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Application Notes FERRITE ISOLATORS and CIRCULATORS

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Application Notes FERRITE ISOLATORS and CIRCULATORS 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. Top: DiTom’s 18-40 GHz circulator passes a signal with a frequency anywhere from 18-40 GHz from port 1 to port 2. Right: DiTom’s 18-40 GHz circulator passes a signal with a frequency anywhere from 18-40 GHz from port 2 to port 3. Left: DiTom’s 18-40 GHz circulator passes a signal with a frequency anywhere from 18-40 GHz from port 3 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. DiTom’s 18-40 GHz isolator attenuating a signal DiTom’s 18-40 GHz isolator allowing a signal anywhere from 18-40 GHz as it passes from port anywhere from 18-40 GHz to pass from port 1 to 2 to port 1. A majority of the energy is absorbed port 2 with little attenuation by the termination on port 3, effectively isolating port 2 from port 1. 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: dB = 10 × log10( ) -0.90 dB = 10 × log10( ) = = 0.81 Pout = (0.81)Pin = (0.81) × (1 Watt) = 0.81 Watt 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. A representation of a bad VSWR - The entire A representation of a good VSWR – Only a small signal is reflected back in the direction of where fraction of the incident signal is reflected back in it came from. the direction of where it came from. 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: R = R = = 0.184 Continue with general reflection coefficient to return loss (dB) conversion: Return Loss = -20 × log10(R) Return Loss = -20 × log10(0.184) = 14.7 dB (-14.7 dB in actuality, as it is a loss) Finish with general dB to power conversion: = Preflected = (Pin) = 0.034(Pin) = 34mW 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.
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