Understanding Gain and its Importance in RF over Fiber
RF over fiber is the method of transmitting radio waves over fiber optic cables. In order to do this, the radio waves are converted into light by modulating the intensity of the light source with the RF signal.
Across the system, it is natural for the signal strength to decrease while travelling from the transmitter to the receiver via fiber. This decrease in signal strength is known as loss, and in order to recover the signal, gain can be added.
What is gain and why is it important?
Gain is the amplification of a signal which can compensate for losses. It is possible to adjust the gain settings in both the transmitter and receiver to produce the correct output signal strength.
Signal losses are unavoidable due to the physics and construction of fiber cable, and in many cases it is important that gain is used to compensate them. The length of the fiber system has the greatest impact on the magnitude of the losses. ViaLite customers can have fiber systems ranging from a few hundred meters to hundreds of kilometers long. In RF over fiber systems, you can expect to lose around 0.4 dB/km for 1310 nm wavelength systems and 0.2 dB/km for 1550 nm systems.
RF signals going through the RF over fiber link should ideally sit within the Dynamic Range of the system. The Dynamic Range is given from the Minimum Detectable Signal (MDS), lowest possible signal, through to the maximum signal which is determined by the P1dB level, i.e. the level where the signal starts to compress from ideal response, see figure 1.
Adjusting the gain can affect the MDS and P1dB levels, i.e. if you increase transmitter gain, this lowers the P1dB and MDS levels and conversely, decreasing transmitter gain increases the P1dB and MDS levels. This effect on MDS and P1dB needs to be considered when compensating for fiber losses and also the signal levels used in the RF over fiber system.
ViaLite Product Signal Gains
There is an extensive range of ViaLite RF over fiber products and the gains applied will depend on the application they are used for. Figure 2 displays the different gain settings for the transmitters and receivers for some of the more common link systems.
All gain variants are capable of being adjusted using a Digital Step Attenuator and have a window of 15.5 dB. ViaLite RF over fiber links are delivered to a factory set default gain setting, with the adjustment window allowing the user to increase or decrease gain as needed. Adjustment can be performed using software set parameters or through a manual switch on the rack card variants.
Breaking down the different signals, Figure 2 shows:
- GPS/GNSS & Signals Intelligence (SIGINT) links have default transmitter settings of -5 dB gain input signal, with receiver gain of +5 dB
- Satcom Inter-Facility Links (IFL) are set with the transmitter having -11 dB of input gain and the output gain at the receiver being +20 dB
- Broadcast and general purpose links are set as -15 dB / +15 dB for the transmitter / receiver gain
- 10 MHz Timing Reference Links will typically have transmitter input gain of around -25 dB and receiver output gain of +25 dB
- For very high signals, input gain will usually sit around -35 dB, while output gain will be +35 dB.
For further help deciding which gain option is the most relevant to a specific application, please see the gain guide; Box 9 of ViaLite product options. Another option is to model the RF over fiber system using the ViaLite System Designer Tool, which can calculate the performance of the solution end-to-end.
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