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Isolators & Circulators
INTRODUCTION: Isolators and Circulators are usually three port devices, and they are
used to force the microwave energy into one direction only. The typical junction Circulator
consists of a stripline circuit, sandwiched between two ferrite discs or triangles, an upper and
a lower ground plane, magnetically biased by permanent magnets located outside the ground
planes. In a Circulator, the magnetic field, applied through the vertical axis of the assembly,
results into a circulation of the microwave energy from one port to the other, depending on
where the energy is coming from.
 Microwave energy entering the device from port J1 is directed
to port J2. Energy entering from port J2,is directed to port J3.
Signals entering from port J3, are directed to port J1, etc. If
one of the ports is terminated into a 50 Ohms load, the device
becomes an Isolator. Signals then only can pass the unit with
low loss in one direction, and only with high loss in the reverse
direction. If e.g. port J3 is terminated into a 50 Ohms line,
microwave energy only can pass the device with low loss from
port J1 to port J2. An Isolator is used to "isolate" microwave
components from each other, or to protect units from receiving damages when working into
an open or short circuit. The output of an oscillator is usually protected by an isolator.
Frequency and Bandwidth: Coaxial and microstrip circulators and isolators operate either
in the bias region above resonance or below resonance. Above-resonance circuits are usually
used for smaller bandwidths and higher power designs, while below-resonance circuits achieve
wider bandwidths. Theoretically, the above-resonance circuits have no lower frequency limit.
Operating Temperature: The performance depends on the magnetic field, applied to saturate
the ferrite material. Temperature compensated magnets and ferrites need to be used where
wide temperature ranges are required. Internal heaters can be installed, where temperature
range and ferrite material do not allow other compensation.
Input VSWR: The input VSWR is a function of the VSWR of the other ports. At an isolator
the higher output VSWR will cause reflected energy towards the terminated port, where it
will be attenuated by the value of the isolation, and the balance is reflected back to the input,
increasing the input VSWR.
Four Port Devices:  Four Port Circulators and Isolators are used where higher directivity is
needed. An Isolator would have the ports J3 and
J4 terminated. In the schematic to the left,
microwave energy is forced from port J1 to J2,
or from port J3 to port J4, when crossing two
ferrite junctions. The high isolation only applies
when two ferrite junctions have been crossed, here
between ports J2 and J1 with ports J3 and J4
terminated with matched loads.
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