Fiber for RF

It’s not just for SONET anymore.

One large federal systems integrator discovered the benefits of using flexible optical switching first hand in its design of a system that can dynamically connect RF receivers to an antennae array. The system, intended for use in a classified application, had to receive signals from multiple 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 feed in the array at different times. At a cost of more than $100,000 per receiver, equipment costs alone could have made the project a non-starter if done in the traditional manner.

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 transmitted radion signal (IF) is routed over the long coax cable runs 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.

Figure 1 – System Design

This design however, required some leaps of faith that flew in the face of traditional RF design thinking:

First, the dynamic range of the system had to be at least 50 dB. Conventional wisdom “was” that you could get only 40 to 45 dB of dynamic range out of fiber.
Second, transmissions from each of the antennae had to be connected and sometimes multicasted between the antennae and the fewer receivers by using a switch.
Third, the need to multicast RF signals and connect them to multiple receivers simultaneously required amplification, adding additional cost and noise.
Fourth, the solution needed to exhibit very low noise figure performance (< 5dB) for signals traversing the 1000’ distance, passing through the RF-to-fiber converters, through the optical amplifiers, moving through the optical switch and terminating at each receiver.

Mead assembled a risk reduction test consisting of single-mode fiber, Optium RF-to-fiber converters, an optical amplifier at 1550 nanometers and a Glimmerglass Intelligent Optical Switch. To assist in this first-of-a-kind test, Glimmerglass sent two engineers along with its switch, to the test site. In turn, Mead invited an ace field engineer, known as “MacGyver” to his peers for his skill and innovative abilities to fix difficult problems with the materials at hand in the field. At the conclusion of the first test, the engineer said:

“This is the finest RF performance I’ve seen in 35 years. We don’t need to do any more testing. They’ll get even better performance in the field.”

“System receiver sensitivities ranged from -110 to -118 dBm, results that exceed what Mead can get by connecting his signal generator directly into the back of the receiver! I have never seen RF performance like this,” said Mead, referring to the cleanliness of the optical signal at his receivers. “The RF path is incredible.”

The system’s RF performance opens up new options for systems designers that have operational areas a significant distance from their antennae. “We are advancing the state of the art,” said Mead. “This end to end system performance utilizing the optical transmission medium is so good that people will no longer use coax. We’re changing the paradigm.”

The combination of fiber, RF-to-fiber converters and a Glimmerglass Intelligent Optical Switch to reduce the number of receivers actually costs less than dedicating receivers to each antenna. Mead’s team was able to successfully design a system using half the originally estimated quantity of receivers.

“We were able to deploy the entire implementation for what a single non-blocking, full fan-out RF switch would have cost.”

From a systems perspective looking at the total cost of implementation, fiber has become considerably cheaper. And it’s now proven to out perform conventional solutions. It costs less and delivers equal or better dynamic range, improved sensitivity and outstanding noise figure performance. That’s a hard combination to beat.”