Circulator

Circulators are important devices used in microwave engineering to deal with devices that use electromagnetic or radio waves. You might have heard of the non-metallic ferrite substances that are very popular due to their magnetic properties. In this article, we will discuss one such substance that uses ferrite material i.e. a circulator. We will study what is a circulator and what are the principles of operation of a circulator.

In this article, we will through what a microwave circulator is, we will start our article with the definition of circulators, and then we will go through the different types such as Ferrite Circulator, Non-Ferrite Circulator, Coaxial Circulator, Self-biased Junction Circulator and Microstrip Circulator, then we will go through its testing and measurements with its characteristics, at last, we will conclude our article with its Applications, Advantages and Disadvantages with some FAQs.

Circulators are two or more port devices made up of ferrite material that are usually in the center of the microwave path. These devices are up to a frequency range of 40GHz. The signal fed at any port of the circulator will transmit to the next port and will generate no output at the remaining.

For a 4-port circulator, if the signal is fed at port 2 then the output signal will be obtained at port 3 and there will be no output at port 1 and port 4. Let us now see how a circulator works.

Let us consider a 3 port circulator to understand the workings of the circulator.

• The signal is fed at a certain port, for example, the input signal is fed at port 1. The central part of the circulator consists of a ferrite material.

• The ferrite materials are highly resistive, non-metallic devices that display great magnetic properties. Due to the magnetic nature of ferrite material, it interacts with the electromagnetic waves that are given as input.

• Depending on the direction of the magnetic field in the ferrite material, the input signal will be propagated in a particular direction. In the above example, the direction of rotation is anticlockwise.

• A three-port device consists of three parts: input, output, and isolated. The input port is the port that receives the signal, the output port is the one where output is generated and the isolated port generates no output.

• It is important to note that the circulator introduces some phase shifts in the signal to ensure that the signal fed at one port can travel to another port without being out of phase. This is how the signal is transmitted in a circulator.

There are various types of circulator listed below. Let us study each of them.

• Ferrite Circulator
• Non-Ferrite Circulator
• Coaxial Circulator
• Self-biased junction Circulator
• Microstrip Circulator

Ferrite Circulator

These circulators include a magnetic ferrite core which is responsible for generating a magnetic field inside the circulator. This magnetic field will influence the input electromagnetic signal. Due to the magnetic core, when the input signal is fed in, the signal moves in a particular path generating the signal at the output port and leaving no signal at the isolated port.

These are the most commonly used circulators.

Non-Ferrite Circulator

These circulators need an external power source to work due to the absence of ferrite material which drives the electromagnetic signal in the ferrite port. They generally operate at a higher frequency and work similarly to a ferrite circulator. The major disadvantage of using a non-ferrite circulator is the high loss associated with them which deteriorates the signal-to-noise ratio. Another disadvantage is their ability to handle power only to a certain extent.

These circulators are not used as widely as ferrite circulators.

Coaxial Circulator

Coaxial Circulator is similar to any other circulator except that it is a 3 port device that is used to direct the incoming signal in a particular direction. It only generates the signal at the output port and completely isolates the other port. Coaxial ports are designed for low-power applications and are used in fields of communications, testing, measurement, and scientific research.

The coaxial microwave circulators in frequencies from 50 MHz to 50 GHz.

Self-biased Junction Circulator

As the name suggests, Self-biased junction Circulators have a self-biased Y-junction which drives the circulator. The one property that differentiates this circulator from other circulators is that it uses the high anisotropy magnetic field in hexagonal ferrites, this eliminates the need for biasing magnets used for using the circulator. These circulators are generally small in size and generally light since they don’t have biasing magnets like other circulators.

Microstrip Circulator

Microstrip Circulator works similarly to any circulator by directing the input signal in a particular direction. These signals are generally microwave signals that flow circularly. The design of the Microstrip Circulator is different from other circulators since it consists of a thin ferrite substrate with metallic deposited over it. This design enhances the overall performance of the circulator.

