Signal Chain Discussion Series: Circulators & Isolators Versus Switches As Duplexers

Circulators/isolators and switches are both commonly used in the RF front-end of communication and radar systems. The purpose of circulators/isolators and switches in RF front-ends is to enable the use of transmission and reception from the same antenna, duplexing. For instance, in a transmit/receive module (TRM) for an active antenna system (AAS) (phased array antenna), the design may only allow for a single antenna element for a TRM. In this case a method of sending transmission signals and receiving low power reception signals without the high power transmission signals feeding into the receive signal path is necessary.

A circulator is a 3-port magnetic component that when properly biased, allows the bidirectional flow of RF signals within a certain bandwidth at some degree of loss similar to a power divider. Intrinsic part of a circulator’s operation is the isolation of the forward and reverse signal at their respective ports, with an antenna port carrying both forward and reverse signals. For continuous wave (CW) radar and other non-pulsed radar schemes circulators are essential to prevent desensitization and possibly damage to the radar’s receiver. For some communications protocols, such as frequency domain duplexing (FDD) where a frequency-selective filter duplexing isn’t practical.

A switch uses either solid state technology, such as diodes and transistors, or mechanical connections between inductors to direct RF signals along different paths. When a switch connection is made, there is a low attenuation path with other switch paths at very high impedance. In this way, RF switches provide high isolation and low loss along the signal path when active. The downside of an RF switch for these applications is that if an antenna is being switched between a transmission path and reception path, then only one path can be active at a time. For some communication protocols, such as time domain duplexing (TDD) used in 4G and 5G cellular technology, a switch may be a viable solution as there is set active timing shared between the reception and transmission signals, so the antenna would not need access to both signals simultaneously.

A benefit of circulators over switches is that they operate continuously over a set bandwidth, and only require a bias signal to function. Switches require active control signals to be sent to the switch for every instance that the signal path needs to be changed. In the case of communication signals, this switching may be incredibly frequent, which can add substantial wear to a mechanical switch type. In cases with frequency TR duplex switching, solid state or microelectromechanical machine (MEMS) switches are generally preferred as they tend to exhibit much higher switching life than typical mechanical switches.

In terms of isolation, switches tend to provide greater levels of transmit/receive isolation than circulators, which are ultimately limited by the quality of the design and magnetic materials used. A circulator’s isolation also depends on the termination quality and the impedance match between the ports. Hence, wideband circulators tend to exhibit lower isolation across the operating frequency range than narrowband circulators. Typical circulator isolation ranges from 16 dB to 40 dB, and in some cases an additional isolator may be used to enhance the isolation of the receive port from high power transmit signals.

Switches, depending on if they are solid state or mechanical, may range in isolation from 30 dB to over 60 dB for electromechanical switches. Switches also have operating frequency ranges far wider than circulators. For higher switching speeds, solid state and MEMS switching technology is often used. These technologies tend to exhibit lower power handling than electromechanical switches and circulators.

Power Handling
• Circulators: watts to hundreds of watts
• Electromechanical switches: tens of watts to hundreds of watts
• Solid State Switches (PIN Diode): milliwatts to watts
• MEMS Switches: milliwatts to watts

Learn more about Pasternack’s extensive lines of RF Switches and Circulators/Isolators.