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Application Notes


FERRITE ISOLATORS AND CIRCULATORS

Ferrite isolators and circulators play a fundamental and valuable role in RF systems. They are passive, ferrite devices that act as traffic conductors for RF energy in a system, routing signals wherever a system designer needs them to go. Their ability to behave non-reciprocally (non-reversible, allowing energy to pass in only one direction through the device) when RF energy is applied to them is very important for a number of applications.
 
RF CIRCULATOR
An RF circulator can be thought of as a merry-go-round for RF energy. Energy of an appropriate frequency that enters port 1 of the circulator (gets on the merry go round) will travel clockwise until it reaches port 2, at which point it will exit the circulator (gets off the merry go round). There is very little attenuation of the signal while it is inside the circulator. Likewise, energy entering port 2 of the circulator will travel to port 3, and energy entering port 3 of the circulator will travel to port 1.
 
RF ISOLATOR
An RF isolator can be thought of as a diode for RF energy. An isolator is simply a circulator with one of its ports terminated with a matched 50Ω load. The device has only 2 ports, and as a result, has only one path for energy to flow without significant attenuation. Energy can only enter port 1 and travel to port 2. Any energy that enters port 2 will be routed to the matched termination on port 3, and quickly dissipated as heat. This behavior heavily attenuates any signal entering port 2 before it reaches port 1, yet allows almost all of a signal entering port 1 to reach port 2.
 
ELECTRICAL SPECIFICATIONS OF ISOLATORS AND CIRCULATORS
When selecting an isolator and/or circulator, it is important to understand some common electrical specifications that tell you how well it is capable of performing in your application. The three basic specifications for isolators and circulators are insertion loss, VSWR, and isolation. Generally speaking, these specs are a direct trade off with bandwidth: the more bandwidth you want, the more degraded set of specs you will get.
 
Insertion Loss
Insertion loss describes how much energy is lost during the process of transferring a signal from one port of an isolator/circulator to another. It is essentially a measure of how much energy it costs a designer to use an isolator/circulator in their system. As stated above, isolators and circulators are passive components, so a signal traveling through them has to do so using its own energy. As in any real system, there will be some attenuation to the signal as it travels through the device. This attenuation is called insertion loss, and it is measured in decibels (dB). The higher the insertion loss, the more energy it costs to use the isolator or circulator. This energy is converted into heat on its way through the device. However, insertion loss specifications are relatively small, so the benefits a system receives from the use of an isolator/circulator are usually worth the energy cost of implementing them. Typical insertion loss specifications are on the order of 0.4 dB for octave bandwidth units, however the specification can be as small as 0.15 dB for narrowband units, and as high as 1.7 dB for certain broadband units.

Example: A customer purchases DiTom’s 6.0 – 18.0 GHz isolator, DMI6018, because it is the best unit available on the market today. The customer wants to know how much of their 1 watt signal it will cost to use this isolator in their system. The DMI6018 has a typical insertion loss spec of 0.90 dB (actual value is -0.90 dB, as insertion loss is always a negative ratio; it measures attenuation, after all).


Start with general dB to power conversion:

From this calculation, the customer now knows that it will only cost 190 mW to use the isolator in their system.

VSWR

VSWR stands for voltage standing wave ratio. It is a ratio of the maximum voltage to the minimum voltage of a standing wave created by an imperfect impedance match where two boundaries meet (typically where a source meets a load). This standing wave is produced by energy reflecting off of the boundary, and traveling back the way it came. In the case of isolators and circulators, VSWR is the measure of how much of the signal that you want to send through the isolator will reflect back towards the transmitter that sent it. A low VSWR spec is always desirable. Lower VSWRs mean that there is less energy reflecting off of port 1 of the isolator/circulator and going where you don’t want it to go.


Standing waves are unattractive because they represent energy going where you don’t want it to go, and very bad things can come from this happening. To visualize this, consider the following situation:

Your goal is to get a tennis ball from your hand to the inside of a house. In this situation, you are the transmitter, the tennis ball represents your RF signal, and the house is the load to which you want to deliver the signal (antenna, amplifier, etc.). You throw the tennis ball as hard as you can at the brick wall of the house. The tennis ball promptly bounces off the wall, and comes flying back at you.

