In designing an RFoF link, one needs to calculate the optical link budget as well as the RF link budget. The 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, there is still some 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 relates to 0.5 dB RF loss 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. It is therefore recommended to measure the optical power end to end. This can be achieved easily with RFOptic’s 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. The gain can be changed easily by using the Tx and Rx 30 dB RF attenuator so in a case of low loss adding gain is done easily and 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.