In an increasingly interconnected world, the demand for high-speed and high-capacity signal transmission is challenging the limits of traditional coaxial cable systems. Recently, people's interest in fiber optic radio frequency transmission (RFoF) has been increasing day by day. This technology combines the low loss and high bandwidth advantages of fiber optic with the multifunctionality of radio frequency communication (Figure 1). The RFoF system transmits RF signals through optical fibers, achieving long-distance, interference free signal transmission in a wide range of applications from satellite ground stations, remote antenna deployments, to 3G-5G infrastructure and defense systems. This article explores the basic principles of RFoF system design.
The main functions of RFoF
Figure 1: Main characteristics of RFoF. (Image source: NuPhotonics)
Long distance transmission - signal strength
The performance of coaxial cables varies depending on the cable configuration. The insertion loss of a typical dielectric SMA cable is approximately 0.25 dB/m (at 2 GHz). The performance of inflatable cables is slightly better, but the cost is much higher. It is precisely this high loss characteristic that makes RFoF technology applicable for transmission distances exceeding 50 meters. In RFoF technology, the most commonly used wavelengths are 1310 nm and 1550 nm. The loss at 1310 nm wavelength is approximately 0.35 dB/km, while the loss at 1550 nm wavelength is only 0.25 dB/km. It can be seen that the loss of this technology is significantly lower than that of coaxial cables.
DigiKey and NuPhotonics simplify the component procurement process
DigiKey has been leading the world in simplifying the procurement process of critical components. Amateur enthusiasts, students, professionals, and large corporations are all purchasing components through DigiKey. As a leading manufacturer in the RF and optoelectronic device industries, NuPhotonics has partnered with DigiKey to provide easy-to-use and easily accessible component products for the industry, which is a natural progression (see Figure 2).
NuPhotonics 10G PIN photodiode tail fiber FC/APC
Figure 2: NuPhotonics10G PIN photodiode tail fiber FC/APC. (Image source: NuPhotonics)
Although there are currently some commercial solutions available, they often lack economic benefits. This article will introduce standard design, enabling users to develop low-cost specialized solutions using NuPhotonics components. The products and solutions discussed in this article can be easily purchased from DigiKey.
RFoF transmitter design -10G DFB laser
The first part of designing an RFoF system is developing the transmitter. For RFoF architecture, it is necessary to modulate the data RF signal onto an optical carrier signal, and then transmit it through an optical link. Distributed feedback lasers (DFBs) can be directly modulated by radio frequency signals, making them an ideal device for converting radio frequency electrical signals into optical signals. The basic principle is shown in Figure 3. Due to the anode side bias method used in the laser, it is also an input terminal for RF frequency. To ensure system safety, the circuit includes a DC blocking capacitor (C2). The value of C2 will be fine tuned according to the desired low-frequency cutoff point. The resistor R1 in the circuit is used to impedance match the 10 Ω DFB laser with the 50 Ω system. The larger the R1 value, the better the matching with the link, but the disadvantage is that it will increase the insertion loss of the optical link. This can achieve precise level control to achieve the required impedance matching and insertion loss indicators. The resistor R2 in the circuit is a current limiting resistor used to limit the current of the laser. Inductor L is a high impedance path for RF signals and also the minimum impedance current path for laser DC bias. Capacitor C1 is an optional device used as a filtering capacitor to filter out power supply noise on biased T-type capacitors.
10G DFB laser with bias T-junction and impedance matching circuit
Figure 3: 10G DFB laser with bias T-junction and impedance matching circuit. (Image source: NuPhotonics)
RFoF Receiver Design -10G PIN Photodiode
The light in optical fibers needs to be converted into more useful electrical signals. For this, photodiodes can be used. When photons with sufficient energy collide with a diode, electron hole pairs are generated. This mechanism is also known as the internal photoelectric effect. These holes move towards the anode (+) and electrons move towards the cathode (-). This effect will generate photocurrent. Due to the involvement of broadband operation in the circuit, photodiodes will operate under reverse bias. When reverse biased, current will only pass through the photodiode under the condition that incident light generates photocurrent. This bias method also has another advantage, which is to improve the linearity of the photodiode. By increasing the size of the depletion layer, the reverse bias response time can be shortened. The increase in depletion layer width will reduce the junction capacity and increase the drift speed of charge carriers in the photodiode. The transit time of charge carriers is shortened, and the response time will also be shortened accordingly.
Figure 4 shows the basic driving circuit of a photodiode. There are similarities between photodiode circuits and laser circuits. Capacitor C is a DC blocking capacitor used to protect RF ports. Inductor L is a low impedance DC ground path that allows current to flow from the DC bias pin to ground, as DC blocking capacitor C has no direct ground path. Choosing R1 and C1 correctly can help improve high-frequency impedance matching.