This situation represents a perfectly bad VSWR for an isolator/circulator – one in which your entire signal went exactly where you didn’t want it to go. It reflected off of your isolator/circulator, headed back towards your most likely expensive transmitter, and either disrupted it, or destroyed it completely. This type of VSWR occurs at an open or short circuit in a system, where the impedance match is the worst. An RF signal on a transmission line will see this impedance mismatch as very obstructive, so rather than conduct through it, it reflects off of it. In the example, the tennis ball sees the brick wall as very obstructive, so likewise, it reflects off of it.

The next day, you try again, and decide you are not going to hurl the ball into a brick wall directly in front of you, but rather try to throw the ball through an open window. You hold your breath, and release the ball. After you open your eyes, you find that the ball is resting comfortably inside the house.

This represents a perfect VSWR for an isolator/circulator – one in which your entire signal went exactly where you wanted it to go, and you have maximum power transfer through the isolator/circulator. This type of VSWR occurs at boundaries where both sides are impedance matched to each other perfectly. An RF signal traveling on a 50Ω transmission line, for example, will happily conduct through an isolator that has been tuned to present the signal with a 50Ω impedance. In the example, the air on the outside of the house and the air inside of the house have the same effect on the tennis ball, so the ball can move easily between the two with no reflection.

A perfect VSWR will never be attained in real life (but here at DiTom, we get pretty close). VSWR is typically expressed as a ratio (1.5:1, 2.5:1, etc.); the lower the ratio, the better the VSWR. DiTom’s typical VSWR specifications for isolators/circulators are 1.1:1 for certain narrowband units, and around 1.5:1 for broadband units.

Example: The same customer that just bought the DMI6018s wants to know how much of his 1W signal will reflect off of the incident port. The DMI6018 has a typical VSWR spec of 1.45:1.

Start with general VSWR to reflection coefficient conversion:

From this calculation, the customer now knows that 34 mW of his 1W signal will reflect off of the isolator when it is used in their system

 
Isolation

Quite possibly the most important spec for an isolator is… the isolation spec! Isolation is a measure of how well an isolator can carry out its main purpose of decoupling energy entering port 2 from whatever is attached to port 1. An isolator has isolation because a termination is attached to port 3 of the device, not a connector. This termination has an element inside its shell called the load element. When matched well to the isolator, this element will dissipate any energy it encounters as heat, stopping it in its tracks. Isolation is dependent on two things primarily: the quality of the termination on the isolator, and the VSWR of port 3 on the isolator.

With a quality termination attached (having a VSWR of about 1.05:1 or better) isolation of 24-26 dB can be achieved for narrowband units, and 15-20 dB can typically be achieved for broadband units. One thing to remember is that your isolation will always be at least as high as your VSWR, and sometimes higher. Just think about it: the spec for VSWR on port 3 will express the absolute maximum amount of energy that will reflect off of port 3 when a 50Ω load is placed on it. Isolation exists only because you have a 50Ω load on port 3, so it follows logically that you will have isolation that is at least equivalent to that VSWR by default. Sometimes, a slight improvement can be made to the isolation spec when a termination is added to the port, as it is tuned by a technician to give the absolute highest isolation possible.

One thing to remember about isolation: Circulators by definition do not have any isolation spec. They are defined purely by insertion loss and VSWR. One can turn a circulator into an isolator by attaching an aftermarket load to the port 3 connector, but be warned…..don’t expect an isolation measurement as good as or better than your VSWR specification right off the bat. The isolation that a circulator provides is completely dependent on the VSWR of the load that you attach.

All of DiTom’s circulators are supplied with an isolation specification for your convenience, and they are tuned using a reference load with a VSWR of 1.05:1 or better. Using a circulator as an isolator by attaching a load to port 3 is completely fine and standard practice, but be mindful when doing so that attaching a load with a larger VSWR than 1.05:1 will leave you with a degraded isolation spec - the termination will be reflecting more energy back into the circulator than the termination that was used to initially build and tune the circulator.

 
POWER HANDLING AND GROUP DELAY

Two more common specifications that aren’t encountered as often but are good to remember for certain applications are power handling and group delay.

Power Handling

Power handling is simply a measure of how much power an isolator/circulator can usefully pass without degrading to the point where signal distortion and/or attenuation render it ineffective. Factors that dictate how much power an isolator/circulator can handle are predominantly heat, voltage level, and spin waves (related to ferrite material).

Heat is generated by the isolator/circulator because of insertion loss. Any energy that doesn’t make it from one port to another has to go somewhere, and that somewhere is heat. Heat is bad for a number of reasons:

  • It degausses the permanent magnets that bias the ferrite
  • It causes thermal expansion of the internal circuit which can overly distort its sensitive geometries, degrading quoted specification
  • It will eventually destroy the unit, as the dielectric will melt and the magnets will completely degauss


It is important to consider heat sinking an isolator/circulator whenever possible when you will be approaching the quoted average or peak pulsed power of the isolator/circulator in your application.

