U.S. Air Force Research Laboratory scientists recently patented a new tunable radio frequency (RF) filter design. The new design is for a smaller, lighter, less-expensive device that requires much less power than current technology. As the Air Force moves to smaller, more “agile” flying vehicles such as hand-launched drones, reduced size, weight, and power (SWaP) for all components become more critical.
RF filters are a standard component in any device that transmits or receives RF signals. A cell phone, for example, operates within a certain frequency range, or bandwidth, assigned by the government. RF filters ensure that a cell phone operates within a particular bandwidth, preventing interference with or from other devices. However, for military applications, the ideal RF filter needs to be tunable.
“If you are on the battlefield, your adversary will likely try to contaminate your signal, something that’s not very hard to do,” said project team leader Dr. Michael Page. “We have the ability in place right now to tune the filter to remove undesirable signals while preserving intended functionality.”
Although the RF filter can be tunable by an operator, the best technology is “intelligent,” that is, it automatically searches for the best “noise-free frequency” and then automatically switches to it. The most common tunable RF filter currently in use is a bandpass filter.
“A bandpass filter is based on an electromagnet,” said Page. “It has a little magnetic sphere about the size of the point of a pen. That little sphere sits inside an electromagnet. When you change the magnetic field of the electromagnet, that changes the way the sphere behaves, which changes the operating frequency of the filter.”
Even though it works well, there are practical problems with the current technology.
“Electromagnets are fairly large,” said Page. “They can’t be miniaturized, and they use a lot of power. On a larger platform, that’s not an issue, but the way the Air Force is moving, we expect to have a lot of smaller autonomous platforms, drones, where you can’t use a big bulky filter with an electromagnet and a big power supply.”