Within RF circuits, there is often the need to open or close one or more circuits based on timing or other triggers. In some cases, this is a single circuit at a time, in others it is many circuits simultaneously. There are now a wide variety of RF switch types, including solid state switches, microelectromechanical (MEMS) switches, diode switches, manual switches, and the venerable electromechanical relay (EMR). EMRs are sometimes called EMR switches, which is a nod to some of the confusing nomenclature in the RF industry, and somewhat redundant. In the case of RF switches, most types of switches, other than manual RF switches, are really relays, in that they can be activated remotely. However, on most occasions, RF switches are generally just called switches, and EMRs are a subclass of that.
EMRs are a type of switch that uses an electromagnetic to open or close a connection via control of a plunger with a contact surface that bridges the circuit path when active or inactive, depending on the EMR type. Normally closed (NC) EMR types are open circuits when activated and normally open (NO) EMRs are closed when activated and otherwise open. EMRs are also made for a variety of other applications other than RF, including industrial automation for robotics, control circuits, power, etc.
The difference between an RF EMR and other industrial EMR devices is that an RF EMR is designed with impedance matched ports and circuit pathway specified from DC to between megahertz to tens of gigahertz operation. RF EMRs also have a limited power handling at various frequencies, generally the higher frequency the maximum operation of the EMR the lower the power handling due to the size of the conductive pathways and contacts internal to the switch.
Key Electrical Specifications & Features of Electromechanical Relays (EMRs)
- Frequency Range (typically DC to a maximum frequency except for waveguide EMRs) [Hz]
- Operating Voltage [V]
- Actuating Current (@ some set voltage) [mA]
- VSWR
- Insertion loss [dB]
- Isolation [dB]
- Input Power [Watts}
- Switching Time [s, ms, us, or ns}
- Operational lifetime (often given as a total number of cycles in millions)
- Hot switching compatibility
- Impedance [Ohms]
A major consideration for many EMRs is hot switching. This is the act of switching while under RF stimulus. In most cases the hot switching capability results in a much lower RF power rating than the typical power handling. Other considerations include the number of switching cycles the EMR is rated for. Typically, RF EMRs are rated to millions of cycles, somewhere between 2 and 10 million. RF EMR switches are preferred over solid state and even newer MEMS switches for their particularly low insertion loss and superior isolation.