What is Power Gain? What is Small Signal Gain? Quick Links. Popular Categories. Our Network. Follow Us :. Need Help Finding a Product? Yes, I would like to receive weekly updates. Thus, this is how a directional coupler works. The direction of flow is determined by the strength and arrangement of the magnetic field inside the enclosure.
Signal flow can be clockwise or counterclockwise, dependent on the configuration of the magnet: flip the magnet upside down in a circulator, and the direction will reverse. Without an isolator, unwanted energy that reflects back to the transmitter could potentially contribute to signal distortions or cause damage to the equipment.
Unwanted signals reflected down the transmission line toward the transmitter could be caused by many factors, such as a broken antenna, shorted or open cable, age, water invasion, incorrect cable length, or strong nearby signal source.
For effective isolation, the isolator port that is to be terminated must be well-matched. Indeed, the isolation of an isolator is only as good as its port match. If a signal were applied at Port 1 the two waves will arrive in phase at Port 2 and cancel at Port 3. Maximum power transfer will occur from Port 1 to 2 and minimum transfer from Port 1 to 3, depending on the direction of the applied magnetic field.
Due to the symmetry of the Y-Junction, similar results can be obtained for other port combinations. Externally the circulator seem to direct the signal flow clockwise or counterclockwise depending on the polarization of the magnetic biasing field. When the ferrite material is magnetized the magnetic moments of the electrons precess at a frequency proportional to the biasing magnetic field.
Ferromagnetic resonance occurs when a rotating RF magnetic field has the same direction and frequency as the precessing electrons in the ferrite material. The maximum coupling of the energy from the RF signal to the ferrite material will occur at ferromagnetic resonance. If the direction of rotation or the frequency of the RF signal is changed, minimum coupling will occur.
A simplistic analogy can be used to explain these phenomena. It is easier for a person to pass items to an individual riding on a merry-go-round if he is running in the same direction and at the same speed while it is more difficult to pass them if both are moving in opposite directions. Biasing the junction circulator at ferromagnetic resonance is not desirable because the circulator would be extremely lossy. High insertion loss can also occur at very low biasing magnetic fields.
This low field loss region arises from the fact that the applied magnetic field is not sufficient to fully saturate or align the individual magnetic domains of the ferrite material. Although high loss occurs in both the low field and ferromagnetic resonance areas low loss operation can still be obtained in the below and above resonance regions as shown in Figure 2.
It should be noted that the following comparison applies principally to strip line junction circulators and isolators and is intended as a guide only.
Although operation above this frequency can be achieved, impractical magnetic circuits are required in order to bias the ferrite material.
Operation at frequencies below 50 MHz is difficult because the magnetic field the demagnetizing factors of the ferrite geometry do not allow proper biasing of the junction. Operation below this frequency is possible but generally more limited in performance. Operation above this frequency is limited mainly by the strip line geometry.
Waveguide circulators can be designed to operate at frequencies greater than GHz. The magnetic properties of these materials change with temperature and are used to compensate for the ferrite junction temperature characteristics. The magnetic properties of the ferrite materials available to build circulators or isolators at these frequencies are extremely temperature sensitive. Circulation of the input signal cannot occur at this temperature. In general, ferrite materials used for the higher operating frequencies have greater temperature stability.
For narrow bandwidths in the 1. High energy product magnets which are now used to bias the ferrite junction have minimized the problem of irreversible change in the magnetic field due to temperature. Extensive temperature cycling is no longer required to stabilize the magnetic field against further permanent change.
As previously discussed, the magnetic circuit will also include materials to compensate for the reversible changes with temperature. Optimization of the magnetic circuit can be done to provide additional magnetic shielding for critical applications where units are mounted in close proximity.
Multiple quarter-wavelength sections of transmission line are commonly used to match the lower impedance of the ferrite disk to the 50 Ohm impedance of the connectors. The VSWR and bandwidth sections determine the number of transformer sections required.
This results in the the signal only being able to travel to the next port around the circulator. The circulator assembly consisting of the Y junction and the ferrite disks has a distinct resonant frequency - the assembly actually forms a resonator. For obvious reasons the circulator is not operated at this frequency, but either above or below it because the insertion loss, i.
There are several applications for RF circulators in a variety of RF circuit design applications. Normally they tend to be used at microwave frequencies and as a result they are often referred to as microwave circulators. Duplexer: One of the most obvious and common applications for an RF circulator is within radar systems or radio communications systems where the transmitter and receiver use a common antenna.
Here the transmitter output is connected for example to Port 1, the antenna to Port 2 and the receiver to Port 3. Accordingly the transmitter power will circulate to the antenna, but not to the receiver, and the antenna received signals will circulate to the receiver. In this way the receiver is isolated from the transmitter but the antenna has power from the transmitter and passes the received signal to the receiver without any mechanical switching. These are useful for protecting a transmitter output amplifier has to run with a high level of VSWR.
As high voltages or current levels can be experienced by the power amplifier if connected directly to the antenna under these circumstances it is possible for the amplifier to be damaged by them.
Often transmitters need to operate over a wide bandwidth and under these circumstances, a good impedance match is unlikely to be maintained over the whole bandwidth and damaging levels of VSWR could be seen. To overcome this issue a circulator can be used to protect the PA from the effects of the reflected power.
For this the transmitter is connected to Port 1 and the antenna to Port 2. If a poor match or open circuit is presented, then the isolation performance will be impaired. The transmitted power is passed from Port 1 to Port 2 and travels to the antenna.
Any reflected power will return along the feeder and pass from Port 2 into Port 3 where it can be dissipated in the load. The load on Port 3 is required to provide a matched impedance: isolators need matched impedances onth e ports to maintain the level of isolation from that port. A common use of an isolator is shown in Fig. The isolator is connected between a signal generator and some device under test DUT.
If all impedances are matched, the signal passes freely to the DUT. The circulator absorbs this signal, protecting the usually expensive signal generator. Explanations for some of the key specifications appropriate to RF circulators are given below:.
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