RF over Fiber is a technology used to transmit Radio Frequency (RF) signals over long distances over Fiber. It is also called RF over Glass, RF Optical Transceiver, and Coax Replacement solution. In this technology the RF signal is converted to Optical Signal by laser diode and converted again to RF signal by photo diode.
An RF signal which is received through an antenna needs to be processed by a receiver. Typically antennas are connected to nearby receivers using coax cables. Even though coax cable weakens the strength of the RF signal, for shorter distances, RF power stays within the acceptable input power range of the receivers. However, there are some applications where using a coax cable becomes impractical or impossible. The decision to switch to RFoF is mainly based on RF link budget calculation. Due to coax cable loss typically for distances longer than 150 ft RFoF. The distance gets shorter as the frequency increases. RFoF is also used where coax cable deployment is difficult or costly due to its size and weight or where the fiber already exists.
RFoF solutions are made of 2 modules. The Tx module takes in the RF signal and using a modulation and laser converts it to optical signal which can have a wavelength of 1310 nanometer or 1550 nanometer depending on the laser type used. At the far end the Rx module takes in the optical signal and converts it back to RF signal which then can be connected to a Receiver. Depending on the frequency range either direct or modulation or external Mach –Zehnder modulation is used . Typically the latter is used in high frequency applications.
In designing an RFoF link one needs to calculate the optical link budget as well as the RF link budget. Optical link budget is used to calculate the loss introduced by the fiber cable. Even though the fiber has substantially less loss than the coax cable, it still introduces signal degradation. Fiber cable typically introduces 0.25 dBo (dB optical loss) per kilometer (km). There is a 1:2 ratio relationship between optical loss and RF loss. 0.25 dBo is 0.5 dB RF loss. The Optical budget is influenced from the system components including internal devices, optical and RF connectors and the condition of the fiber cable. It could be a huge difference between the theoretical calculation and the real loss due to many parameters so it is recommended to measure the optical power end to end. This can be achieved easily in RFOptic programmable RFoF series. Once the optical link budget is calculated, now the end to end RF link budget has to be calculated. For this it becomes important to know the specs of the RFoF solution that will be used. RFoF units based on the frequency range they support can introduce varying degrees of loss or gain. RFOptic also offers RFoF solutions with nominal gain which means 0 dB S21 system gain. In RFoptic’s programmable RFoF series gain can be easily be changed by activating the internal LNA and the adjustable 30 dB RF attenuator so that the link budget can be adjusted to the required value. Example: When using an RFoF solution that has a nominal gain (S21=0dB) if the input signal, S1, is -10 dBm, then the output signal, S2, will still be -10 dBm. In this scenario if the Tx and Rx modules are connected with a 10 km single mode fiber, the fiber will introduce 10 x 0.5 dB = 5 dB loss, S2 will be -15 dBm. The system gain, S21, will be -5 dB. When calculating link budget, one should also consider if there are any fiber patch panels and/or connectors on the fiber line. Typically, each connector will introduce about 0.5 dB loss.
RFoF system gain depends on the RF frequency signal. If the RFoF system gain doesn’t change much for the whole bandwidth the unit supports, it is said to have a flat response. System gain is important for deployments where the input signal frequency changes. RFOptic units are very flat meaning there is not much variation in system gain. For example our programmable 3 Ghz RFoF solutions has a flatness of ±1.5 dB between 0.5MHz to 3 GHz while our 6 GHz solution has a flatness of ±2.5dB between 0.5MHz to 6GHz.
This parameter is especially important to determine the max RF input power into an RFoF solution. A signal that is stronger than the P1db point of the RFoF system will be compressed and distorted. This will cause the RFoF system not to be able to produce the same RF signal at the far end. RFOptic provides an option to its customers to adjust the P1dB compression point to fit their needs, In the Programmable RFoF units P1dB point can be adjusted by modifying the S21 gain levels. In higher frequency units (8 Ghz – 20 Ghz) RFOptic can still adjust the parameter by incorporating a pre and/or post LNA (low noise amplifier) based on the customer needs. Here is the place to put a picture.
One of the challenges of installers is to know that the RFoF link is that the optical and RF functions are in order. This imposes to bring RF measurement tools such as signal generator and spectrum analyzer and optical power meter. RFOptic solves this issue of fast diagnostic by adding an injection of pilot signal from the Tx to the Rx,. The installer can check the functioning of the transmitter alone, the receiver alone and the link (transmitter and receiver) by injecting 15MHz pilot signal and measuring the output signal in comparison to the input. This off course not accurate like RF measurements tools but it is ideal for field installation without the need to bring heavy and expensive equipment.
The ODL is an electric-optic-electric instrument. It performs fixed time delay(s), between few nanoseconds up to several hundred microseconds, for RF signals from 0.1 up to 20 GHz and more (there are low frequency ODL versions 0.1-5 GHz, and high frequency ODLs versions: up to 8GHz, 15GHz and 18 GHz). ODL is also called Fiber Delay Line The RF input signal is converted into an optical modulated signal. The optical signal is transmitted into a long single mode fiber, usually at a 1.55-micron wavelength or similar. Passing the fiber, the optical signal is converted back into an electrical RF signal. An Optical delay lines system (ODL), incorporates high performance lasers such as DFBs, optical modulators for high operation frequencies, photodiodes, and optionally other components such as optical dispersion compensators, optical switches, optical amplifiers and Pre and Post RF amplifiers, to provide exceptionally high performance. The ODL optical system supports very high bandwidths of analog signals, high sensitivity with wide dynamic range, for various delays.