Grass Valley Rattler 4 Feb 24 2015 - Manual (Page 18)

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Rattler 4 Operation
Fiber Optical Channel Monitoring using the LED Display
Fiber Optical Channel Monitoring using the LED Display
The Rattler 4 Rx receiver units provide direct digital readout of the Fiber Optic Link signal
strength for signals received at the unit. This readout is presented in 5 dBm ranges of dBm
units . Both the dB or decibel and the dBm or decibel referenced to one milliwatt.
The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a
physical quantity (usually power or intensity) relative to a specified or implied reference
level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless,
relative unit. A decibel is 1/10 of a bel, a seldom-used unit. Typically dB has been employed
in Audio Measurement and Fiber Optics among many uses.
Proper measurement of signal strength requires an absolute measurement and the dBm
provides this measurement. Since it is referenced to the milliwatt, it is an absolute unit,
used when measuring absolute power. By comparison, the decibel (dB) is used for
quantifying the ratio between two values, such as signal-to-noise ratio
The Rattler 4 operates within a defined range of Fiber Optic Link signal strength. The
minimum recommended signal strength is -20 dBm or better. Typically the system should
operate at levels between -8 dBm and -20 dBm. Cable length affects signal strength as does
the number of connections between the two Rattler 4 units. Each connection, interconnect,
or patch cable will produce a measurable signal loss that will contribute to decreasing the
overall link optical loss margin and attainable distance.
The maximum fiber distance is defined by the optical loss margin. The RX signal must be -
20 dBm or better. Losses on single mode fiber are approximately 0.5 dB/km or less. CWDM’s
account for about 5 dBm of loss per pair and must be considered when computing your link
loss budget.