Most of the circulators include a ferrite material. Here is the cross-section of a circulator showing the ferrite part of the circulator.

Let us study some properties of ferrite material

• The ferrite materials are made up of oxides like iron oxide. This material when subjected to an external magnetic field aligns its magnetic moments in that particular direction which leads to the non-reciprocal behavior of electromagnetic waves.

• The ferrite material is the non-metallic, highly resistive part of the circulator which has great magnetic properties needed for the signal to propagate in one direction.

• The magnetic field of ferrite material is controlled and designed in such a way to isolate certain ports. This isolation between the input and output is necessary for an isolator.

A directional coupler is a 4 port device that is used to couple the microwave power. This power can be incident or reflected power. We can use the circulators as directional couplers using some modifications. Let us see how

The circulator can direct signals from one port to the other either clockwise or anticlockwise. To use it as a directional coupler, we can terminate the loads or the ports. Consider a 3-port device, the coupling of port 1 and port 2 is used for directional coupling, and port 3 is isolated.

Since port 3 is an isolated port and no signal is generated from that port, we can analyze port 2 to see the strength of port 1. This is how we can use a circulator as a directional coupler.

As the name suggests, compact and integrated circulators are circulators that have been designed in smaller sizes to ensure that they can be integrated into large-scale circuits. Here is a picture of a compact and integrated circulator that is used in industrial circuits.

Compact Circulators

Compact circulators are designed by the simple process of miniaturization. They are designed using technologies like miniaturized ferrite materials, compact waveguide structures, or surface-mount technology to ensure that they occupy less space without letting their performance be affected. It is necessary to ensure that these devices generate accurate outputs therefore, efficient techniques like microfabrication or precision machining are used for designing them.

Integrated Circulators

After a compact circulator is designed, it is important to integrate it into a larger circuit using different fabrication techniques. Integration is done to eliminate the need for any separate packing of the circulator. Integration is done by efficiently using the space available on the substrate which results in monolithic structures. Some devices that use integrated circulators include amplifiers, filters, or antennas, to form highly integrated RF/microwave modules.

Let us see how we can test and measure the properties of a circulator to compare their characteristics

• Loss Measurement: Circulators of better quality minimize the loss associated with them. We can measure the insertion loss associated with a circulator to measure the performance of the circulator. For this, we compare the power generated at the output port and the power that was sent at the input port. The difference of power is termed the ‘insertion loss’.

• Isolation: Isolation of ports is an important characteristic of circulator. A good isolator must be able to isolate its ports. To measure the isolation of a circulator, we can send a signal at any port and measure the power levels at the other ports. The greater the signal leakage, the lower the isolation.

• Bandwidth of Operation: This refers to a circulator’s associated frequency operation range. This is tested by analyzing the performance of the circulator over a range of frequencies to test the frequency response of the circulator.

• VSWR Measurement: This method can be used to measure the impedance matching feature offered by the circulator. VSWR is measured by measuring the ratio of maximum voltage to the minimum voltage allowed.

• Calibration: A circulator is better only if it gives accurate and reliable readings. Therefore, it is necessary to adjust the measuring equipment like power meters, spectrum analyzers, and network analyzers and ensure accurate results are generated.

Some of the Characteristics of circulators are

• Insertion Loss:This Parameter represents how much signal power is lost when the circulator is inserted into a transmission line or circuit and it is measured in decibels (dB). In an ideal circulator insertion loss is zero meaning all the signal power entering one port exits through the desired output port without attenuation. However, in reality, there’s always some loss due to factors like imperfect coupling between ports, material absorption, and impedance mismatches. A low insertion loss is typically less than 1 dB and is desirable because it minimizes signal degradation and ensures efficient transmission of signals through the circulator.
• Isolation:It refers as the ability of the circulator to isolate one port from the other ports and it is also measured in decibles.High isolation between ports is important for preventing interference and maintaining signal integrity in multi-port systems. Isolation is achieved through the circulator’s internal design, such as the use of ferrite materials and magnetic fields to direct signals along specific paths. Isolation typically ranges from 20 dB to 50 dB or more depending on the circulator’s design and application requirements.
• VSWR (Voltage Standing Wave Ratio):It can be defined as how well a load is matched to a transmission line. It quantifies the amount of reflected power due to impedance mismatches between the circulator and the connected components or transmission lines.VSWR is a unitless ratio that is represented as a number greater than 1. A VSWR of 1 indicates perfect impedance matching that means that all the power is transmitted into the load without any reflection. A VSWR greater than 1 indicates a portion of the power is reflected back. A VSWR less than 1.5 is considered good which indicates low reflection and efficient power transfer.

