 250,000 hours typically) and can pass frequencies from 0.5 MHz to 18 GHz depending on model. Solid-state switches have no mechanical wear, so the insertion loss repeatability is excellent. On the other hand, solid-state switches have lower isolation (40-80 dB), lower power levels and higher harmonic distortion than their EM counterparts.
Figure 2 compares the specifications for a solid-state and an EM RF switch. Both are single-pole, double-throw reflective switches, but there are significant differences in performance that may determine the best fit for a particular application.
RF Switch Glossary of Terms
Here are definitions for some common terms:
Reflective switch: Switch ports are high impedance (open) when turned off.
Absorptive switch: Switch ports are internally terminated with an impedance when turned off. The impedance is 50 Ω or 75 Ω, depending on model.
Self-terminating switch: Another term for absorptive switch.
Failsafe switch: With no power applied, the common port is connected to a specified output port. In a 1P2T switch, this is the “NC” port.
Failsafe, absorptive switch: With no power applied, the common port is connected to a specified output port and all other ports are internally terminated with the specified impedance.
Normally open (NO) switch: With no power applied, the common port isn’t connected to any of the output ports.
Cold switch: No signal is applied to the switch while the switch is changing states.
Hot switch: The switch changes states while a signal is still being applied to the switch.
Switching speed: Port turning on equals 50 percent transistor-transistor logic (TTL) control to 90 percent of the RF signal. Port turning off equals 50 percent TTL control to 10 percent of the RF signal.
Rise time: Port turning on equals 10 percent of the RF signal to 90 percent of the RF signal.
Fall time: Port turning off equals 90 percent of the RF signal to 10 percent of the RF signal.
RF Switch Frequently Asked Questions (FAQ)
Here are the answers to some frequently asked questions about RF switches:
Q: What RF switch configurations are available?
A: Not every configuration is available in every frequency range or power level, but here are the choices in the JFW product family:
Electro-mechanical switches: 1P2T (single-pole, double-throw); 1P3T; 1P4T; 1P6T; 1P8T; 1P12T
Solid-state switches: 1P1T; 1P2T; 1P3T; 1P4T; 1P6T; 1P8T; 1P10T; 1P12T; 1P16T
Q: What control options are available?
A: Depending on model, EM RF switches offer TTL logic-level control, or +12, +15 or +28 V DC control. Solid-state switches use TTL logic levels, and one model has a USB interface.
Q: Are the RF switch attenuators bi-directional?
A: All of JFW’s RF switches are bi-directional unless noted differently on the specification sheet.
Q: I need a custom RF switch designed. How much are your typical NREs (non-recurring engineering charges)?
A: JFW does not charge NREs. Please send your custom requirement to firstname.lastname@example.org.
Q: Are your 18GHz electro-mechanical switches available with indicators or as latching?
A: Typical options: latching, self-terminating, indicators, TTL, RoHS and non-RoHS.
About JFW Industries
JFW Industries offers a large number of RF switches for low-, medium- and high-power applications. Other product families for RF testing include: fixed and programmable attenuators; terminations; power dividers and combiners; attenuator switching systems; and systems to test cellular, radio transceiver and other RF applications.
Find out more at www.jfwindustries.com.