by Introduction to RF Isolators and Circulators
RF isolators and circulators are passive devices that are commonly used in microwave systems to control the direction of signal flow. They are constructed using ferromagnetic materials and perform an important role in applications that require the separation of transmit and receive signals, such as radar and wireless communication systems.
Isolators are two-port devices that allow signals to pass through easily in one direction, while blocking them or providing high attenuation in the opposite direction. Circulators, on the other hand, are multi-port devices that route incoming signals to specific output ports depending on the point of entry. Both isolators and circulators have various uses in microwave systems where control over signal propagation paths is required.
RF Isolators and Circulators Design and Operation
An RF Isolators and Circulators consists of a cylindrical ferrite core placed inside a conductor housing with two connecting ports. The basic design involves cladding a cylindrical ferrite rod with a metalized film or winding thin metal wires that serve as the input and output ports. A static DC magnetic bias field is applied to the ferrite core using magnets placed outside the housing.
When a signal enters one port of the isolator, the ferrite’s non-reciprocal properties transmit the wave through to the other port while blocking it from propagating backwards. This phenomenon occurs due to the magnetization induced in the ferrite material by the DC bias field. The polarized ferrite presents very little opposition for signals propagating in one direction but absorbs waves trying to travel in the reverse path.
Typical isolators provide isolation levels of 20-40 dB across microwave frequency bands like S, C and X. Higher isolation exceeding 60 dB is achievable using multicell designs with multiple ferrite-conductor stages placed in series. Isolators find widespread use to protect sensitive receiver circuits from strong transmitted signals in areas like antenna systems and amplifier outputs.
RF Isolators and Circulators Design and Working
Circulators have a similar isolator construction involving a ferrite cylinder fitted inside a metal housing but contain three or more ports instead of two. The ports are placed at symmetric intervals around the circumference and coupled magnetically to the ferrite core through built-in windings or film patterns.
A clockwise DC bias magnetic field is applied while circulator operation is based on the non-reciprocal gyromagnetic effect in ferrite materials. When a signal enters from any port, the ferrite’s non-reciprocal permeability causes it to propagate to the next adjacent port in the clockwise direction only. This circulates incoming signals from one port to the next in a controlled rotating fashion.
Three-port circulators have ports numbered 1, 2 and 3. A signal entering port 1 is routed to port 2, from port 2 to port 3 and port 3 back to port 1, thereby restricting its path. Similarly, four-port and six-port circulators transfer signals in a rotating clockwise cycle between ports. Circulators find use for duplexing antennas in radar and microwave communication links.
RF Isolators and Circulators Applications
Power Amplifier Protection
Radar and communication transmitters use high power solid-state or traveling wave tube amplifiers that need protection. Amplifier inputs are fitted with isolators to block any reflected signals. This protects the amplifiers from damage due to impedance mismatches.
Antenna Duplexing
Radar and cellular base station antennas need to simultaneously transmit and receive signals. Isolators and circulators placed between antennas and transceivers allow transmission during one time slot and reception during another by isolating or circulating signals.
Frequency Multiplexing
In electronically frequency scanned radar arrays, transmit/receive modules share the same antenna elements. Circulators allow time-division multiplexing of different frequencies for transmit and receive functions through each antenna.
Receiver Front Ends
The front-end low-noise amplifiers in receivers are protected from strong out-of-band transmitter interference using isolators fitted at the input. This improves receiver sensitivity and dynamic range performance.
Duplexers and Diplexers
Multi-band communication systems employ isolators and circulators in filter-based duplexers or diplexers to separate transmit and receive signals at multiple frequencies through common antennas.
Oscillator Stability
L-C tuned oscillators in transmitters require isolation from any feedback loads. Isolators serve this purpose, providing stable operation by protecting oscillators from variations in the load impedance seen by following amplifier stages.
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1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it