Let us study some applications of a circulator.

• Duplexer: Duplexer is a device that uses a single antenna to transmit and receive a signal. The issue is that a single antenna can’t provide enough isolation which can result in the mixing of signals. Therefore, we use a circulator to put the transmitter at the input port and the receiver at the isolated port to ensure proper isolation.
• Isolator: The circulator can be an isolator since it can isolate ports. When one port of all three ports of a circulator is terminated in a matched load, a signal can travel in only one direction between the remaining ports. This is the exact function of an isolator.
• Reflection Amplifier: The circulator ensures reflection amplification by controlling the direction of the receiving signal. The circulator ensures that the reflected signal is directed towards the receiver, thereby ensuring minimum signal attenuation.
• Radar Systems: The circulator is primarily used for signal processing in radar systems. It ensures that the transmitted signal is directed toward the antenna to ensure minimal loss and does the same when any reflected signal is received.
• Signal Reception: Another common application is the integration of circulators in various devices to ensure proper signal reception. The signals received by antennae of certain devices can be weak due to scattering through the medium therefore, a circulator is used for amplifying the received signal.

Let us study some advantages and disadvantages of Circulator :

Advantages of Circulator

• Circulator provides the necessary isolation of ports. A 3-port isolator consists of an input port, an output port, and an isolated port which is useful in many applications.

• A circulator provides full control over the direction, we can decide the direction in which the input electromagnetic wave will go therefore it is even used as a directional coupler.

• Circulator is the best device to deal with microwave signals that involve electromagnetic waves and radio frequency signals.

• Circulators are compact and durable devices. They occupy minimum space therefore, they can be integrated into other circuits and they are durable in the long run.

• Circulators can operate over a wide temperature range without showing any major effect on their performance. This stability is necessary since they can be used in different environmental conditions.

Disadvantages of Circulator

• The cost of circulators can be high and increases with the customization done according to the requirements, therefore, it is not the best choice for small-scale projects.

• Circulators introduce some ‘insertion loss‘ in the input signal. This attenuation can degrade the signal as it passes through the attenuator.

• Circulators require a certain amount of maintenance since they are prone to wear and tear with time due to the environmental conditions.

• Integrating circulators with other circuits can be a complex task since many factors like insertion loss, and impedance matching must be considered while designing.

• It is difficult to design circulators that can operate over a large frequency range therefore, it can lead to limited availability of circulators for a particular frequency range.

We have seen how circulators form an integral part of microwave engineering. They are not only used for isolation and amplification but can also be modified to work as duplexers in certain devices. We have seen how the circulators can be categorized into two types depending on the core material used in designing them. It is necessary to understand the properties of circulators to design an efficient circulator that can minimize loss and ensure maximum power. Readers are suggested to refer to the frequently asked questions in case of any doubt.

What is meant by the integration of a circulator?

A circulator alone can serve only a few purposes therefore, the circulator is joined with other elements like antennas to use it as a duplexer, an isolator, and many more. This is known as the integration of circulators in other circuits.

Can a circulator have a single port?

No, the circulator can’t have a single port. Circulators need a minimum of one port on each side and such circulators are called isolators.

What are the factors that must be considered while selecting a circulator?

While selecting the best circulator for your device, you must look for factors like frequency range of operation, power required, insertion loss, isolation, size and weight constraints, temperature stability, and compatibility.