Editing Signal-to-noise ratio (section)

==Optical signals==
Optical signals have a [[carrier frequency]] (about {{val|200|u=THz}} and more) that is much higher than the modulation frequency. This way the noise covers a bandwidth that is much wider than the signal itself. The resulting signal influence relies mainly on the filtering of the noise. To describe the signal quality without taking the receiver into account, the optical SNR (OSNR) is used. The OSNR is the ratio between the signal power and the noise power in a given bandwidth. Most commonly a reference bandwidth of 0.1&nbsp;nm is used. This bandwidth is independent of the modulation format, the frequency and the receiver. For instance an OSNR of 20&nbsp;dB/0.1&nbsp;nm could be given, even the signal of 40 GBit [[DPSK#Differential phase-shift keying (DPSK)|DPSK]] would not fit in this bandwidth. OSNR is measured with an [[optical spectrum analyzer]].