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HomeLifestyleTech & gadgetsRF over fiber: overcoming an inherent transmission-line problem, part 2

RF over fiber: overcoming an inherent transmission-line problem, part 2

Learn the basics of how to use RF over fiber. You have many options.

Coaxial cable is widely used as it has many favorable attributes, but it is not the only or best choice in some cases. Part 1 covered the fundamentals. In part 2, we look at design issues.

RF waveguides

Figure 1. Waveguides are very effective at conveying and confining RF energy but are awkward to install and arrange, and also to reroute if needed. (Image source: Pasternack Enterprises)

Q: What can be used instead of a coaxial cable?
A: A very low-loss option is to use rigid waveguides (Figure 1) rather than coaxial cables — which are flexible waveguides for RF energy — but these are harder to set up, place and route, or re-route While waveguide dimensions get smaller as frequency increases making them somewhat easier to handle and place, they are still costly and awkward to use. Nonetheless, they are a very low-loss RF transmission-path solution.

Q: How does optical fiber fit into to RF transmission-line story?

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A: There advantages of optical fiber are the well-known: immunity to EMI/RFI, security against signal interception, lighter weight, flexibility, thinness compared to waveguides, and overall cost-effectiveness for a given level of performance and distance. But they are designed to carry electromagnetic energy in the optical spectrum, not electromagnetic energy in the radio spectrum.
Note that from a physics perspective, optical fibers are waveguides for optical energy, just as metal waveguides perform that role for RF energy. The use of optical fiber in place of coaxial cable to convey electromagnetic energy in a confined path is just another manifestation of Maxwell’s equations; after all, whether the energy is in the radio spectrum or in the optical spectrum, it is still governed by those equations.

Fiber optic transmitter receiver

Figure 2. The laser-diode transmit side and the photodiode receive side of the link each require sophisticated circuitry to ensure linearity and compensate for unavoidable imperfections and thermal effects. (Image source: Optical Zonu Corp.)

Q: Those attributes are well known, but that’s for digital signals on optical fiber. So how does RFoF work?
A: Those attributes still apply, with a twist. The RFoF signal chain looks straightforward but with two unusual blocks: a laser-diode electrical-to-optical converter (E/O modulator) at the transmit side and the complementary phototransistor-based optical-to-electrical converter (O/E demodulator) at the receiver side.
The RF signal intensity modulates the laser directly without any frequency shifting, creating a linear optical signal which represents the original analog RF signal, except now it is a modulated optical signal, Figure 2. Impedance-matching circuits are used to maximize power transfer and minimize reflections of RF signal power at the transmitter, or photodiode output at the receiver back to the source, which would cause distortion and nonlinearity.
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Q: Is it just a matter of controlling the optical-signal amplitude?
A: Yes in the general sense, but not in the technical sense: this is intensity modulation rather than amplitude modulation. That’s because you can’t change the amplitude of a photon, which is constant and solely a function of the photon’s frequency (wavelength). What you do control and modulate is the number of photons created, with each photon having that pre-defined energy value commensurate with its frequency.

Laser diode photodiode

Figure 3. Understanding the transfer function of the laser diode and the photodiode is critical to achieving the desired overall performance. (Image source: Optical Zonu Corp.)

Q: What’s the critical circuit function here?
A: The linear performance of the laser diode of the E/O modulator is most critical, as is the complementary function at the photodiode, Figure 3. This linearity is achieved by, among other techniques, careful control of the current drive to the diode (LEDs and laser diodes are current-driven devices) as well as temperature compensation, as their performance is highly temperature dependent.

Q: Are any other circuit functions needed to enhance performance?
A: An automatic gain control circuit (AGC) with a typical dynamic range of 40 dB is used at both transmit and receive sides to further improve performance. There’s some historical continuity here, as AGC is a circuit function that has been used since the very earliest days of “wireless” links and even unstable wired links (the 1920s) to even-out signal strength which shifts over a wide dynamic range.

Q: What distances are possible with RFoF?
A: Due to the low-loss nature of advanced optical fibers, distances of several hundred kilometers are feasible, which is far greater than with coaxial cable or waveguides.
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RFOptic optical RF transmission set

Figure 4. This off-the-shelf RFoF system is designed to transparently support the frequency range of 1 to 40 GHz. (Image source: RFOptic)

Q: Is an RFoF system largely a custom, do-it-yourself scheme?
A: Not at all. Many vendors provide all the items needed as off-the-shelf components in the form of modules, chassis, and complete systems. Among the vendors who offer standard RFoF products are ViaLite Communications, RFOptic, Huber+Suhner, Amtele Communication AB, Optical Zonu Corp., and DEV Systemtechnik. Their websites show their product listings and data sheets for the electro-optical and optical-electrical modules, generally packaged in standard form-factor PC boards or enclosures, supporting bandwidths as high as 10 GHz and even higher, Figure 4. They also offer various auxiliary functions and blocks needed for a complete end-to-end system.

Q: Up to what RF frequencies can RFoF operate?
A: Many standard systems are available up to 10 GHz, and there are even some reaching 40 and 50 GHz.

Q: Are RFoF system functions and interfaces standardized?
A: Yes and no. Most systems are proprietary to each vendor, but the industry is in the process of setting interoperability standards. That will likely expand applications’ interest and attractiveness to users.

Q: What does the RFoF future look like?
A: Seeing these specifics also reinforces a trend of increased system-level analog and digital integration of electronic and optical functions, especially with increased integration at the component and even chip levels. We now have MEMS-like optical waveguides, filters, interferometers, and other functions in solid-state materials such as lithium niobate, for example.

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Intensive R&D efforts are working towards developing active, integrated optical components as hybrid devices and ultimately as monolithic ones in silicon, but there are some difficult basic-physics barriers to overcome. Perhaps we are at the early stages of an inflection point, roughly analogous to how discrete transistors became integrated circuits and mixed-signal devices, and which subsequently changed everything.

References

  • James Burke, “Connections”
  • Vitalite Communications, “What is RF over fiber technology, and what are the benefits?”
  • Vitalite Communications, “Application Note 037 – Fibre Optic Link Budget”
  • Huber+Suhner, “RF-over-Fiber series”
  • Everything RF, “What is RF Over Fiber?”
  • RFOptic, “RF over Fiber Converters and Bands”
  • Wikipedia, “Radio over Fiber”
  • Amtele Communication AB, “RF over Fiber”
  • Optical Zonu Corp., “RF over Fiber”
  • DEV Systemtechnik GmbH, “Wideband RF Over Fiber Systems by DEV”




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