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Practical Use: Circulators and Isolators
Circulators and isolators are commonly used in RF work. You should know how they work and the appropriate applications for each.
A circulator is a three-port device that has an interesting characteristic. (See Figure 1.) When the RF signal is applied to port 1, it rotates through the device and will exit via port 2. Likewise, any RF signal applied to port 2 will rotate through the device to port three; and an RF signal applied to port 3 will rotate through the device to exit port 1.
Now take a circulator and place a termination on port 3. The RF signal applied to port 2 will exit and be terminated at port 3. The better the termination on port 3, the better the isolation will be back to port 1. By adding this terminator, the circulator has just become an isolator.
What happens inside a circulator though? From Nova Microwave, we have a reasonable explanation. (Italics are mine.)
"A junction circulator is a three-port device formed by a symmetrical Y-junction strip line coupled to a magnetically biased ferrite material. A ferrite disk and the intersection of three transmission lines from the Y-junction is where the actual circulation occurs. A signal applied to a ferrite disk will generate two equal, circularly polarized counter-rotating waves that will rotate at velocities ω+ and ω-. The velocity of a circularly polarized wave as it propagates through a magnetically biased microwave ferrite material will depend on its direction of rotation. By selecting the proper ferrite material and biasing the magnetic field the phase velocity of the wave traveling in one direction can be made greater than the wave traveling in the opposite direction. 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 seems to direct the signal flow clockwise or counterclockwise depending on the polarization of the magnetic biasing field."
Applications for isolators (a circulator with a termination on port 3) are many. A common use is in the output of an HD Radio transmitter. Here the idea is to shunt RF from the analog transmitter that is present in the HD Radio antenna for one of many reasons in to the termination, as opposed to directly back in to the RF output of the HD Radio transmitter, where it could easily cause intermodulation products to be generated.
Having an isolator in the output of a UHF or VHF transmitter is not uncommon when the desire is to take all the various signals that are induced in the antenna that the transmitter is seeing on the far end of the transmission line, that subsequently come down the transmission line, and shunting them in to a termination (port 3) so they do not get back in to the output amplifiers of the transmitter, where they could easily generate intermodulation products, that would subsequently travel back up the transmission line and in to the antenna, and into space afterward.
A third application is in the output of an amplifier that is sensitive to its load conditions. A tube transmitter that has a solid-state IPA with VSWR fold-back is a perfect example. Often, when brought up from a cold start, the final amp of a tube transmitter will present a considerably different load then when it is warm. A solid-state IPA will sometimes suffer from VSWR foldback in this condition -- leaving you with a transmitter that won't produce enough (if any) RF. The use of an isolator can cure this problem by effectively letting the IPA see a fixed load, even as the tube warms up. You can optimize the grid tuning while the tube is warm and not have to worry about it presenting a bad load (at the same tuning points) to the IPA when it is cold.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at firstname.lastname@example.org.
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