Optical Switching Comes Of Age For RF

The benefits of using fiber for data transmissions are significant and undisputed. However, for years, RF engineers have believed that fiber optics delivers lower dynamic ranges with higher cable costs compared with coax. Naturally, this belief has limited fiber's deployment to RF applications where long-distance transmissions were beyond coax's capabilities. Today, a new generation of coax-to-fiber converters combined with all-optical switches are producing extraordinary results, delivering better dynamic ranges with very low noise figure levels – all at a lower overall systems cost. From a system perspective, getting superior results at lower costs has the potential to revolutionize the way engineers connect their antennae to receivers.

One large federal systems integrator recently discovered the benefits of using this fiber-optic technology combination firsthand in its design of a system that can dynamically connect RF receivers to an antennae array with each antenna array having multiple octave band outputs. The system, intended for use in a classified application, was facing budget constraints. The system had to receive signals from multiple octave band antennae feeds from multiple antennae in an array for a broad spectrum of frequencies up to 18 GHz. The system had to connect up to four receivers to any single octave band in the array at different times. At a cost of more then $100,000 per receiver, equipment costs alone could have made the project a non-starter if done in the traditional manner in which the microwave frequency receivers are co-located close to the antennae.

Senior Technical Manager Bruce Mead theorized that, by using fiber and sharing assets via an optical switch, he could use fewer total receivers in a pooled asset configuration. The fiber and optical switch would replace the conventional solution in which the receivers and an RF switch are placed close to the antenna and the IF is routed over the long coax cable runs through an IF switch to the processing subsystem. The longest distance between an antenna array and the operations center was 1000 feet. Other runs were shorter but also of significant distance. Fiber, unlike coax, can easily transmit RF cleanly over this distance without sustaining massive losses. This enabled Mead to bring the RF signals back to receivers enclosed in the relatively benign environment of the operations center, versus positioning the expensive receivers out by the antennae arrays. This design however, required some leaps of faith that flew in the face of traditional RF design thinking.