Electronics

New RF circulator to run rings around old technology

The new RF circulator could revolutionize microminiature communications devices (Photo: Cockrell School of Engineering at UT)
The new RF circulator could revolutionize microminiature communications devices (Photo: Cockrell School of Engineering at UT)
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The new RF circulator could revolutionize microminiature communications devices (Photo: Cockrell School of Engineering at UT)
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The new RF circulator could revolutionize microminiature communications devices (Photo: Cockrell School of Engineering at UT)
The new device uses a circuit created from inexpensive, surface-mount, active semiconductor components (Photo: Cockrell School of Engineering at UT)
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The new device uses a circuit created from inexpensive, surface-mount, active semiconductor components (Photo: Cockrell School of Engineering at UT)

In the world of electronic components, there are many devices out there that do their job well and reliably, but are almost never heard of – even though they may be vital to equipment that plays a role in our technology-driven lives. The radio frequency (RF) circulator is just such a device: it has simply done its job as a nondescript box of gubbins buried in radio communications systems, quietly directing radio frequency signals to the places they should go. Now researchers at the University of Texas have given the RF circulator a makeover. Not only is the new prototype smaller, lighter, and cheaper, it's also claimed to be easily adapted to different frequencies on the fly, which is something the old style circulator cannot do.

A standard RF circulator is a three-port ferromagnetic passive device used to control the direction of signal flow in a circuit. In simple terms, magnetic fields are used to channel electromagnetic flow in a specific direction, thereby providing two-way communications on the same frequency channel by allowing, for example, two transmitters to use the same antenna. The downside, according to the researchers, is the bulk and weight of a standard circulator.

The new RF circulator uses a circuit created from inexpensive, surface-mount, active semiconductor components. The team predicts that the current prototype is just a start to the miniaturization; the size of future RF circulators may be scaled down much further to sub-miniature versions of the components on an integrated circuit. As a result, the researchers say that this should lead to improvements in cost and size and allow the incorporation of circulators in cellphones and other microelectronic communications devices that may result in faster downloads, fewer call drop-outs and clearer communications.

"We envision micron-sized circulators embedded in cellphone technology," said Nicholas Estep, lead researcher and a doctoral student in the Department of Electrical and Computer Engineering. "When you consider cellphone traffic during high demand events such as a football game or a concert, there are enormous implications opened by our technology, including fewer dropped calls and clearer communications."

The new device is reported to work by copying the method used in magnetic circulators to route radio waves. In the standard RF circulator, a signal injected into one of three ports produces a magnetic field that flows in the direction forward of the signal. Any subsequent wave on the next port is then forced to follow the direction of the first. Something akin to stirring a cup of coffee quickly in one direction and then pouring in milk. The milk can only flow in the direction of rotation of the liquid in the cup.

In the new circulator the same effect is accomplished, but the magnetic field traveling in a ferromagenetic core has been replaced with an induced traveling radio wave spinning around the circuit of the device. In effect, the active electronic components create three symmetrically-coupled resonators whose resonant frequencies are modulated by external signals with the same amplitude to effectively "pump" radio frequency waves around the circuit.

As the resonators employ a "tank" circuit consisting of an inductor and variable capacitance diodes (varicaps), the new circulator has a real advantage over standard circulators in that it can have its RF tuning altered in real time across a wide range of frequencies.

"With this technology, we can incorporate tunable nonreciprocal components in mobile platforms, " said Estep. "In doing so, we may pave the way to simultaneous two-way communication in the same frequency band, which can free up chunks of bandwidth for more effective use."

No announcement was made on further developments of the RF circulator or its likely release as a consumer device.

This research was supported by the Defense Threat Reduction Agency and the Air Force Office of Scientific Research and was recently published in the journal Nature Physics.

Source: The University of Texas at Austin

7 comments
flink
It is official. My years of high-end electronics training from the 70's and 80's is now entirely obsolete. RF Circulator? WTF. Well, at least I don't need diapers and a drool bib, yet.
myth318
Colin, I think you got some of the details wrong here. Circulators are a piece of lab equipment used to route a device under test's antenna to multiple pieces of test equipment. They would never need to be included in an actual radio. Even if they created a "micron-sized" version, it would be non-nonsensical to put it in a commercial circuit any time in the foreseeable future. Mostly because there is no advantage to simultaneous communication on the same frequency other than to save bandwidth but this is already being done using time-division duplexing.
Ormond Otvos
Think about Professor Proton, and his new research into nano vacuum tubes!
ringo the Baptist
@myth318 - Nothing wrong with Colin's details as far as I could see. You may only have used them as lab equipment - but they have been used in certain radios for decades. Time Division Multiplexing between RX and TX must effectively halve the bandwidth, as the "conduit" is only available for half the time! (And then there is also the time taken for the switching, which means that even less than half the bandwidth will be usable) The article is good - circulators such as this new "frequency-agile" variety do look set to get into all manner of future portable devices. Great stuff Colin. One wonders what we will be be able do next!
Snert
@Ringo the Baptist - I'm with you, all the details are absolutely spot on with what I know about RF circulators (and their use as diplexers when you ground one input, as well). I've seen some gigantic ones in multikilowatt installations that actually get hot to the touch! I'd like to see what equipment we can make smaller with this new tech.
Jakes B
@Myth318 " it would be non-nonsensical to put it in a commercial circuit any time in the foreseeable future" Welcome to the already distant future. Most RFID readers have a circulator of some description. The transmitted CW signal excites the tag and the received modulated backscatter signal are obviously at the same frequency.
Bob Ehresman
Circulators at VHF and above frequencies are quite useful for maintaining constant impedance at the output of a high power transmitter in the face of varying (a good euphemism for mobile) environmental conditions. But are not particularly broadband. A common use case is television broadcast. Circulators that work well at HF frequencies are sort of a holy grail for SIGINT and jamming applications because most currently viable designs have ridiculously high Q and no frequency agility what so ever. This could be huge for all of communications.
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