What are RF Relays?
RF relays are electrically operated switches that route high frequency signals through various transmission paths. These relays have lots of applications and are found in computers, test equipment and radio broadcast systems. The need for RF switching circuits has been increasing with wireless communications that operate at higher frequencies and require wider bandwidths. RF relays/switches are mostly integrated in microwave test systems for signal routing between instruments and devices under test (DUT).
Types of RF Relays
RF Relays can be categorized into two types - Solid-State Relays and Electro-mechanical Relays.
Electro-mechanical RF Relays: These are based on electromagnetic induction. Their switching mechanism relies on mechanical contacts. Electromechanical RF Relay switches are further categorized as terminated or unterminated. An RF Relay is termed as terminated when all closed paths are terminated with 50-ohm loads thereby resulting in less reflections. Unterminated RF Relays are just left open and not terminated with 50 ohm loads so these ones reflect power.
Solid-State RF Relays: These are electronic switching devices which are based on semiconductor technology such as MOSFETs and PIN diodes. Most modern RF Relays are solid state. They are smaller, more reliable, have a longer life and consume less power. These are categorized as absorptive or reflective. Absorptive switches integrate a 50-ohm termination in each of the output ports to present a low VSWR in both the OFF and ON states. Reflective switches conduct RF power when the diode is reverse biased and reflect RF power when forward biased.
Key Parameters of an RF Relay Switch
Configuration: This can range from SPST (Single Pole Single Throw) to SPnT (Single Pole ‘N’ Throw) where n is the number of paths or switching states a relay can support. Transfer switch relays are also an option. So this would be 2P2T which would have four ports with two possible switch states and have the capability to switch a load between two sources.
Type: Solid State or Electro-Mechanical
Frequency: Typically, frequency is always application dependent. Here, for solid-state RF Relays the frequencies range from DC - GHz, and for electro-mechanical RF Relays, range from kHz – GHz.
Insertion loss: Insertion loss is a critical parameter for RF Relays. The lower the insertion loss the better. It is represented in dB. Electromechanical RF Relays usually provide the lowest possible loss as compared to solid state relays.
Return Loss: Return loss is normally caused by an impedance mismatch between circuits. RF Relay switches typically provide good to excellent return loss performance, hence, ensuring optimum power transfer through the switch and the entire network.
Repeatability: Repeatability reduces sources of random errors in the measurement path and therefore accuracy. The repeatability an RF relay similarly improves measurement accuracy and can cut the cost of ownership by reducing calibration cycles. Electromechanical RF Relays exhibit good repeatability, whereas solid-state RF Relays have excellent repeatability.
Isolation: Isolation is a very important parameter for an RF Relay. It is a measure of how effectively a switch is turned off. High isolation reduces the influence of signals from other channels, endures the integrity of the measured signal, and reduces system measurement uncertainties. Electromechanical RF Relays exhibit excellent isolation, whereas solid-state RF Relays have good isolation.
Switching speed: Switching speed is the time required for the transition of a switch port state from "ON' to "OFF" or from "OFF" to "ON". This period can range from several microseconds in high-power devices to a few nanoseconds in low-power, high-speed devices. An electro-mechanical RF Relay has its switching time in milli-seconds(ms) and a solid-state RF Relay switching time is expressed in nanoseconds (ns).
Settling Time: Another important factor in RF Relays is settling time, it is mostly highlighted in solid state RF Relays. Settling time in a solid-state RF Relay is less than 1 µs, and less than 15 ms in case of electro-mechanical ones.
Power handling: Power handling or the ability of a device to handle power is normally dependent on the design and materials used in the device. Electro-mechanical RF Relay switches hence have a higher power handling capability and a solid-state Relay has a low power handling profile. The power handling differs for different RF Relays such as hot switching, cold switching, average power and peak power. Hot switching happens when RF/microwave power is present at the ports of the switching at the time of the switching. Cold switching arises when the signal power is removed before switching.
Operating Life: This defines the operating life of an RF Relay and tells us how many cycles the relay can go through in its lifetime. This is an important field in electro-mechanical relays. This is not that relevant when it comes to solid state relays, as their cycles are usually much higher so we don’t consider it a factor.
RF Relay Packages: RF Relays are available in a variety of package sizes and connector configurations. The type of connector is usually based on the frequency of the RF Relay. Some RF Relays are also available with waveguide interfaces, these are usually for very high frequencies or very high power levels. Package styles can range from commercial grades which are not environmentally sealed, to highly reliable mil grades which are hermetically sealed to withstand harsh conditions.
