Some quick rules to be aware of in your Fibre Optic designs and discussions As the optical input level is increased by 1 dB, the RF output level increases by 2 dB. 2 As the RF level increases by 1 dB: a) The CNR increases by 1 dB b) The CSO decreases by 1 dB c) The CTB decreases by 2 dB What does all this mean? Simply that when you read the equipment specifications, the data is referenced to a particular optical input level. Now if you design to some other optical input level you can use these simple rules to compute what the actual performance levels will be.