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Acoustic Circulator

March 3, 2014

Circulators are common microwave devices distinguished by being passive (no power required) and non-reciprocal. The non-reciprocal property means that radio frequency energy in a circulator travels on a one-way street; that is, you can easily conduct signals from point "A" to point "B," but the reverse signal direction is blocked. The operation of a three-port circulator is illustrated in the figure.

Operation of a three-port circulatorIn a circulator, radio frequency energy "circulates" in either a clockwise or counter-clockwise fashion.

You can conduct signals between adjacent ports in just one direction.

(Illustration by the author using Inkscape.)

Such an operating principle is possible, since circulators contain materials for which electromagnetic wave propagation is not rotationally symmetric. For radio frequencies, this is ferrite, biased in a magnetic field. For optical circulators, there's also a magnetic field, but the material is an optical Faraday rotator, such as terbium aluminum garnet. These materials cause a phase shift between left and right circularly polarized counter-rotating waves injected at an input port, so signals will reinforce in one direction, and cancel in the other. This means, also, that signals will be conducted from "A" to "B," but not "A" to "C."

One obvious application of a circulator is as an isolator, which can be used, for example, at the output of an oscillator to prevent its being detuned by downstream circuitry. The most useful application is as a means of using a transmitter and receiver with the same antenna. With proper port connection, signals from a transmitter will be conducted by a circulator to the antenna, only, and not to a receiver with connection to the same circulator.

If you're working with low frequencies, which may even be as high as a 100 MHz, it's possible to build a circulator with commonly available components. The circulator circuit, below, published by Charles Wenzel in 1991, is easy to build.[1] It would be a homework problem for students learning SPICE, or another analog circuit simulation program, to verify its operation.

Charles Wenzel operational amplifier circulator circuit.Charles Wenzel operational amplifier circulator circuit.[1]

In the original circuit, R = 100Ω, R' = 323.6Ω, and the operational amplifiers were type CLC406.

(Illustration by the author using Inkscape.)

Once a concept has been proven to be useful in one application, scientists and engineers try to extend it to others. Researchers at the Department of Electrical and Computer Engineering, the Applied Research Laboratories, and the Department of Mechanical Engineering of the University of Texas at Austin (Austin, Texas), have demonstrated a circulator for sound.[2-4] Their achievement was notable enough to be the cover feature of a recent issue of Science.[2]

As can be seen in the photograph, small computer cooling fans are an integral part of the circulator construction. These "Mini Kaze Ultra" fans from Scythe Corporation were fixed in the circulator cavity at 120 ̊ intervals between the three ports.[5] The air flow of these fans established the directional bias of the circulator by circulating the fluid (air) in which the sound waves propagate. This bias causes the counter-propagating acoustic waves to experience different resonant frequencies in the cavity. Careful tuning of the air flow results in interference between these modes to allow 30-40 decibel nonreciprocal isolation between ports.[2-3]

University of Texas at Austin acoustic circulator.University of Texas at Austin acoustic circulator.

The circulator is about 20 cm in diameter.

(University of Texas at Austin photograph, used with permission.)

Says Andrea Alù, associate professor of Electrical and Computer Engineering and project leader,
"It is just the right spin of fluid (air) coupled with the strong resonance of our ring cavity, which makes our design powerful... These two combined mechanisms create strong nonreciprocity in a compact device. Sound waves are routed in one direction only — always contrary to the direction of the airflow."[4]
Unfortunately, this is a resonant device that only operates at its design frequency of about 800 Hz, but the research team believes that the design is scalable for other frequencies. The University of Texas at Austin has filed a provisional patent application for this device.[4] The Texas team is further working on an acoustic circulator that won't need moving parts.[4]

As I discussed in several previous articles, heat is conducted in solids via acoustic vibrations called phonons. Such research may have an application to development of thermal diodes. Research for the acoustic circulator was supported by the Defense Threat Reduction Agency and the Air Force Office of Scientific Research.[4]

References:

  1. C. Wenzel, "Low Frequency Circulator/Isolator Uses No Ferrite or Magnet," RF Design, July, 1991.
  2. Romain Fleury, Dimitrios L. Sounas, Caleb F. Sieck, Michael R. Haberman and Andrea Alù,, "Sound Isolation and Giant Linear Nonreciprocity in a Compact Acoustic Circulator," Science, vol. 343, no. 6170 (January 31, 2014), pp. 516-519.
  3. Steven A. Cummer, "Selecting the Direction of Sound Transmission," Science, vol. 343, no. 6170 (January 31, 2014), pp. 495-496.
  4. UT Austin Engineers Build First Nonreciprocal Acoustic Circulator: A One-Way Sound Device, University of Texas Press Release, January 30, 2014.
  5. Supplementary Materials for Ref. 2.