Spin waves are related to heat, and can also be a problem encountered when dealing with high power signals. When RF power levels approach a certain critical point, spin waves (think of it as noise) within the RF signal begin to get excited, and can saturate the main resonance line width of the ferrite material. This saturation disrupts the uniform mode that is driven by the applied RF signal. Spin waves usually have the same frequency as the uniform mode, or some harmonic derivative of said frequency, but they are frequently out of phase with the uniform mode. This causes the saturation of the main resonance line width, and leads to non-linear absorption within the ferrite itself. All of these negative effects of spin waves essentially lead to a unit having more insertion loss than it was originally designed to have.

Isolators and circulators have both an average power handling spec, and a peak power handling spec. Average power represents the maximum power that the isolator/circulator can handle when power is applied continuously. This can be the average power of a continuous wave signal, or the temporal average of the power of a pulsed signal (radar or beacon application). In the case of a continuous wave signal, the average power of the isolator/circulator must be greater than the average power of the signal. In the case of a pulsed signal, the average power specification must be greater than the temporal peak of the signal multiplied by the signal’s duty cycle.

Peak power represents the maximum power than an isolator/circulator can handle when pulsed. For an application where a pulsed signal will be transmitted, the temporal peak must be less than the peak power handling of the isolator/circulator.

Isolators have an additional power specification called reflective power handling. This specification refers to the power that the termination attached to the isolator can dissipate. This can be customized during manufacturing by selecting a termination that has the power handling that a customer wants. The important thing to remember here is that the termination needs to be able to handle any foreseeable amount of energy that may either reflect off of or enter port 2 during operation when something goes wrong further down the transmission line (short/open circuit). In the event that the termination sees too much power for too long, it will burn up.

Circulators do not have a reflective power spec because they do not have a termination attached to them. If you are using a circulator as an isolator, refer to your termination’s datasheet to find your reflective power handling – it will be the same as the peak power handling of the termination.

Group Delay

Group delay can be defined as the amount of real time that it takes for a signal to exit port 2 after it has entered port 1. Group delay is calculated by taking the derivative of the phase. This value, typically expressed in picoseconds or nanoseconds, is an indicator of the linearity of the isolator/circulator. It is desirable to have a group delay value that is constant relative to all frequencies in the band of interest. A constant group delay indicates a linear phase over the entire band, so large fluctuations in group delay indicate phase non-linearities caused by the isolator/circulator. Any non-linearities in a signal that are a result of an isolator or circulator are undesirable, as they indicate that the signal has been degraded by the isolator or circulator in some way.

 
APPLICATIONS

Common applications for circulators are as simple duplexers and as high reflective power handling isolators. Isolators are typically used to protect active components from distorting or potentially damaging reflective power.

Duplexers

A duplexer is anything that allows two directions of communication over a single channel. For example, a duplexer allows you to both transmit and receive a signal using the same antenna, reducing the number of parts in a system while at the same time reducing the cost of the system. The non-reciprocal nature of a circulator fits this application perfectly.

By attaching a transmitter to port 1, an antenna to port 2, and a receiver to port 3, you can effectively use 1 antenna to perform two tasks. This reduces the number of antennas and cable runs by one half, saving space and money.

High Reflective Power Handling Isolators

Another popular application for circulators is as isolators with high reflective power handling. Attaching a high power termination to port 3 of the circulator allows the user to use it as an isolator in higher power applications. This can be particularly useful for base station applications where you need a termination that can dissipate more power than a standard, lower power termination would be able to dissipate.

Signal Generator/Transmitter Protection

The standard application of an RF isolator is as a form of protection for sensitive equipment that is connected in a chain. By placing an isolator on the output of a transmitter, for example, you can protect that transmitter from harmful reflections that would have otherwise made it back to the transmitter output, potentially contributing to signal distortions or damage to the equipment.

Dual Junction Isolators/Circulators

An isolator/circulator can be made with more than 3 ports if desired. A dual junction isolator/circulator has 4 ports, and functions exactly the same as a 3 port isolator/circulator. The additional port can be beneficial for certain applications. For example, an additional termination can be placed on the extra port if more isolation is desired between the input and output of an isolator, or in a duplexer application, additional isolation can be added between the receive and transmit port.

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